It’s Just A Pickup

[truck72454]

This is one outstanding truck, very detailed build post, just hopes mine will turn out half as good as this truck is

[yfs200p]

After 26 pages and 700 posts, it all goes back to the first words you read on this post; "It's Just a pickup." Some people would call us crazy for putting so much blood sweat and tears into just an old pickup truck. But once its done, its a damn nice pickup truck and its something that will make peoples heads turn when they see it. Youve done an amazing job since the word go, and you should be awfully proud of the way that thing came out. Even if its just a pickup....

[Bruce88]

Happy New Year to all

A change to this build thread. In this build thread in the past I was showing you what I did to put all the parts together in the build and my approach to overcome some of the problems that I’ve run into. All the parts are together and now its time to get them to work together, what I would call tuning to what I have.

I’ve learned a few things over the years regarding tuning what I have sitting in front of me with a fare amount of success. As there are very few if any that have the same parts put together as I do, any specific information that I may give regarding what I have sitting in front of me may not be of value to others trying to tune there engine or setup.

With that being said, why would you want to follow this thread and what benefit is there in it. I’m going to try to show you an approach that I would take to tuning the engine and setup sitting in front of you. There has to be some limitations and there might be some crossover information that would be of benefit to other setups but you’re going to have to figure that out yourself.

Limitations
A normally aspired engine V8 or 6 carbureted (not computer controlled, supercharged or turbocharged) this is a engine that’s not expected to exceed 5000 – 6000 RPM
Street performance (not all out racing or to meet emission standards)

End goal
Good performance and reasonable fuel economy thru normal street RPM of 600 – 5000
I look at every engine as it has potential and I’m working to get 90 – 95% of its potential out of it. As there is always a give and take between performance and fuel economy and drive ability I try to shoot for a middle ground and striving for 100% potential is not practical or you would be in perpetual tuning 3 or more times every day.

Emission standards and this tuning approach don’t work together to reach the potential of the engine that you have. Every state has there own approach to emissions for 1972 & older cars and trucks and it is to your benefit to do the research and find out what’s expected of you by the state that your in before wasting your time and effort and then finding out that you don’t pass the requirements to drive on the streets in your state.

Where to start
First you need to find out everything that you can about the engine sitting in front of you. I know that is easy to say, but some times it’s almost impossible to gather all the information. Any information that you can gather together will make the tuning easer and will end in better results at making every thing that you have work together. This is not the time to think about changing parts, rebuilding, upgrading, or going to a more performance oriented item (where working with what you have). This is not to say that you won’t be rebuilding or making a decision to change parts to achieve the end goal just not at this time and hopefully we will find out as early as possible if that might be necessary. Most of tuning is checking that all the parts are doing what they should be doing and optimizing them to work together.

If you’re working with a crate motor or engine built by a reputable supplier you should have the build sheet and a lot of the information is on that sheet. If the engine came with the car/truck or you bought it from someone, don’t trust what was told to you or a sticker on the valve cover or on the air cleaner (things might not be what they seem to be). You need to start checking block and head part numbers and figure out what you have, plus any parts that you can identify that have been changed such as carburetor, manifold, or distributor. You’re not disassembling the engine or parts you’re just gathering what information from what you can see with everything together.

Thanks to the internet and search engines in aiding in gathering information. Sometimes it’s as simple as imputing a part/model number and manufacturer and you can get the information that you want, other times you might have to go to a web site that has the information in it. (Example) Chevy Mania http://www.chevymania.com tipped off to by bruceman1968 (a forum member) aiding in deciphering stock block, head, manifold and carburetors.

There’s too much information and variables to list. I think that most of the engines out there in your trucks have been modified from stock in some way. When your tuning you don’t want to be looking around for the information and having it in one place makes it easer, plus the next time you stop down to the parts store needing a part and they start asking you a bunch of question as to year/model size or part number for there crossover chart you have it. I’ve included three pages of a blank form (Engine Specifications and tune up guide) that you can copy and print and then fill them out. There might be other items that you think should be added, it’s your sheet add them this is just a generic form. If you want to see what kind of information you’re looking to include on the form I think the crate motor guide by GM is a good example of the type of information. Hear is a link to one in a PDF format (Adobe Acrobat). http://www.gmperformanceparts.com/_r...ngine_Base.pdf Any information regarding when a part was used and its intended operating range in RPM, PSI or vacuum is definitely a nice thing to have on the form.







The next post is going to be dealing with some basic tools that you will be needing and possibly some not so basic tools that you might consider.

Enjoy Your Build

[greystoke]

this is going to be some very useful information, thanks for taking the time Bruce. You da man !!!

[Bruce88]

Tools, tools, tools, some say you can’t have too many
But some times there’s no workaround for not having one (smile)

I just bought some new tools, not that the old ones no longer worked but I’ve had them since 1970 and I was looking for some that where more universal and simpler to use or maybe better words updated with added features. I’m not one to recommend a particular brand or manufacture in tools but I think the ones below will meet my requirements.

Compression Tester ---- All the tuning that you can do is not going to make an engine run great if the engine is not in good condition. Even a brand new engine can have problems that need to be address before you start tuning. On a new engine I would do a compression test after breaking it in for about 500 miles. This break in time I just set the timing to 5 – 10 degrees BTDC and adjust the carb to idle fair. This is not the time for wide open throttle (WOT) acceleration runs but there’s plenty of information that can be gathered during this time. The primary thing that I’m looking for in a compression test is that each cylinder is doing its part, the high and low cylinder should be within 30 -35 PSI of each other. There’s way too many problems that can be uncovered by a compression test to list hear and they need to be addressed before you start tuning.

Vacuum Gauge ---- There’s no way that you can do tuning with out one. So many things are based/controlled by vacuum and you need a gauge to determine if there working at the right time and optimizing them. Plus it’s a good tool to diagnose problems with the basic engine condition. I prefer a large (3 inch diameter) one to use because it’s easy to read plus one that works as a vacuum and pressure gauge. The pressure gauge side for checking/setting fuel pressure at the carb. I prefer to have a tach and vacuum gauge permanently install in the truck cab, but you can get by with additional vacuum hose and a vacuum gauge temporarily installed in the cab during the tuning. You need it hooked up to fulltime vacuum on the engine. Its easy to identify a full time vacuum port, if you plug the vacuum hose on a port and it has vacuum at idle it’s a full time vacuum port if no vacuum it’s a timed port. If you can’t find an open port on the carb or manifold you will need to tee off a full time vacuum hose. You can find all different size vacuum tees and caps down at your parts store, I prefer staying away from teeing off the PCV valve port or hose.

Hand Vacuum Pump ---- I’ve had trouble with plastic vacuum pumps in the past, they seem to last a few years then start leaking and not holding vacuum. Even after cleaning, lubing seals and tightening fittings they still leak, maybe it has something to do with going from hot to cold weather. I’m going to try this metal one out. A vacuum pump is good for checking that the distributor can is working right and setting the right vacuum operation of the can. Plus you can use it for vacuum bleeding the brakes with some special cups or checking out vacuum switches and valves used on other areas of the cars and trucks.

Timing Light ---- This timing light has mutable functions. It can advance or retard the timing mark in degrees, RPM reading, battery voltage, charging voltage and checking dwell. If your timing light does not have the additional functions you will also need a RPM meter and a dwell meter if you’re working with points. To advance or retard the timing mark is a nice thing to have but not necessary you can use a timing tape on the harmonic damper. One problem you might run into with a timing light is erratic readings or no readings due to EMI/RFI (Electro-Magnetic Interference/Radio Frequency Interference) interference from the spark plug wires if there (solid core wires, racing wires, or off-road wires) along with some aftermarket ignition systems. The solution might be as simple as temporarily install a street spark plug wire on number one cylinder or you might have to contact the aftermarket ignition manufacture to see what they recommend. I personally have not run into this problem with any hi-performance street ignitions that I’ve used.



Some other things you might need or are nice to have.

Timing Tape ---- If your timing light does not have the ability to advance or retard the timing mark in degrees you’re going to need some timing tape. The timing tape is based on the diameter of your damper and there are a few different diameter dampers out there so you need to measure what you have. You can usually find one at a performance shop or JEGS/ Summit, a nice one is the timing tape by MSD and it covers a number of different damper diameter sizes. Your damper might have the degrees marked on it, if you can read them with a timing light you don’t need the timing tape, the number of degrees that you might find on your timing tab is not enough to take all the readings that are necessary.

Multimeter ---- Is nice to have any time you’re dealing with electrical items for trouble shooting any problems as to why it’s not working. The one I use is a fluke model 115 but it’s probably more expensive one than is necessary for working on cars and trucks. If you need a RPM/Dwell meter I would be looking for an Automotive Multimeter that includes a RPM and dwell function as well as all the standard multimeter functions.



Air/Fuel Meter ---- Not that you would find it in most home/garages, it falls within the status of a luxury and not falling in the category of a must need to fine tune your car or truck. There are plenty of articles/write-ups out there in tuning and it’s been for a long time without an air/fuel meter and there’s plenty of good information. Performance street tuning has always had problems, when you make an adjustment/change to one system it might have an effect on another or how it performs on the street and cause a problem. I’ve found through the years that when things just didn’t seem to be working right, I fall back on my basic understanding of what it should be and how do I measure it. I’ve tried some other air/fuel meters (home/garage units) in the past and not found a lot of benefit to me in using them, mainly because they where not very accurate and would give false readings. All I can say about this air/fuel meter system at this time is I’m going to give it a try and see if it will be accurate enough and a benefit.



The next section will deal with some more basic system operation. I know (yuck) a lot of people just want to set the timing and fiddle with the idle adjustment screws a little and go out and hit the road. This sometimes works but most of the times not, where looking to optimizing every thing and ignoring some of the basics can cause problems down the road.

Enjoy Your Build

[69gmcc10]

Bruce, I can’t wait to find out what I am going to learn from the next stage of your build, keep it up!

P.S. Thanks again for the help on the 700r4 kit.

[jchav62]

What an inspiring and educational build thread. I'm in AWE!! You sir, are a pro.

Beautiful truck... My favorite part is how you engineered the gas filler neck behind the frenched taillight. Absolutely amazing...

[Bruce88]

Sometimes we or others create problems, and then try to solve them by adjusting other systems
Most of the problems are easy to identify, check and solve with the basic tools and your eyes.

Fuel system ---- I’m not directly talking about the carburetor at this time, all that feeds the carburetor and engine. If something is out of standards or not doing its part it can have an effect that is not easily solved by adjusting other systems.

Fuel ---- I prefer to tune with the fuel that the engine is designed to run on (regular, premium or higher octane). The primary factor that determines the grade of fuel to use for a particular engine is compression ratio. It’s simple to figure out, see the chart below from (Advanced Engine Technology by Heinz Heisler) to figure out what fuel you should be tuning with. Generally speaking a street engine is 7 -1 up to 11 – 1 compression ratio, de-tuning a engine to run on a lower octane than it was designed to run on is a waste of effort and you end up with a very poor performing engine that will not last for the long run. There is no benefit in running a higher octane fuel than the engine was designed for, you’re not going to create more performance. Yes every one can find them self with a bad tank of gas or old gas that has a low octane level, adding some octane booster or a higher octane fuel to compensate is a solution and will not hurt any thing, but it’s not the fuel to tune with.



Gas Cap / Venting ---- The fuel tank needs to be vented in some manor in order for the fuel to flow, if it’s not vented the fuel pump will be creating a vacuum in the fuel tank preventing the fuel from flowing. For tuning my primary concern is that the vent system is working and not allowing a vacuum to be created in the fuel tank. It’s simple to check, just take a drive for about 15 to 20 minutes, stop and immediately remove the gas cap and note if you hear any air being sucked into the gas tank. If you hear air being sucked into the gas tank the vent system is not working and needs to be fixed, it might be the gas cap or a blocked vent tube, what ever it is needs to be addressed. Leaving the gas cap off or a rag in place of a gas cap is not a solution (smile). Note, some modern day cars are designed to have a slight vacuum/pressure in the fuel tank and when you go to fill them up you might hear some air at the gas cap, this is not necessarily a bad thing for them but is for an older carbureted engine.

Fuel Pump ---- They can be wore out, high performance/racing one, electrical or mechanical one. The question is, are they delivering the right fuel pressure for the carburetor. The carburetor is very sensitive to float level/fuel level in the carburetor, and the float level is directly related to fuel pressure being supplied to it. It’s easy to check just put a tee fitting in the fuel line between the pump and carburetor and attach your vacuum/pressure gauge, start the engine and read the pressure, pressure should be 5 – 7 PSI MAX 8 PSI, anything above or below the limits need to be corrected. This could be replacing a low or high PSI pump to one that delivers the right PSI or possibly a pressure regulator to reduce the high PSI pump to bring the pressure down to acceptable limits.

Fuel Filters ---- Often not thought about until there fully clogged and the engine won’t run. Normally changed by number of miles used as normal maintenance, often neglected during or thought about during those times. If there’s any doubt as to when the last time it was changed, put a new one in there cheap. On older cars and trucks there’s no way to know how well they’ve been treated and there is another filter that might need changing, it’s more or a screen in the fuel tank on the pickup tube. There’s no rule as to when to change that screen but it can deteriorate and fall a part allowing larger particles/dirt into the fuel line. All the changing of the inline fuel filter is not going to overcome feeding it dirty fuel from the fuel tank. This screen is just something that you might want to think about if you’re running into fuel filter clogging.

Positive Crankcase Ventilation System (PCV Valve System) ---- A lot of problems can be generated by an inoperative/disabled or some modifications that might cause it to not operate as it was intended. I’m talking about the PCV system not just the PCV Valve by it self, the valve could be right and the rest of the system causing the problem. There’s very little you can test in your garage to make sure the system is operating right except a visual inspection that all the parts to the system are there, if any of them are missing or disabled you have a problem. Most of the problems I’ve seen center around aftermarket valve covers and air cleaners, not that there a bad item, It’s just that they where not installed right or provisions where not made to keep the PCV system operating. I have a minimum list of all the parts of the system that need to be there and checked that they are there for a system to operate. What I would consider as obvious such as a capped off PCV port on the carb or damaged or pinched hoses or clean filters are not included on the list.
(First) Where is the filtered air coming into the crankcase? Air has to come in somewhere for the system to work. You can find this on the opposite valve cover to the PCV Valve or oil fill tube on V8’s or a tube down into the crankcase on I6’s, the filter can be a standalone on the valve cover or be connected to the air cleaner with a hose or tube, a separate filter in the air cleaner or using the carb air cleaner as the filter.
(Second) Where are the baffles in the system? They have to be there to prevent sucking liquid oil thru the system. Normally found in the valve covers, they work to separate the oil from the gases passing thru the system. They can be as simple as a plate or more elaborate baffle preventing oil from splashing up into the PCV Valve or air filter. Easy to check that their there, simply look down the hole where the PCV Valve is and the filtered air is going in, you should not be able to see the rocker arms in the valve cover. If you see the rocker arms the baffle is not installed and needs to be or the valve cover needs to be replaced with one that has baffles or provisions for one.
(Third) Where is the PCV Valve? I like to look at the PCV Valve as a controlled vacuum leak, drawing air thru the crankcase to eliminate gas buildup in the crankcase. There’s no test to check the PCV Valve except a visual inspection that it’s clean and the spring inside is working, spring not broken or restricting the free movement of the valve inside. As to what PCV Valve to use very little information is available to you or me regarding the flow rate/CFM of one valve over another. So the best choice for a valve is to find a valve for your particular engine or engine type and size down at the parts store thru there long list of valves to chose from. Generally you can find the PCV Valve in the valve cover or in the hose from the valve cover to the carb PCV port or maybe in the intake manifold going into the oil valley under the intake manifold.

This is a very simplistic look at the PCV System, there might be some additional items that can be found but this list meets the basic items that need to be there or it won’t work properly. Carburetors from 1968 and on or having a PCV port on them require a properly working PCV System or there will be troubles in tuning the carb.

Not looking at the parts/systems listed thru out this post can cause you problems and when your making adjustments to the ignition or carburetor your not trying to compensate for the above items not doing there part.

The next post will be addressing distributors and their curves, a must in understanding to bring the most potential out of a particular engine.

Enjoy Your Build

[camrycurt]

This tuning information is great, and I can't wait for the rest of the write-up.

Clear, understandable information like this on how the systems under the hood operate is gold for the suburban, in-over-their-head, non-mechanics like me. If I stare at it long enough I'll be able to figure out how to patch the rust in my fender, but I'll never be able to noodle through how a carburetor works and what I need to do to get mine humming.

Thanks so much for your invaluable help!

[Bruce88]

Originally Posted by camrycurt

This tuning information is great, and I can't wait for the rest of the write-up.

Clear, understandable information like this on how the systems under the hood operate is gold for the suburban, in-over-their-head, non-mechanics like me. If I stare at it long enough I'll be able to figure out how to patch the rust in my fender, but I'll never be able to noodle through how a carburetor works and what I need to do to get mine humming.

Thanks so much for your invaluable help!

Hi Curt

I’ve seen some good mechanics that got what I would call tunnel vision and say there problem with a particular engine tune is lets say the (carburetor) and didn’t check out all the systems that effect it performing right and end up making adjustments or changing carb parts trying to solve it. I’m writing this from the perspective (hopefully) of you starting a tune on your engine in your own garage. The simple tests that I talk about, I believe are within your ability to do with some basic tools and your eyes and if you perform them before you start making adjustments to your carburetor, the odds are that very minor adjustments will be needed to make the carburetor do its part. Some times its a little tid bit of basic information and how it applies and (kabam) you will understand how your carburetor works and be able to figure out what adjustments you need to do. So don’t write your self off as never being able to do it. There’s always the possibility (smile)

[Bruce88]

Distributor curves ahead
Yes there will be math, but trust me its not difficult (smile)

There are many different distributors that you can find on your engine. All the distributors have a mechanical advance, what I would call fly weights controlled by springs. Not all distributors have a vacuum canister, controlling timing advance based on engine vacuum. For street performance you need a distributor with both the mechanical & vacuum canister advance. If you don’t have a vacuum advance canister on your distributor, you need to replace the distributor with one that has one.

We are going to be checking and setting the distributor advance curve. Keep in mind that all the distributors work the same concerning how the timing advance is accomplished, but the mechanical advance might not be as easily accessible on all the different distributors. For example some of the L6 engine point distributors have the mechanical advance hidden under the point’s plate, a little bit harder to get to but it can be done. I’m going to be using pictures based on a SB Chevy engine point & HEI distributors but the information applies to all mechanical/vacuum controlled distributors.



Every distributor that I’ve seen for the street has the mechanical advance springs and a vacuum advance canister, the mechanical advance might not be as easily seen as in the pictures but it’s in there, you just have to look for it. The springs and the canister are the two primary items that where going to be looking at and possibly making adjustments to in order to bring the distributor curve to match your engine for street performance.

The largest problems I see is the lack of information by some distributor manufactures either stock or custom relating to timing curves. Or the different numbers used by some to accomplish the same curve but makes understanding and changes questionable and confusing. So the next paragraph of information is very important to understand before you start making adjustments.

As you’re looking at the specification sheet for a distributor or parts like springs or canisters, the first question I ask is the information in distributor degrees/RPM or in engine crankshaft degrees/RPM. The two different kind of numbers can not be used together to figure out your timing curve, you need to convert the distributor degrees/RPM to engine crankshaft degrees/RPM for the tuning described below. To convert distributor degrees/RPM to engine crankshaft degrees/RPM multiply the distributor degrees/RPM times 2, this will convert it to engine crankshaft degrees/RPM. The distributor rotates at half the speed and degrees and the measurements/specifications are often listed as what you would find when using a distributor machine. We need them in a measurement that we can use and verify. If the measurements that your looking at don’t include what they are, your going to need to figure it out, using the wrong numbers will not add up to a good outcome (smile).

Hear are a few examples of timing curves that you might see from factory settings. There are pages upon pages of different distributor curves you might find categorized by years. The first picture is a few different Chevy years that I put together obtained from (Petersen’s all-new 1982 edition, Big Book Of Auto Repair). The 1970 year was the last year before emission changes to timing curves, 1971 was the first year emission changes was introduced, 1976 was one year after the HEI ignition was introduced, and 1979 was the last year that a true mechanical/vacuum advance was used by the factory, in 1980 and on you could find computer controlled distributors (this picture is a list of car distributors only but the truck distributors would be similar). The second picture is out of (1972 Chevrolet 10 – 30 Series, Truck Service Manual). You can see how degrees/RPM’s are listed differently, the first one by distributor degrees/RPM and the second one by crankshaft degrees/RPM (note all ignition timing/initial timing columns on both charts are in crankshaft degrees).





Time and time again I see people set the initial timing with no consideration or expecting that all distributors are the same regarding advance curves and what’s going on inside the distributor, and then wonder why there engine is not performing as expected. We need to look at all the individual components and how they add up together to create your timing advance. There are only a few degrees/RPM’s that we need to look at/measure in order to see the total picture. The first part of the puzzle that where going to be looking at is how many mechanical degrees your distributor has.

With the vacuum canister capped off so that it’s not functioning and the idle set at or below 800 RPM, set your initial timing to what ever number of degrees that you feel works the best for you. I find 5 or 10 degrees works for me, this degree is just for taking measurements, not your final setting and this setting needs to be written down. This degree setting number will need to be subtracted from your next reading in order to determine your mechanical advance degrees. Shutoff the engine and remove the springs from the mechanical advance weights, with the springs removed the weights will move out to full advance with the engine at idle. Start the engine and read the timing advance, ether off the timing tape or with an adjustable timing light. You don’t need to raise the RPM higher with the springs off. Subtract the initial timing that you set at the beginning from your reading and this is the amount of mechanical advance that you have for your distributor.

You don’t have to remove the springs to figure out the number of mechanical degrees that your distributor has, if you have lighter springs than stock spring on your distributor. But you will have to run the engine at a higher RPM to see the advance on the timing mark. I personally don’t like to take an engine above 4000 RPM in a no load, sitting in the driveway condition. You can see in the upcoming graph that most of the stock distributors with stock spring would need to be taken to a higher RPM to see the mechanical degrees that a distributor has. If you put lighter springs on your distributor before measuring the mechanical degrees, I would recommend increasing the RPM in 500 RPM steps taking the degree reading at each step along the way and writing them down so you can plot them on the upcoming blank graph. With the springs removed it’s a good time to check the fly weights pivot hole and post for wear. A little bit of wear is normal but if it becomes to much the distributor needs to be rebuilt or replaced. How much wear is to much its hard to say in an exact statement, suffice to say if it looks bad to you or something just doesn’t look right rebuild or replace it (smile).

The term Total Timing of a distributor is the degree number that I’m most interested in, and this is the degrees that I’m going to be setting my distributor to. Total timing is the distributor mechanical crankshaft degrees + initial timing degrees added together. The number I’m shooting for is total timing of 32 – 36 degrees. My distributor has 19 distributor mechanical crankshaft degrees and I picked the total timing of 33, to find out what my initial timing will be I subtract the 19 from the 33 and that equals 14 degrees that I’m setting my initial timing to. Now if I make adjustments to my initial timing, I know how it’s going to effect my total timing, to stay within my range of 32 – 36 degrees my initial timing would be between 13 – 17 degrees. Note some times you might find some talking about a total timing up in the 50s, this is adding on the vacuum advance to the total timing that I describing above (I will be talking about the vacuum advance in the next post).

What is your distributor’s number of mechanical advance degrees? With out it you can’t set your initial timing correctly. With all the different mechanical advance degrees that can be found on different distributors (stock or custom) you just can’t assume that there all a set number or the same. Some distributors have means to adjust the number of degrees with bushings, changing weights or center cams, normally I find no need to play around with this possible adjustment, you can if you like but check/measure it after you make any adjustments.

Now where going to look at when the total advance is applied to the distributor this is based on RPM. This is controlled by the springs that you chouse to install, lighter springs equals advance at a lower RPM and heaver springs equals advance at a higher RPM.

Finding the right springs for your distributor can be a problem. I find that for the stock stile distributors the Crane Cams (PN 99601-1 Crane Vacuum Advance Kit for Delco Points or PN 99600-1 Crane Vacuum Advance Kit for GM HEI) work good to meet my requirements, they can be found at Summit Racing or JEG’s. The vacuum canister that comes in the two kits is not my favorite to use but the springs work for me (I will talk about picking a vacuum canister in the next post), sometimes you have to buy a kit with items your not going to use to get the ones you need (smile). If you have a distributor other than a stock stile distributor you will need to contact that distributor manufacturer and see if they have springs for there distributor that will allow you to adjust it to meet the RPM requirements. When all else fails to find the right springs, you will need to start measuring the springs that are in your distributor, first at what RPM do you have total advance in, next do you need to go up or down (heaver or lighter) springs, then measure the spring dimensions (out side spring diameter, spring length, spring wire size), then you will need to find some springs one wire size up or down or more depending on how much RPM you need to change. You might find that you will need to use two springs of different wire size to hit the RPM range, this is OK to do. Where this last approach can fall into the category of trial and error with plenty of checks to see if you’ve achieved the right RPM, it can be done. One place that you might find some springs for this approach is at McMaster-CARR in there Ultra-Precision Extension Spring section.

The target RPM that I want total timing in at is 2500 – 3500 RPM this is considered to be the range that normally would be found to be cruise RPM at 60 -70 MPH in third or forth the gear with normal gear ratio rear ends and tire size (not in overdrive). My truck driving down the road at 65 MPH in third gear (not in overdrive) is 2900 RPM, I picked the springs that would have total timing in at 2800 RPM. The distributor that I have came with spring in it that had total timing in at 2000 RPM. Sometimes having the total timing coming in at a lower RPM can cause problems in the off idle up to about 3000 RPM range.

The final check is to make sure that all my settings are what I want them to be by running the engine up to 2800 RPM and checking that the total timing is 33 degrees on the timing tape or with a adjustable timing light.

Now with the vacuum canister still capped off, take the truck out for a wide open throttle (WOT) run and check for knocking/pinging. If you run into a pinging problems start backing off the initial timing mark one or two degrees until no pinging occurs. If your total timing is at 30 – 31 degrees or below and you still have pinging the problem is not in the timing of having too much timing. Start looking at the gas octane level and the compress ratio or possibly air fuel ratio at WOT. Normally you are not going to be running into a pinging problem with your total timing with in the target area that you can see in the graph. Total timing that an engine can use is directly related to cylinder head combustion chamber shape/configuration and all the Chevy engines of this time 1965 – 1979 have similar combustion chambers.

Some history and a visual aid. With all the numbers being thrown around concerning distributor curves I like to plot them on a graph to keep everything straight and see what changes that I’m making, I like to see the difference to a standard. The one standard that I use is what did the factory do. If you look at the below graph all the red dots to the right of the graph are the average crankshaft total timing/RPM by years for the small block Chevy (note if big block or L6 engines where averaged in there, there would be very little difference). The black line is the crankshaft average total timing/RPM for the years before emission changes and the blue line is the average crankshaft total timing/RPM for the years after emission changes. You can see the differences just by year as well as pre-emissions and post-emissions and the general differences that you can run into regarding mechanical crankshaft degrees/RPM. The orange line is the timing curve that I set my distributor to in crankshaft degrees/RPM. The Target Area is what I was shooting for (described above). You can see that the crankshaft total timing degrees in the Target Area and the factory crankshaft total timing (pre-emissions) are a similar range, the primary thing that’s changes is at what RPM the timing is completely in. The yellow line is the average vacuum advance degrees added to the average total timing (pre-emissions) and the green line is the vacuum degrees that I used with my distributor setup (I will be taking about setting up the vacuum canister in the next post).



Hear is a blank graph with the Target Area marked on it so that you can plot your distributor curve, possibly a help to you in keeping track of all the different numbers that you run into. You might be asking where this Target Area came from? Over the years hot roders and performance enthusiasts have been looking at improving the performance of the Chevy engines and this Target Area has found its way to being a good compromise for street performance and a benefit to increased performance over stock settings. The Target Area is averaging out (on the conservative side) of a number of different experts concerning street performance curves, some of them are Lars Grimsrud and John Hinckley and you can find some of their article concerning distributor curves by searching the web. They are definitely worth finding and reading concerning distributor curves and performance.



Regardless on what you set your distributor curve to a more conservative or aggressive curve to the one shown above I believe this would be a good starting point for the GM engines and a way that you can figure out what you have and any modifications that you do to your distributor. Knowing what going on inside your distributor can pay big benefits in performance and a good running engine.

In the next post I’m going to be continuing on with the distributor curves but focusing on choosing a vacuum advance canister to work with your engine.

Enjoy your build

[69gmcc10]

Nice write up Bruce! I couldn’t agree with you more on mechanical timing curves and knowing where you really are. There is allot of discussion and debate about this subject and through all of my searching this is the most correct and precise explanation I have read.

To find the total mechanical advance on my HEI transplant I measured the total circumference of the balancer and multiplied by .094 to find the distance from the notch on the balancer to where 34* is located and marked it with a sharpie. Then with my lovely assistant (wife) I had her hold the rpms at 2800, where it is completely advanced, and aligned the sharpie mark I made on the balancer to 0 degrees.

I had to do this twice because the springs in the first advance kit I bought (Accel) weren’t consistent in the advance and after about 1000 miles the springs weakened and the advance was kicking in during idle. I also found that the weights in the Accel kit would not allow the distributor advance all the way. I have since bought the Crane kit and that is one quality kit, great springs and the stock weights work just fine.

I can’t wait to read your write-up on vacuum canisters. Keep up the great posts!

[y5mgisi]

EXcellent write up!

[Bruce88]

Figuring out what your engine timing needs are is relatively easy
Finding the parts to accomplish it might be a little bit harder

After you set your initial timing based on total timing described in the previous post, make sure your carburetor idle RPM is close to normal between 600 – 800 RPM and a quick idle screw adjustments for best idle. This is just to get it close to the final settings I will be talking about carburetor tuning in the next post.

A vacuum gauge and a hand vacuum pump as described in a previous post #703 is essential to have to tune your distributor or carburetor, in fact I have a few vacuum gauges sitting in my tool box for the times when having more than one simplifies taking readings. In fact if you don’t have one there’s no need to read any further, you would only be guessing what’s happening and what you need to do. For this section you need a vacuum gauge hooked up to manifold vacuum and in the cab so you can read it when you’re driving down the road, this doesn’t have to be a permanent installation unless you want it to be.

First I want to figure out what I want, then what I have, and then bring the two together. Time and time again I see articles describing what we want but very few that tell you how to determine if it fits your needs, and then to make some kind of adjustment that might be a good adjustment or bad one and not checking it. I always want to verify what I’m doing and the change is actually happening the way I expect it to.

The biggest problem with vacuum readings is that they keep changing because of a number of different reasons, but they are of a value for you to know for where you are at and changing conditions just to set things up. The mechanical/vacuum controlled distributor is not able to compensate for all the changes that happen to changing conditions that could effect what should be happening, so we need to set up a compromise to help it out and somewhat of a standard to take your measurements at.

The two things that effect vacuum readings the most are temperature and altitude, in a more precise statement Pressure Altitude. The day that you are doing the tuning and taking the readings should be about 60 degrees Fahrenheit, between 55 – 65 degrees. This should put you in the middle ground as far as temp is concerned, higher temps equals thinner air (less dense) and lower temps equals thicker air (more dense). As far as altitude you need to do the tuning at the normal altitude that you plan at driving at, plus or minus 500 feet, you can find out what your altitude is at your local airport. Normally you’re going to tune to your altitude and the occasional drive to a higher or lower altitude is not a concern, but if you relocate to a higher or lower altitude of plus or minus 1000 feet as a normal driving area a re-tuning is recommended.

General information about vacuum readings and altitude. For every 1000 foot altitude change the engine vacuum will go up or down 1 inch of vacuum reading (Hg). So an engine that makes 18 Hg at idle at sea level, when its relocated to 4000 foot higher will only generate 14 Hg at idle. Also as you go up and down in altitude the air fuel ratio of your carburetor changes, as you go higher the air fuel ratio will go richer and as you go lower it will go leaner. Not much you can do about the change except pull over to the side of the road and re-jet/re-tune your engine, not very practical (smile), just be aware of it and as the change happens your engine is not going to be running at optimum settings.

The vacuum canister adds additional timing on to the total timing talked about in the previous post. This additional timing is used during lighter than WOT loads and is where your going to find increased fuel mileage and a cleaner running engine with good throttle response thru the different load conditions.

How much vacuum advance timing do I want to add? If you look at the previously posted graph you see the two upper red lines at 52 & 56 degrees. This is the range that’s generally considered a good target area for the vacuum advance timing. I prefer to stay around the 52 degree plus or minus 1 degree line (51 – 53 degrees). To much timing at cruise RPM going down the road is not a knocking or pinging its like a very slight hesitation very minor feeling, it could be overlooked by some or just feeling a little odd and as you start applying a little more throttle and your vacuum drops a little it goes away. You take your total timing and subtract it from 52 degrees and this is the amount of vacuum advance timing that you want plus or mines 1 degree. My set up is 52 – 33 Total Timing = 19 so I want a vacuum advance with 18 - 20 degrees. (This is crankshaft degrees, keep that in mind)

How much vacuum advance do I have (in the vacuum canister on the car/truck)? There are three numbers that I interested in. (1) At what vacuum Hg does the vacuum canister start to advance the timing? (2) How many degrees does the vacuum canister add to the total timing? (3) At what vacuum Hg does the vacuum canister have all the degrees applied? With the engine at operating temp, vacuum port on carb capped off, and mechanical timing set as in previous post (initial timing set). Hookup your vacuum hand pump to the canister and timing light, with the engine running at idle start slowly applying vacuum to the canister and watch the timing mark. You want to write down at what vacuum Hg does the timing mark start to advance/change (this would be generally between 3 – 8 Hg). Continue applying vacuum to the vacuum canister, you want to find out how may degrees the canister applies and at what vacuum Hg it’s applied. Write this information down (this could be any where between 10 – 20 Hg). You might have to apply the vacuum a few times to determine at what vacuum Hg things happen, try to be as accurate as possible.

One problem you could run into when trying to measure the vacuum canister in the above paragraph. If the engine RPM increases as more timing is added to the initial timing when you go above 800 RPM the mechanical advance starts adding timing. We just want to know how much the vacuum canister is adding, not what the additional mechanical advance might be adding because of increased RPM. A workaround to this problem is to set your initial timing (advancing timing) until no RPM change is noted on the timing mark and adjust the idle RPM below 800 (this is just a temporary setting to take the readings, not for driving around). Generally you’re not going to run into this problem when your initial timing is set to 15 – 18 degrees for your distributor, but if your initial timing is about 5 degrees you could see this problem. After you take your readings or any time you have to use this workaround to take measurements, reset your initial timing to what I would say is required to meet your total timing as described in the previous post. At the end of this post I will talk about chousing a distributor for your engine, all this information is in setting up what you have sitting in front of you on your engine and don’t try to read into it that you need to change your distributor to increase performance, all the different distributors can be made to work for street performance.

Next you need to hookup the vacuum line from your carb to the vacuum canister and take it out for a drive to gather more information. The vacuum canister that you have in your car/truck should be good enough for you to gather the information. You need the vacuum gauge and RPM gauge in the cab to take the measurements. There are two measurements that I want. (1) What is the vacuum Hg reading at WOT? This is thru 1000 – 3500 RPM probably in 2nd or 3rd gear (not in overdrive), normally you will find this number to be about 1 – 3 Hg. (2) What is the vacuum Hg reading at cruise? This is cruising down the road on level land (not going up or down hill) at your cruise MPH (mine is 65 MPH). This is in overdrive with the torque converter locked up if so equipped, how the car/truck would normally be used at your normal MPH cruise.

More general information about the two vacuum Hg readings above. First the readings that we obtain we are going to use to determine the right vacuum canister for the car/truck sitting in front of you at your location. I think most of you have seen the charts on how to pick a carburetor size by cubic feet per minute (CFM) for a particular engine cubic inch displacement (CID). In general an engine with a certain CID has the potential/ability to generate/create a set amount of vacuum at sea level (yes a cam size can affect this). The factory generally matches the carburetor this way and depending on how the engine will be used a smaller CFM carburetor may be used on an engine. With a smaller carb CFM you might see a higher vacuum reading Hg at WOT they could be up about 8 Hg, with a larger carb CFM you could see vacuum Hg readings down at 0 Hg at WOT. With a larger carb CFM than a particular CID engine can handle it creates problems in street performance tuning, not that it can not be overcome it just makes it a little bit harder unless you’ve gone to big. Keep in mind if you change your carburetor to a larger or smaller CFM one that refiguring of your vacuum canister needs might be required. Every car/truck has a different load to maintain a steady cruise speed going down the road and at this light load condition we want all the additional timing that the vacuum canister is supplying. Be careful about taking this reading that you’re on a level surface and not driving into a head or tail wind, drive both directions on the same road and the vacuum readings should be with in ½ Hg in both directions.

We now have all the information we need to figure out what vacuum canister we need to fit the engine and setup. I’m going to be using my numbers for the example below you should be using your readings and plugging them in to figure out your needs.
I want a vacuum canister with 18 – 20 degrees advance (see above on how to figure)
I want the vacuum canister to start advancing the timing at about 2 – 4 Hg above WOT vacuum reading (my WOT reading is ½ - 1 Hg) that means I want my vacuum canister to start increasing the timing at 3 – 5 Hg.
I want all the vacuum advance degrees in at 1 – 2 Hg below cruise MPH vacuum Hg (my cruise Hg is 14) that means I want a vacuum canister that has all the degrees in at 12 - 13 Hg.

Now to find a vacuum canister to meets my needs. By outward appearance all the vacuum canisters look similar but the difference is in how they would meet my needs regarding the above specifications I generated listed above. The above tolerances you might have to expand a little to fit a particular vacuum canister choice but I recommend staying as close as possible in chousing a vacuum canister for your engine and set up.

There are a number of different vacuum canisters to chouse from for the stock stile distributors (point or HEI stile), but over the years the specifications have been striped out of the parts books and when your down at the parts store the parts man just wants to know what car/truck and engine your using it on (not much help there). On custom distributors adjustable vacuum advance canisters are commune place and retro fitting to the stock stile distributors also exist. Regardless on what canister you decide to use check it before you install it, there’s no need to install it just to find out that it’s not going to work for your setup/specifications. You can’t bench check how many crankshaft degrees that a particular canister will give you but you can check at what Hg it starts to advance and at what Hg is the entire advance applied. Hookup your hand vacuum pump to the vacuum canister you have in your hand and start applying vacuum until the canisters advance rod starts to move, this is when the degrees will start to advance. Continue applying more vacuum until the canister advance rod just contacts the full advance stop, this is the Hg that all the advance is accomplished. Personally I want to check the vacuum canister out before installing it, even if I know what the specifications are for the one I have sitting in my hand, I never just trust it, always check.

(Adjustable vacuum advance canisters) There are a number of them out there by different manufactures. The adjustment is normally accomplished by inserting an alien wrench in the vacuum port and turning it a number of turns (clockwise all the way in and up to 10 turns counterclockwise all the way out). Some adjust at what vacuum the vacuum canister starts to change the advance and at what vacuum all the advance is applied, others might change the number of degrees that are applied by turning the alien wrench. Then some also have the ability to change both the degrees applied and at what vacuum. All I can say about them is if you can’t adjust them to meet your specifications that you generated in this article there no use to you and you need to look for another one that meets your needs. I was surprised by the adjustable vacuum advance canister that came with my JEG’s distributor (no markings on the canister to identify it by manufacturer), I was able to adjust it to meet my needs/generated specifications.

Any time you change a vacuum canister in your distributor you need to make sure that the initial timing didn’t change, if it did you need to reset it plus check it out as described above to make sure it meets your generated specifications. Always check to make sure it’s doing what you want it to do.

Hear is a chart that might help you. In the top chart it shows my generated specifications and the canister specifications that I’m using. The bottom chart is blank for you to plug in your numbers and help figure out what canister you need.



Hear is a link to an article written by Lars Grimsrud and Duke Williams (Distributor Vacuum Advance Control units Specs and facts for GM Distributors) That I would recommend reading for better understanding of vacuum canister selection. The link is to a PDF format file and you need Adobe Acrobat to read it http://www.lbfun.com/warehouse/tech_...ance_Specs.pdf There are minor differences between the specifications that I have written above but the primary thing that I believe would be for you to measure what you have and chouse the vacuum canister based on what you have sitting in front of you at your location. I took the vacuum canisters that are listed in the article and looked to see if there still being carried in the NAPA catalog (Echlin part numbers) and adding ACDelco part numbers for the ones that are no longer being carried by NAPA. (See the list in the below pictures) Keep in mind that the vacuum canister degrees are in distributor degrees and you need to multiply them by 2 to convert them to crankshaft degrees.









One question that continues to crop up is do I connect my distributor vacuum advance canister to full time vacuum or ported vacuum? As far as I’m concerned it all depends on the distributor that you’re using and if your engine likes the additional timing at idle. Your initial timing setting is based on total timing, with a distributor that has its initial timing down around 5 degrees or below I would consider hooking the vacuum up to full time vacuum and with the initial timing at about 15 degrees I would use ported vacuum. Ported vacuum becomes full vacuum at about 1000 RPM as the throttle is increased so I don’t think that having additional advance between let’s say 600 – 1000 RPM is that important unless your engine likes it. I’ve found that at idle RPM with the maximum vacuum Hg reading under a no load condition that the small block Chevy likes about 18 degrees initial advance setting but as the load on the engine is increased not to WOT but some where in-between you could find yourself in a knocking/pinging condition when lugging the engine. This is a transition area from idle (no load) to an increased load as your accelerating, the vacuum canister removes the additional timing as the vacuum goes down and the RPM’s go up but the air fuel ratio that the carb is supplying might not be rich enough to support the additional timing. I think you’re going to find that an initial timing of about 15 degrees is around the right area to compensate for the transition plus delaying the additional timing being added till after 1000 RPM will prevent the problem.

For a street car/truck with a street engine I think the HEI distributor is the best choice in distributor to chouse. They generally come with about 20 crankshaft degrees of advance timing and require less maintenance than a point distributor. All the different distributors will work and can be adjusted and work just fine for the street and if you want to stay with a stock look the point distributors will work just fine for street performance.

I spent a lot of time in this article taking about vacuum and taking readings. One thing that needs to be added is before you start doing any of the checks above you need to make sure that you don’t have any vacuum leaks around your carb and manifold. Vacuum leaks can throw your readings off.

I hope you have a better understanding of what’s happening under your distributor cap as you drive your ride around and how to measure it & modify it to achieve improved performance. I don’t think that it is as mysterious as some might want you to believe.

Enjoy your build

[y5mgisi]

YAY! Been waiting for part two! Looks like it was worth the wait! I'll have something FUN to do in my down time at work tonight! Thank you so much!

[ChevyNewb]

Just wanted to subscribe to see this thing thru. I really feel extremely inadequate right now Thought I had done something magical when I painted the frame...you sir are a freaking genius!

[Alan's Classic]

Thanks Bruce for taking the time for all your write ups. I'm at the tuning stage right now and I'm sure this thread will help.

[y5mgisi]

Finally finished reading! Cant wait to start tuning my distributor!

[69gmcc10]

Bruce! You are a wealth of information and you know just how to explain things so they make sense in a clear and concise way. You are correct, allot of the articles I have read tell you what they did, but not how or why they came to that conclusion. Thanks for knowledge and keep it coming, please.

[MAC71]

Outstanding information!

[longbed67]

I think we need a sticky for the info here!

[FLDBRED]

Awesome....what are we going to do when your done?

[Bruce88]

Originally Posted by FLDBRED

Awesome....what are we going to do when your done?

Maybe a Flux Capacitor for our trucks with simple hand tools and things just lying around the house, as in the Back to the Future film about a time machine (smile). I hope your out on your ride putting it togerther and enjoy driving it down the road. I’m trying to give you the basics with the understanding of why and how, so you can set up all your parts that you have so all of them will be working together. We all run into problems on a build and when I do, I generaly fall back to the basics of what I’m trying to do and how am I going to check that its doing what I want it to do. I hope with what I’m wrighting that it can be understood by you and used to solve problems that might arise during your build.

Enjoy your build

[Palf70Step]

Agreed...Lots of good stuff here to keep it near the top.

[70LonghornCST]

It would be nice if there was a "Build Hall of Fame" section to which threads such as these could be stuck. Too much great information to be lost on the bottom of a forum. The new members need to enjoy this too
-Richard