Questions about using a timing light

[dmjlambert]

Hi, I have a question about a timing light, please. I bought the fancier of the two Bosch lights offered at O'Reilly. I am not trying to set the timing right now, just observe what my timing is set to, and learn some. I have a 1969 CST/10 with 350 engine and quadrajet, and it has an HEI distributor from a later model year. My vacuum advance is connected to a manifold vacuum port on the carburetor (has vacuum at idle) during my observations.

I connected the timing light and it is set it for 4 cycle by default when turned on. I start the engine.

I point the light to the timing marks and pressed advance up button on the light until the mark on the balancer aligned with the 0 mark on the timing tag. The timing light reads 30.0. Then I continue to adjust the advance up button until the mark on the balancer lines up 5 marks to the right near the R. The timing light new reads 40.0.

Does each mark on the timing tag represent 1 degree? I do not have the type of timing tag with notches, the edge is flat as shown in the picture.

When I set the timing light to 2 cycle, the reading is 15.0 when the mark on the balancer aligned with the 0 mark on the timing tag, and the reading is 20.0 when lines up with 5 marks to the right near the R. So I am thinking 2 cycle setting is the appropriate one to use with this timing light and this engine, and I have 15 degrees total advance at idle. Do I understand this correctly?

[Jcfcamaro]

Normally they have numbers going up from 0. And I do believe each Mark is 2 degrees. You should have your vacuum advance unplugged with the other end at the carb/intake capped off when you check it. I also think you don't have your trimming light on the right setting.

[Killer Bee]

before you get too carried away with that fancy light, I'd recommend some basic functions first..

first of all, your engine is a 4 cycle or 4 stroke [suck, squeeze, bang, blow] so disregard the 2 cycle setting for now.

the R on your timing scale stands for retarded as ATDC or after top dead center..

your timing is probably somewhere in the BTDC range or before top dead center..

the scale may have more numbers on the BTDC side from 0 that the ATDC..

you should clean off the scale, [engine off of course] and rub some chalk or marker paint of some kind in the lines and numbers so you can see them more easily..

same with the groove on the balancer, you may have to bump or bar the engine around to get that groove in an accessible location..

somewhere along the bottom of the engine while looking up is usually easiest for the balancer..

to see what your base timing is, disconnect the vacuum advance hose and plug the port on the carb..

a short length of hose with a smooth pin or something wedged in it will do or you can buy an assortment of vacuum plugs from any auto parts store..

place the inductive pickup on number 1 plug wire, away from exhaust, and leave the timing light settings to zero..

start the engine and shoot the light at your scale, you should be reading something in the neighborhood of 8°-12° BTDC..

some applications only have a zero indicator and an advance light is required to get the reading by dialing the light to the 0 mark..

also used for finding/adjusting total timing which are typically values much higher than the base timing scale provided..

the values I'm using are round numbers in a typical, stock V8 engine.. your engine may perform better at different settings..

for stock applications, the base timing adjustments can often be found on a decal underhood if not missing or in a common tune-up manual..

that's all I got for today, please let us now how you make out, good luck!

[dmjlambert]

OK folks, thanks. The engine was rebuilt a couple years ago with 1995 crate motor heads and GM HEI distributor and other components stock. I measured the circumference of the balancer and divided by 360 and found the marks on the timing tag are indeed 2 degrees each. The marks on the timing tag are "A 0 R" and other engines have "Before 0 After".

I detached the vacuum advance hose, plugged the port on the carb, set timing light set to 4 cycle and 0 advance, blocked the wheels and set the brake and put the truck in drive, verified with the timing light's tach function the idle is at 600 RPM. The timing mark shows 3.5 marks or 7 degrees before. The tune up sticker for this truck calls for 4 degrees before. It is in poor condition, but enough of it remains for me to figure out which new sticker to buy. So, it appears the timing is a little advanced.

[Killer Bee]

now you're cookin', good job!

you may find those minor modifications or deviations from stock may like a little timing tweaking to find the engine's sweet spot, but you're heading in the right direction..

a little engine performance testing, drivability patterns, starting quality, fuel octane, or even engine builder data may lead to a slightly different timing value than the factory decal..

and while I was away from drafting my reply, some voices of experience posted up!

good luck!

[geezer#99]

It’s advanced just a little but no way even close to what it should be. You should be able to run at least 12 initial if not a bit more.

[rockyrivermark]

when i did upgrades on my 350 (HEI, carb, intake, etc)
I just put a vacuum guage off the carb and advnanced timing until I got peak vacuum.
You can also just do it by ear. adjust dizzy until engine sounds smooth and highest rpm.
then tighten dizzy nut but leave it just loose enough that you can still move the dist with some pressure. then tweak from there as driving.
for me 13 BTDC was best.
honestly you don't even need the timing gun unless you are curious what it's at.
RRM

[Alex V.]

The factory timing numbers are way conservative - start at 8 and look to get more. Look up discussion of timing by ear/feel. To find the sweet spot you'll have to go past it, but as long as you're paying any attention and bump it back down a little before you drive hundreds of miles it won't hurt the engine at all.

[dmjlambert]

I started advanced and have been retarding it a little at a time. I got to the point of having just a little bit of pinging, and adjusted it again and took it for a test drive. It is running smooth and all the pinging is gone. So, I just wanted to see where I ended up by using a timing light. I ended up at 7 BTDC. My vacuum advance provides 23 degrees of advance when connected to manifold vacuum. I have connected the distributor to ported vacuum and I'm going to drive it that way for a few days and see if I like it.

[Steeveedee]

The reason for the factory timing settings being conservative is to save the engine for the warranty period. Part of that consideration is that the manufacturer has to plan for inferior fuel which may have a low enough octane rating that detonation would occur. Advancing the timing may be OK, if the rest of the system is held so that any potential detonation doesn't destroy any pistons.

Just because you don't hear the ping doesn't mean that your engine isn't experiencing detonation effects.

[dmjlambert]

So you're advising just set the timing to factory setting, is that right?

[Killer Bee]

because your engine has some upgraded modifications and fuel technology is better regulated than it was 50 years ago, I think it's safe for you to continue with your drivability and performance testing you mentioned at the slightly advanced timing value you're at..

detonation, while not always audible to a level that grabs your attention, does generally affect other performance aspects that will give you clues that your combustion rate [with regards to timing in this discussion] is not optimal.. your queries on the subject indicates that you're paying careful attention to a lot more than how much rubber you can burn off the light..

here's an article I found on a quick search, better info is out there, but this will give you a basic understanding of the affects of detonation and common causes..

Eliminating Pre-Ignition & Detonation – Pounding Pistons

good luck!

[nightrider388]

I would love to explain it in detail for you, but this write up from badasscars.com is the best I have seen.


Where should I set the timing on my performance engine?


There really is no specific initial timing specs for any performance engine. You certainly can't go by the factory specs when your engine isn't all bone stock and isn't running all factory components, especially when running an aftermarket distributor, or even a stock replacement distributor. This is because the factory KNOWS how much advance your distributor has in it (because they built it) in conjunction with the amount of initial timing they tell you to set it at. Replacement distributors, whether they are performance or stock, are all different than your original distributor so you obviously can't use the factory setting, besides, stock engines, stock distributors and stock timing specs weren't designed for optimum performance, even in factory muscle cars they were kind of mediocre. Also, (and pretty obvious), performance engines aren't stock, they run differently so they need different timing settings. You really have to time an engine of this sort solely by the "total timing". Total timing is a combination of your initial timing (on the crank) and the amount of mechanical advance you get from your distributor.

Most naturally aspirated engines like a total timing of 34 to 36 degrees BTDC, (Before Top Dead Center) AKA "Advance". Nitrous and supercharged engines usually run less than that, unless you plan on blowing the heads off the engine or blowing holes through your pistons. usually a typical supercharged or nitrous engine with a moderate amount of boost ora moderate shot of nitrous will have a total timing of about 28-32 degrees. But that is variable on the type of fuel you are using and the amount of boost or nitrous you are running.

To check and set the total triming, you need a timing light with a timing offset built-in. You can tell this kind of light from a standard light because it will have a dial on the back side. These lights are usually a few bucks more than the regular lights but they do a lot more.

Most street engines use either a vacuum advance and/or a mechanical advance (the weights inside the distributor). More serious performance engines "usually" only use the mechanical advance, and most true race engines are usually "locked-out" and have no advance in the distributor at all. They also have no vacuum advances mounted on them in most cases.

There are several types of timing that you need to know about. First, there's the initial timing, which is also known as "idle timing", which is where the timing is at an idle without the help of the mechanical weights inside the distributor, and without the vacuum advance hooked-up. Then there's the mechanical advance, which is the advance you get from the weights inside the distributor. As the engine RPM comes-up, it's the initial timing AND the mechanical advance that gives you your TOTAL timing. For instance; say your distributor has 26 degrees of mechanical advance built into the weights inside and you want a total of 36 degrees of total timing; you would need to set the initial timing (idle timing) at 10 degrees (10 degrees on the crank and 26 degrees from the mechanical weights = 36 degrees of total timing).

The trick is; for performance use you want as much advance as you can get on the crank and the least amount from the mechanical weights in the distributor, hence why race distributors have no mechanical advance at all. To really make a trick street set-up you should only have about 10 degrees or so of mechanical advance in the distributor and about 24 - 26 degrees of initial timing on the crank to obtain your total of 34 - 36 degrees of total timing. This will launch you out of the hole much harder, and give you great jack rabbit starts from street light to street light. You NEED advance at low RPM to make that engine pull hard. It'll have MUCH better throttle response, a better idle vacuum signal, and it'll run cooler & cleaner. Unfortunately stock engines & distributors are set-up completely the opposite so they need to be re-curved to fix this problem. This is why some guys can tune an engine and make it run killer (like we do), and other's just can't seem to get their car to get out of its own way.

Now; to find and set the total timing all you need to do is set the dial on your timing light to 36. Now rev your engine up to about 3,500 RPM (to insure that the mechanical weights are fully activated) and watch your timing mark on the harmonic balancer. Now rotate your distributor until the marks line-up at "0" on the crank. When it reads "0", (yet the light is set at 36), you have a total timing of 36 degrees. Make sure you do this with your vacuum advance NOT hooked-up. If you don't run a vacuum advance, then don't do anything, just leave everything as it is and tighten it down and you're good to go. Your engine isn't really at "0" or Top Dead Center. The timing light is offsetting the light beam by 36 degrees, so you should be reading "0" on the crank.

The total advance will change if you replace your distributor or install an advance curve kit, so always check it and KNOW where your timing is at! It's REAL important on more radical engines, especially on high compression / supercharged and/or nitrous engines! One degree off on the timing can mean anywhere fron 20, 30 or even as much as 40 horsepower loss in some engines!

Now, if you use a vacuum advance, (which is pretty much only there for part throttle economy), hook it up AFTER you have set the total timing. Of course when you are driving down the highway at part throttle, the vacuum advance will pull anywhere from 10 to 15 degrees MORE advance (beyond your total timing), BUT under hard acceleration, the vacuum advance doesn't work anyway, so you're back on your total timing when you're on the throttle. Personally? I almost never use a vacuum advance on performance engines, although a vacuum advance can also be used to help engines get rid of "run-on" or "Dieseling" problems when you shut the engine off. You can do this by hooking it up to a manifold vacuum source to pull more advance when the engine is idling. For those of you that can follow along with that concept, the reason for doing this is to make that extra timing being pulled at an idle to increase the idle RPM, which in turn allows you to back-off on the idle speed screw, which in turn closes-off the throttle plates so when you shut the engine off, it can not "mechanically" continue to pull fuel and air into the engine causing it to Diesel.

Remember this; a correctly timed engine produces the most horsepower. An engine with timing too late (retarded) will have a low idle vacuum, have slow throttle response, feel like a turd at low RPMs and will run hotter than normal. An engine with the timing too soon (advanced) will have a high and erratic vacuum signal. It might have a snappy throttle response but it won't pull very well under a load, and it will have pre-ignition (detonation) problems, sometimes called "pinging", which will certainly lead to either a blown head gasket and/or serious piston damage. It will also idle rough, like it has a bigger cam than it actually does.

Keep this in mind too; once you set your timing and you don't physically move the distributor, the timing will pretty much never go out. The first thing people do (who have no clue what the hell they're doing) when their engine starts running funny, is to start twisting the distributor around and screwing with the timing. The timing will NOT affect any ONE particular cylinder. It can ONLY affect ALL of them at once, so if you have a back fire or a missfire or a dead cylinder, there is no need to start messing around with the timing. Again, if you don't move it, it'll stay set pretty much forever.

I had someone ask me; "If I set my total timing at 36 degrees at 3,500 RPM, will it still be 36 degrees at 7,000 RPM? If not, how much will it change?"

Below is the answer I gave him:

Well that depends. If you've set your distributor up to get full mechanical advance at or below 3,500 RPM, or have at least checked it to see that it’s all in by 3,500 RPM, then your answer is yes, once you hit full advance it won’t keep advancing as more RPM comes up. Once the mechanical advance weights sling-out at their given RPM, which is set by the type of weights it has, and more-so by the stiffness of the springs to give you your “mechanical advance”, they don’t (and can’t) come-out any farther. Most stock distributors are set to get full mechanical advance at about 4,000 RPM. Brand new MSD distributors come with ridiculously stiff springs in them that don’t see full advance until upwards of 4,500 – 5,000 RPM. You want all of your advance in at about 2,400 RPM. 3,000 is too high, 2,000 “might be” a bit low, so somewhere in there is where you want to be. This is why re-curving your distributor is essential. Not having full advance at or below 3,000 RPM is pretty useless considering an engine needs its advance to run at its best and to get that car moving. Stock, or un-curved, distributors that don’t see full advance until 4,000 – 4,500 RPM are a bit late in most cars and WILL cause your car to not move off the line or accelerate very well below it’s full advance RPM. But like I said, once the weights have slung-out… they’re out and won’t give it any more advance. The trick is to know how much advance you have, and WHEN it comes in. If it comes-in too late, then you need to change the springs and maybe even the weights to make it come-in sooner. If it has too much advance, then you need to limit the amount of advance inside the distributor and give it more “initial” timing on the crank to achieve the 34-36 degrees of desired total. I usually like to see about 10 – 14 degrees in the distributor and the rest on the crank in most performance engines.

[dmjlambert]

Bringing this back to ask another couple questions please. This is for my understanding, not because my engine is running poorly. OK, so my GM HEI on a 350 engine has initial timing set to 7 BTDC. That is the most number of degrees BTDC I can go without causing knocking. The vacuum advance gives 23 degrees of advance when it is connected to vacuum. I have it connected to manifold vacuum, so that means I get 30 degrees BTDC timing in idle.

The tune-up sticker says to set initial timing to 4 degrees BTDC, so I suppose that 3 extra degrees I'm giving it is what it can run with modern gas, the modifications done to the engine, and tweaking the conservative settings the truck rolled out of the factory with. But then I realize the tune-up sticker was not printed with HEI in mind, and the stock distributor should have had 13 degrees of vacuum advance according to the manual.

Now for the questions: How can anybody advise set initial timing at a particular value, such as 12 to 14 degrees BTDC without knowing how much advance the vacuum advance gives? Obviously that would not work on my engine.

Is adjustment of timing just something that should be done by ear and the follow up with a light just so you know where it is set for future reference?

Would it be advantageous in any way to have a vacuum advance that gives a smaller amount of vacuum advance, such as 15 degrees, and then have a more initial advance, such as 15 degrees, so the total amount of advance at idle is still 30 degrees ( same as 23 degrees from the vacuum advance + 7 degrees initial that I have now).

Note, this is not a performance engine, just rebuilt stock except newer heads, HEI, and a 1985-86 Quadrajet. And it's not driven by a lead-foot, just going to the store and work and back on city streets most of the time and on the freeway occasionally.

[Steeveedee]

GM's engineers agonized over those details, believe me. Now, all bets are off, as the fuel and other emissions requirements are completely changed. If you have ready access to a dyno, and a nice parts guy who will let you experiment with a number of vacuum advance assemblies, you could probably end up with a nicely drivable truck that doesn't suffer from detonation from over advanced timing, yet has enough advance to provide optimum mileage. I can't steer you in any direction on this, as I am in the same boat myself. I use the highest octane local fuel when towing the trailer, but am not convinced that I'm not doing damage. I want to rebuild/replace the engine in my truck, and I'm going to have to do a lot of homework to get it right. The current setup seems to be pretty good, but the rebuild has close to 100k miles on it.

[davepl]

Originally Posted by dmjlambert

Now for the questions: How can anybody advise set initial timing at a particular value, such as 12 to 14 degrees BTDC without knowing how much advance the vacuum advance gives? Obviously that would not work on my engine.

Really, it should be done on the dyno with a four gas analyzer. That's what I do. Or you can advance it until it pings or is hard to start, that's pretty standard :-)

The more retard, the more heat winds up in the cylinder head and exhaust (and hence, engine). So it'll even run cooler with more initial. I think the reason GM did this "too little initial advance" was to reduce NOX emissions.

Generally, with today's fuels, the engines like to run 12-16 degrees of initial advance. And that's =probably= OK in combination with the vacuum advance, though if it pings you might need an adjustable can. I'm running 14 and no issues.

However, your mechanical distributor might have 28 degrees of mechanical, and if they assumed 4 initial, that's 32 total (no vacuum at WOT). So you want to stay around 32 total, which means if you advance your initial to 16, you need to lock out or limit the total mechanical that the distributor will add.

My shop has a "distributor machine" but you can even do it in car with little roll pins, though it's been years since I bought a lockout kit.

- Dave

[Ol Blue K20]

Your vac advance should be hooked to ported vac not manifold vac. The total advance is for higher rpm and not idle.

[68 P.O.S.]

Read the following links to learn in-depth about timing and vac advance. It's really good info and dispels some of the misinformation being passed around about it.

http://67-72chevytrucks.com/vboard/s...d.php?t=274806
http://www.camaros.org/pdf/timing101.pdf
http://www.stl-vettes.com/65Vette/co...ng_Advance.pdf

[RichardJ]

>>Now for the questions: How can anybody advise set initial timing at a particular value, such as 12 to 14 degrees BTDC without knowing<<

Answer: they don't.

from nightrider388's post #13
>>Now, if you use a vacuum advance, (which is pretty much only there for part throttle economy),<<

People that advise using manifold vacuum don't seem to understand that manifold vacuum is highest at idle, 16-20" hg. When you stab the throttle, manifold vacuum drops to 2-3'hg. Manifold Vacuum retards the timing by as much as 15* when you step on the throttle. It stops off-idle pinging and it makes them happy.

Your method of retarding the timing two degrees at a time until it stops pinging is as good as it gets for a stock street engine. Did you do that with the vacuum advance removed/plugged? You should.

>>Would it be advantageous in any way to have a vacuum advance that gives a smaller amount of vacuum advance, such as 15 degrees, and then have a more initial advance, such as 15 degrees,<<

Just simply get rid of the 23* can. Something closer to 15* or 10* or an adjustable can.

The engine is going to ping during WOT, acceleration or when lugging(which you should not do) on a hill or when in the wrong gear. The vacuum advance should only be adding timing during higher vacuum such as when cruising.

[dmjlambert]

This thread took an interesting turn, talking about manifold vs. ported vacuum. I'm not worried about how well the engine runs at idle, it works just fine with ported or manifold vacuum, and does not ping at idle or when moving the throttle to various engine speeds when parked. It was only pinging when I was accelerating somewhat rapidly between 35 and 50 mph. So I backed the timing down to 7 BTDC. Under that condition (acceleration), it would not make any difference if I was connected to ported or manifold vacuum. But with the load I was giving it while accelerating, if the vacuum advance was retarding the timing at all, the 23 degree can would have retarded it even more than a 15 degree can. Apparently it was not retarding the timing enough to prevent the pinging under load, so a 15 degree can would not make it retard more under load would it?

Originally Posted by RichardJ

Your method of retarding the timing two degrees at a time until it stops pinging is as good as it gets for a stock street engine. Did you do that with the vacuum advance removed/plugged? You should.

Yes, with the vacuum advance removed and plugged, adjusted until it stopped pinging. Then took it for a test drive and came back and adjusted to further retard it until it didn't ping when accelerating while driving. I didn't think of towing anything or putting a heavy load in the bed and repeating the same test. If I do that, I will probably end up at the factory setting of 4 BTDC.

Originally Posted by RichardJ

Just simply get rid of the 23* can. Something closer to 15* or 10* or an adjustable can.

What would that do for me?

[68 P.O.S.]

Originally Posted by dmjlambert

What would that do for me?

Stop pinging

[dmjlambert]

It's not pinging.

I have 7 BTDC initial plus 23 from the can = 30 at idle and add mechanical advance through most range of engine use. If the vacuum drops on WOT to remove the vacuum advance, I would then have 7 BTDC initial plus mechanical advance.

If I change from a 23 degree can to 15 degree can:
I could have 15 BTDC initial plus 15 from the can = 30 at idle and add mechanical advance through most range of engine use. Same amount of total advance as above with the 23 degree can. If the vacuum drops on WOT to remove the vacuum advance, I would then have 15 BTDC initial plus mechanical advance.

So it seems if anything I would have more pinging with a 15 degree can, because it would retard the timing less in WOT. I may need to bring the initial down to 12 BTDC to keep it from pinging under load, which would then make my total timing less than I was getting with a 23 degree can.

What I'm trying to do is figure out why one amount of vacuum advance may be better than another. In what circumstances is less vacuum advance advantageous?

[Steeveedee]

Under certain conditions, detonation can occur even at part throttle. If one looks at the list of vacuum advance cans available, they have different amounts of advance, and different levels of vacuum required to gain that advance. There are a lot of variables that GM's engineers labored over to make each vehicle work most efficiently. At this point, we're all just guessing unless they have access to a dyno, which is an expensive proposition. Question- which is more expensive- dyno tuning or replacing a piston or crankshaft because of detonation issues?

[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.

[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.