Questions about using a timing light

[68 P.O.S.]

Originally Posted by rsgt

I have read a lot about this on the team Chevelle forum and this seams to be the most popular way to set up the 350. I have done this to my 72 truck and it has never run this good. You have to make or you can buy a vacuum advance limiter. Attached is a picture of the one I made from a piece of aluminum. Crane sells one. You want to limit the vacuum advance to 8 to 10 degrees. Set your initial to 10 or 12. This gives you 20 or 22 at idle. A lot of the forums talk about this.

You are correct, this has been a thing here in the past too, as well as many other forums, and it has worked with great success. It seems the topic has dropped off here and people have forgotten about it. It’s really simple and works wonders: Buy the Crane 99600-1 adjustable vacuum canister kit, replace the current springs in the dizzy with 1 blue and 1 silver spring (if I remember the colors correctly) so the mechanical is fully advanced by 2500-3000 rpm, limit the vac can to 10 degrees by placing the stop on the opposite side of what the instructions say, adjust the vac can 4 turns out from bottomed out (should pull at approx. 7 in of vac), advance total timing to 34-36 degrees with vac can disconnected and plugged, check timing at idle to see where your initial timing number falls for fyi purposes, plug the vac advance back in to manifold vaccum, NOT ported, and adjust idle to 600rpm in gear with your foot on the brake (usually works out to 750-800 rpm in park), and then you should be in there like swimwear.

[Warrens69GMC]

Info-

Originally Posted by HEI451

There are a number of stock GM HEI vacuum canisters available, over 15 or so. Selecting the RIGHT one is tough at best, and takes someone that KNOWS curves and settings to make it all work.

That said, you want the vacuum advance to work on FULL MANIFOLD VACUUM SOURCING, vacuum strong and present at IDLE, and NOT ported vacuum. This source should be at the BASE of the carb, NOT above the throttle plate.

It is also correct that all vacuum advances produce too much timing, IF the limiter bushing isn't installed on them. The limiter bushing is a rubber tube, mostly just rubber vacuum tubing, placed over the pull pin, to limit the pin travel, which reduces the over all degrees of vacuum advance crankshaft timing. This said, the "24" referred to on the canister IS the crankshaft amount of vacuum advance degrees that canister will produce, far too much for our needs.

A nice idle timing we would like to see os between 20 and 22 crankshaft degrees, but, if the initial is set there, the engine is usually too hard to start, and this is where CORRECT vacuum advance souring comes in. You state you now have 8 crankshaft degrees of initial timing, and the canister has 24 available, IF the stop busing is in place. IF the canister is on full manifold vacuum, as it should be, the vacuum advance will take the idle timing to 32 degrees, tar too many degrees. If the rubber limiter isn't there, as they degrade and fall off, never to be replaced when a new advance is mounted, then, the sky's the lkimit for idle timing.

Now, IF we set the initial to your 8 degrees, and make a simple sheet steel limiter and attach it to the vacuum advance mounting bar with two 6/32 machine screws, we can set the pin travel to .120 inch travel, for 12 crankshaft degrees, and make sure the canister is full manifold vacuum sourced, we now have an idle timing of 12 plus 8, 20 degrees, exactly what that engine wants.

We also have a situation where every stock canister has a different vacuum pull, along with different curve numbers, and this is why there are so many of them available, who knows what is what. The extremely simple solution to all this is the Crane 99600-1 adjustable vacuum advance kit, has 3 sets of mechanical advance springs, a pin limiter stop and a fully adjustable canister, giving EVERY stock GM setting, And everything in between them. The only modification is, the Crane stop plate needs to be mounted NOT where Crane says to put it, but on the OTHER side of the pin, towords the canister, NOT the end mounting screw. This is very easily done with one threaded hole and an 8/32 machine screw. The Crane method changed the vacuum pull spring strength with every degree adjustment, my modification separates the vacuum rate from the number of degrees selected, making vacuum advance tuning work, instead of compromising with every adjustment change.

If you are interested, I have a series of pictures and an outline on just how to correctly modify both the GM stock, and Crane advances to the correct way the stop plate should be, for both HEI and stock point distributors, along with the simple home made stops as well. There are even pictures on how to mount the Crane limiter plate to a stock GM vacuum advance, points or HEI.

If you want a picture set, there is no obligation, no ad, just simply e/mail me and ask for the one that fits your distributor, in this case an HEI, and I will send them to you.

Use this e/mail address, please. info@davessmallbodyheis.com and I will send same.

Also, I really don't advocate using ported vacuum, if there are those that still do use it, they have NO clue as to how a "load compensator" (vacuum advance) actually works in a non-EGR equipped internal combustion engine. The ONLY engines that require ported vacuum are those with a functioning EGR valve for EMISSIONS use, and there aren't many of them still running around these days.

Thread
http://67-72chevytrucks.com/vboard/s...d.php?t=486908

[68 P.O.S.]

There you go Warren, I was too lazy to search for that thread haha! Dave is the HEI master. The Crane adjustable vac can has every GM vac can setting built right in

[davepl]

Here's my theory, take it for what it's worth.

1) Vacuum advance should be ported
2) Lots of people have poorly tuned motors with vacuum leaks, etc
3) More timing crutches those symptoms
4) Running vacuum advance to manifold gives you a bunch of initial timing

So yeah, plugging the vacuum advance into manifold vacuum will give you a bunch of initial timing that will make it idle better, but it's not the right way to do it.

There are SOME things (like TCS - Transmission Controlled Spark) that are just nonsense emissions crap that was a bad idea even then. But vacuum advance is a big deal for driveability (and mileage if you care).

[68 P.O.S.]

You only use ported if your engine is running an EGR valve for emissions. Straight from the GM developmental horse’s mouth: ““Ported Vacuum” was strictly an early pre-converter crude emissions strategy and nothing more. Don’t believe anyone who tells you that ported vacuum is a good thing for performance and drivability – it’s not. Anyone with a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy…”. Call me weird or call me crazy, but I'll believe the GM engineer who helped develop the system.

[dmjlambert]

I would like to thank everybody for your input on this thread. I think it is getting pretty interesting and I'm learning.

Originally Posted by 68 P.O.S.

I take it you didn’t read the links I posted. Vacuum advance is only meant to be supplemental for when it is needed (in low load situations like idle and cruise so it can burn the lean mixture more efficiently, keep the engine cooler, and improve mileage and streetability), not used as your primary means of advance. That’s what the mechanical advance is for. What distributor do you have and how many degrees of mechanical advance does it have? How much vacuum are you pulling at idle? The crate engine heads you have on it, are they vortec or standard heads?

I did read the links. I recognized all of them and I've had them bookmarked before. I can't say I understand all of it, but I think perhaps I understand a little more each time I read them. I'm a novice, and I show it at the beginning of this thread where I didn't have much of a clue about how to use the timing light or the timing marks and didn't know how many degrees per mark.

I have a GM HEI distributor that a friend gave me. I don't know what vehicle it came out of. I've been looking for a part number stamp on it but don't see it. Perhaps it is inside. I will look some more. The vacuum advance can was damaged and it didn't work. Another friend shade tree mechanic put the current vacuum advance on it, and I don't know how it was selected, I just know it was new from O'Reilly Auto Parts. He set the timing, had trouble figuring out the timing light, and the engine was working OK but pinging sometimes while driving. Then I picked up from there and started this thread late last year.

I am getting more involved with the mechanics of this truck. Now you guys are getting me interested in tuning. I don't have a vacuum gauge but now I'm interested in getting one. I don't know how much mechanical advance the distributor has. I don't know what type of heads the engine has. The casting number is 10147898. After reading all the great info on this forum that you guys offer up, I have recently moved my vacuum advance from ported to manifold vacuum.

When I brought this thread back to life the other day I was interested in knowing the technical details about: if most total advance you can go with your timing without causing detonation is desirable, why is higher amount of vacuum advance not better than smaller amount of vacuum advance?

Please tell me if I figured out the answer from input from you guys and the links provided that I have been reading. I want the maximum total advance you can get at wide open throttle and heavy loads without causing detonation. If I have a lot of vacuum advance, and adjust the initial timing as advanced as possible to avoid detonation at idle and mid-throttle, it will be the appropriate amount of total advance at idle and mid-throttle. But the total timing will drop more than I want at wide open throttle, because all of the vacuum advance goes away at wide open throttle. On my engine, since I have 23 degrees of vacuum advance, I will lose 23 degrees of advance from my total at wide open throttle. Optimum performance may be realized if I "lost" a lower number of degrees. Do that by lowering vacuum advance and increasing initial advance by about the same amount. The engine with stock parts was designed for minimum warranty period repairs, pretty much without regard for performance, and severely limiting advance under load reduces the chances of my engine having any detonation problem, regardless of quality of fuel. Reducing the amount of vacuum advance is part of hotrodding the distributor.

[ncpetersc]

As you are well aware, I am a novice too. But I am understanding it the same way I think.

It sounds like your total advance is giving you good performance at idle/cruise right now. But perhaps not during acceleration or WOT (due to so much advance loss, vac advance). Solution would be to change vac advance with different canister either adjustable or fixed that provides less advance. Then adjusting timing to achieve same total advance for idle/cruise, but less would be lost during acceleration or WOT.

I'm learning too, but I think I am interpreting this the same as you are.

[davepl]

Originally Posted by 68 P.O.S.

You only use ported if your engine is running an EGR valve for emissions .... call me crazy, but I'll believe the GM engineer who helped develop the system.

I'll run them as his boss specified them to be run. If Zora wants ported, Zora gets it.

Obviously they had vacuum advanced for at least 10 years before EGR!

What I will grant is that in later years, particularly once they started to care about idle emissions and ESPECIALLY once you have EGR and cats then they're doing all kinds of goofy things like running ZERO initial advance and no vacuum to keep the chamber temps where they need to be.

But you tell me why vacuum advance is ported in 1967 then, if not for driveability.

I'm sure there were people who thought PCV was evil and road draft tubes were good enough too....

[68 P.O.S.]

Originally Posted by dmjlambert

I would like to thank everybody for your input on this thread. I think it is getting pretty interesting and I'm learning.



I did read the links. I recognized all of them and I've had them bookmarked before. I can't say I understand all of it, but I think perhaps I understand a little more each time I read them. I'm a novice, and I show it at the beginning of this thread where I didn't have much of a clue about how to use the timing light or the timing marks and didn't know how many degrees per mark. Good, I'm glad you read the links. I didn't realize how new you are to this, my apologies. I had to read it a few times and mess with the truck as well to fully grasp it. Once you understand it, it's actually very easy. Everything on these trucks is like that. I'd hate to see you learn it the wrong way right off the bat. You're in a great place to learn from a lot of great people.

I have a GM HEI distributor that a friend gave me. I don't know what vehicle it came out of. I've been looking for a part number stamp on it but don't see it. Perhaps it is inside. I will look some more. The vacuum advance can was damaged and it didn't work. Another friend shade tree mechanic put the current vacuum advance on it, and I don't know how it was selected, I just know it was new from O'Reilly Auto Parts. He set the timing, had trouble figuring out the timing light, and the engine was working OK but pinging sometimes while driving. Then I picked up from there and started this thread late last year. The HEI is a nice upgrade. Make sure your spark plug gaps are increased to 0.045 with HEI. Since the HEI is unknown, consider it an emissions stocker. So you’ll probably need a different set of springs and the weights and centerplate probably are incorrect too. The new vac advance can you have probably isn’t right also, but may be fixable. You’ll need to figure out how much mechanical advance the distributor has too. To do that, you’ll have to throw the timing light back on it. Unhook your vac advance and verify you still have it set to 7 degrees. It doesn’t sound like you’ve had the distributor cap off, so yank it and make sure the weights are lubed up and see what numbers are stamped into the weights and center plate. Then take the springs off. Be careful, they will shoot off on you and you don’t want to lose them. Throw a rag on top while you’re doing it. Put the cap back on, start it, hold it at about 1500 to 2000 rpm and see what the timing is at. Taking the springs out ensures the mechanical advance is all in and you won’t have to rev it up crazy high. Take the number you just got, and that’s your total timing. Now subtract your 7 degrees initial timing, and that gives you your mechanical advance degrees. Then plug the vac advance back in to manifold vacuum. Let us know what timing you see.

I am getting more involved with the mechanics of this truck. Now you guys are getting me interested in tuning. I don't have a vacuum gauge but now I'm interested in getting one. I don't know how much mechanical advance the distributor has. I don't know what type of heads the engine has. The casting number is 10147898. After reading all the great info on this forum that you guys offer up, I have recently moved my vacuum advance from ported to manifold vacuum. The 898 heads are standard stock crate engine heads, so they’re really not performance heads. They’ll work just fine for what it sounds like your purposes are though. Great, glad you moved it over to manifold vacuum.

When I brought this thread back to life the other day I was interested in knowing the technical details about: if most total advance you can go with your timing without causing detonation is desirable, why is higher amount of vacuum advance not better than smaller amount of vacuum advance? A higher amount of vac advance isn’t desirable because you can only go so far with advancing before it counteracts itself and starts pinging and causing damage. Vac advance is only meant to be supplemental, at idle and cruise, when there is high vacuum/low load on the engine and the fuel mixture is lean in order to allow for efficient combustion.

Please tell me if I figured out the answer from input from you guys and the links provided that I have been reading. I want the maximum total advance you can get at wide open throttle and heavy loads without causing detonation. If I have a lot of vacuum advance, and adjust the initial timing as advanced as possible to avoid detonation at idle and mid-throttle, it will be the appropriate amount of total advance at idle and mid-throttle. But the total timing will drop more than I want at wide open throttle, because all of the vacuum advance goes away at wide open throttle. On my engine, since I have 23 degrees of vacuum advance, I will lose 23 degrees of advance from my total at wide open throttle. Optimum performance may be realized if I "lost" a lower number of degrees. Do that by lowering vacuum advance and increasing initial advance by about the same amount. The engine with stock parts was designed for minimum warranty period repairs, pretty much without regard for performance, and severely limiting advance under load reduces the chances of my engine having any detonation problem, regardless of quality of fuel. Reducing the amount of vacuum advance is part of hotrodding the distributor. Sounds like you pretty much got it. You want your total timing (initial timing + mechanical advance) to be 34-36 degrees and your vacuum advance to be 10-12 degrees. Using my truck as an example, my dizzy has 22 degrees of mechanical advance in it, so I run 14 degrees initial to get to 36 total. Most aftermarket and GM Performance dizzys have 20 to 22 degrees in them. The springs you put in the dizzy should allow for full mechanical advance by 2500-3000 rpm. My vac advance is limited to 10 degrees and you’ll have to adjust the rate at which it comes in based on your vacuum level at idle. When you check your timing at idle with the vac advance hooked up, you should see around 20-24 degrees. That’s what the engine wants.

DMJ, I replied to your message above in blue.

[68 P.O.S.]

Originally Posted by davepl

I'll run them as his boss specified them to be run. If Zora wants ported, Zora gets it. Can you show me where this is at?

Obviously they had vacuum advanced for at least 10 years before EGR! A lot longer than that

But you tell me why vacuum advance is ported in 1967 then, if not for driveability. No idea. I can't go back and read the engineer's minds. It was a different time back then with different knowledge and different technology...no one truly knows but them. Unless they have spoken out, such as John Hinckley has.

Dave, see above

[68 P.O.S.]

Dmj, you ever get this figured out?

[dmjlambert]

Hi, yes, from information gathering and learning standpoint, I believe I have it. Thanks very much for the education.

I have a stock distributor with a stock amount of vacuum advance (a lot). The initial timing is set more advanced than stock, to the highest BTDC I can go while preventing knocking. This allows for efficient and easy starting with no kickback, and adequate idle and mid-throttle economy and performance. At wide open throttle, my timing is not sufficiently advanced for high performance and efficiency. This is because the timing is retarded by the vacuum advance can. The timing curve is set up for conservative stock performance and maximum engine safety.

I have the vacuum advance running on manifold vacuum. My engine works equally well on ported or manifold vacuum, it doesn't seem to make any significant difference except when switching from one port to the other I need to adjust idle speed. And it only affects the engine at idle. So, I'm not convinced it makes any big difference which to use. My logic leads me to choose manifold because it just burns more of the gas in the cylinder before sending it to the exhaust, and therefore must be more efficient. I don't have an A.I.R. system or EGR, so nothing is going to happen with any unburned gas on my engine, except going out the tailpipe. Therefore, I can't find a reason to retard the timing at idle by using ported vacuum.

I will probably get into hot rodding this HEI at some point, or at least mapping the curve so I know what I have for future reference. I can't tell when because I have my interior cab work to do and A/C to get going, shock installation and other maintenance to work on. In 1969 there was no starter brace, and I don't want to start breaking things, so I want to add the brace before messing with the timing curve.

The guy who put in my exhaust put in glass packs, so I want to work on correcting that annoyance fairly soon, too. He asked what kind of tone I want, and I told him "no tone at all, I don't want to be able to tell the engine is running." Apparently to him that means glass packs welded in. I was mad, but just wanted to get the truck out of there and back to my side of Texas. I've now had a couple of years of listening to it every day, so it is getting old. Like everything on my truck I need to do it twice, that is my curse. That kind of stuff slows down my progress.

[Steeveedee]

When I put mufflers on my truck, it's going to be a pair of those babies that go on the 500 CI Cadillacs. They obviously flow well, since they usually only get one for the whole 500 cubes, and they're plenty quiet. I'll also have an H-Pipe.