Let's talk timing

[rich weyand]

I posted these messages in a discussion of ignition timing on another forum. I thought I would post them here in the hope they would help somebody out.

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OK, let's talk timing.

You want to be at or near 36* BTDC with the mechanical advance all in and the vacuum disconnected, which is anywhere above 2500 rpm. You also want the most base timing you can get without knocking. That's the sweet spot for low end power. 16*BTDC is usually safe on a stock 350 with the 76cc heads. It ought to start knocking under lugged conditions (low rpm, foot in it, up a steep hill) between 18 and 22 BTDC, so that's why I say 16* BTDC is usually OK.

If you can get 16* base timing, then 20* mechanical advance is perfect to get to 36* all in. If you can only get 14* base timing, because it starts knocking under load at 16* and you want a couple degrees safety margin, then you would want 22* mechanical. Try to get 16* base timing first. That's your ignition timing at launch, 1500 rpm and full throttle. If you get knocking, back off 2 degrees and try again.

This is why being able to dial in more mechanical advance on performance kits is important. With high-compression pistons and heads, you may only be able to run 12* BTDC base timing, so you need 24* mechanical advance to get to 36* all in. But 24* mechanical advance by itself is not a performance setting. You are better off with the highest base timing you can run and as much mechanical as it takes to get to 36* all in.

Base + mechanical of 36* BTDC is your best timing on all GM V8s in the upper half of the rpm range, 2500 rpm to redline, and full throttle.

Vacuum advance is to compensate for low cylinder pressures at closed throttle positions, such as at idle. Less intake manifold pressure (that is, more vacuum) means less mixture to compress in the cylinder, lower cylinder pressure at ignition, and a slower burn rate of the mixture, so you need to light it sooner. Vacuum advance also adjusts your timing at cruise, when you are above 2500 rpm and at constant speed, like interstate cruising at 70 mph. 44-52* BTDC is a good range. More than 52 is probably too much. Some people like to run low in the range. So lots of variation here. Adjusting vacuum advance for smoothest idle (on manifold vacuum!) is probably your best setting, wherever it is in that range.

If you have 36* BTDC base + mechanical, another 8 to 15 degrees of vacuum advance puts you in the range of 44-52* BTDC at cruise. Note that all of these are in CRANKSHAFT degrees. Vacuum advance for historical reasons is usually specified in CAMSHAFT degrees, where mechanical advance is not. Don't ask me, I didn't make the rules. So 4 to 7.5 camshaft degrees of vacuum advance is in the range. The stock HEI that came on our trucks had the vacuum can set for 7.5 degrees. Adjustable vacuum cans allow you to adjust down from that value, to achieve smoothest idle.

As an example, I am running 16* BTDC with the stock HEI, which gives me 16* BTDC on launch, 36* BTDC above 2500 rpm at full throttle, and 51* BTDC at cruise. The stock HEI works for me with a mostly stock engine, because I get the right numbers per the above discussion. An adjustable vacuum can would probably allow me to smooth out my idle a bit. If I could not get 16* BTDC, I would have to adjust the mechanical advance to get 36* BTDC all in.

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Oh, one more thing. Our stock truck engines pull a lot of vacuum. Having the vacuum advance come in very early (at low vacuum, like 5") means you have to have the pedal completely planted to get the advance out. Not so good. The stock advance comes in at about 11". The 1970s low-compression 350s pull 15+ inches of vacuum, so when you open the throttle plates, the vacuum advance drops out earlier. This is a better match for the internal cylinder pressures at ignition with partially open throttles than a vacuum advance set at 5".

When people build high-performance engines, optimized for high-end horsepower, they may only pull 10-12 inches of vacuum at idle, or even less. A vacuum advance set to come in at 11" of vacuum may not come in at all on such an engine, or the vacuum advance may "hunt", kicking in and out of advance as the manifold vacuum hovers around the 11" mark. Setting vacuum advance to come in at 5" is necessary on these engines to get them to settle down at idle. Not a biggy, because if you are racing, you only run the engine wide open (0" of vaccum) or at idle waiting for the Christmas tree to light up.

So setting the vacuum advance to come in at 5" is also considered a performance setting, but it is not a performance setting per se, it is a setting made necessary by other issues with a high-performance engine. Half-throttle performance in a street engine pulling 15+" of vacuum will be better if the vacuum advance comes off earlier, at a higher vacuum, like the stock 10" to 11" value common to GM vehicles of the 1970s.

The vacuum advance should always be set at least 2" lower than the vacuum at idle, to prevent hunting. So measure idle vacuum. If it's 10", your vacuum advance should be set to come off no sooner than 8"; 7" would be even better. But if you idle at 15" of vacuum, an 11" drop off will give better part-throttle performance than a setting of 5".

The thing is, as the throttle plate opens and cylinder pressures come up, you want to get out of the vacuum advance so you don't get knocking in transition. Bigger numbers -- 11" as opposed to 5" -- get the advance out SOONER as the pressure comes up, and get you to your launch mode base timing sooner.

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Let's see if I can explain why I set it the way I do.

When the engine is at idle, the car stopped, and you tromp the throttle to launch the vehicle, there is almost no air flow through the engine, so no resistance through the carb and air cleaner and all, and the manifold pressure goes to 1 bar. At the same time, the engine has high load, because the drivetrain is at dead stop. The cylinder pressures go way up. This is the most likely point where an engine will detonate if the timing is too advanced.

For racing applications, they avoid this situation by running a torque converter with a higher lock rpm. The engine gets to wind up before it sees the full load of the drivetrain. This also lets them run cams that don't make as much torque down low; they wind the engine up to where the power band is before the torque converter locks. High-rpm torque converters are also a PITA to drive on the street, and put high streeses on transmissions.

In a street application, we have the engine at low rpm, the manifold at one bar, and the drivetrain load on the engine is a maximum. This is where detonation will occur if ever. The maximum power you can get from the charge in the engine at this point is with the earliest timing that will not cause knocking. This is why all the old geezer hot rodders and engine tuners I know say to advance the base timing until you get knocking, then back off 2 degrees to give some safety margin against variations in gasoline, atmospheric conditions (temperature, humidity, barometric pressure), and other variables.

As the engine speeds up, more air flows, and the induction system introduces some pressure loss, the vacuum comes up a bit and the cylinder pressures come down. The timing, which is set in degrees, gets shorter in milliseconds: 15 degrees of advance is 2.5 milliseconds at 1000 rpm, but is only 1.67 milliseconds at 1500 rpm and 1.25 milliseconds at 2000 rpm. If we already determined that 2.5 milliseconds was the detonation limit at 1000 rpm, then we have some head room now. We can add advance -- 2.5 milliseconds at 1500 rpm is 22.5 degrees, 7.5 degrees more than the base timing of 15 degrees, and 2.5 milliseconds at 2000 rpm is 30 degrees, 15 degrees over the 15 degrees base timing. The mechanical advance adds the extra advance we need to keep the engine in time but stay short of the detonation limit.

That's with the throttle plate wide open. Closing the throttle plate adds air resistance in the induction system, resulting in a lot of manifold vacuum and reduced cylinder pressures. These thinner mixtures raise the detonation limit a lot, because thinner mixtures burn more slowly and because compression heating of the charge is much less, and so we can add a lot more advance without stepping over the limit. The vacuum advance monitors the intake manifold and if set up properly adds additional timing based on the lower cylinder pressure.

This doesn't affect performance -- torque from launch through redline at wide open throttle conditions -- and so race cars don't run vacuum advance. Extra parts to break, and if it sticks advanced, the heads will go through the hood on launch. For a street car, though, where you are usually not at wide-open throttle, the vacuum advance affects driveability and mileage. Setting up the vacuum advance for smoothest idle, and setting the rpm it comes in at to enable vacuum advance at constant-speed cruise, should result in the best driveability and mileage. Having the vacuum advance drop out too early (higher number) will reduce mileage at cruise, having it drop out too late (lower number) risks going over the detonation limit in transition, such as cruising at speed and then stomping the throttle to pass.

Anyway, that's why I set mine the way I do.