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Old 11-22-2015, 11:51 AM   #1
rich weyand
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Ignition Timing for First-Generation GM V-8 Engines

I pulled all my ignition timing rants from multiple forums together into one post for another forum, so I thought I would post it here as well.

Ignition timing, including base timing, total timing, mechanical advance, and vacuum advance, which can be either ported vacuum or manifold vacuum, drives people crazy. There are as many opinions out there as there are people, maybe more. It's complicated by the fact that our trucks in many cases are no longer subject to a pollution controls regime, so we took that stuff off, and the "official" settings are no longer the best ones. I have been working on cars for almost fifty years, from a 1961 Chrysler Newport forward, and have talked to hot rod designers, drag racers, dirt-track racers, mechanics and engine theory guys to get the best understanding I can of how best to approach the problem. That said, it's still one guy's opinion.

Also, this article applies to our trucks with more or less stock first generation SBC or BBC engines, as well as to GM cars of the same period. Big cams and dual-plane manifolds and headers won't change this advice. Turbochargers, superchargers, nitrous and other serious shenanigans will, however, as will later engines that were designed to incorporate emissions considerations in their designs rather than having them added on (and removed!) later.

Let's get some terms out of the way:

- base timing. The timing at idle with the vacuum advance disconnected. This is usually the value you set with your timing light. (Yes, you need a timing light.) Base timing is expressed in degrees before [the piston is at] top dead center, such as 12*BTDC.

- mechanical advance. The amount of extra advance that is added by the ignition system when the engine is running above 3000 rpm. It usually starts much sooner than that, and ramps in, and it may all be in by 2500 rpm or so.

- total timing. The timing above 3000 rpm. This is the base timing plus the mechanical advance. You can measure this with a timing light, too, with the vacuum advance disconnected and while revving the engine.

- vacuum advance. The amount of extra advance that is added by the ignition system based on how much vacuum the engine is pulling. When the throttle plate is wide open, the vacuum the engine is pulling is very low, a couple of inches of mercury (such as 4"Hg), and when the throttle plate is closed, the engine is sucking on the closed carburetor and generating high vacuum (such as 18"Hg). 30"Hg is one atmosphere.

- manifold vacuum. This is the vacuum in the intake manifold of the engine, below the carburetor.

- ported vacuum. This is the vacuum taken just above the closed throttle plate position in the carburetor. At idle, the plate is below the port, and there is no vacuum, but as soon as the throttle plate opens a little bit, ported vacuum becomes the same as manifold vacuum.

- cruise timing. This is the ignition timing when cruising at a constant highway speed. In this situation, the throttle is almost closed, but the engine is running at rpm, so all the timing is in: base timing plus mechanical advance plus vacuum advance.

- top dead center. The point where the piston stops at the very top of its stroke before heading back down. There are two top-dead-centers every cycle, one between the compression stroke (up) and power stroke (down), and one between the exhaust stroke (up) and intake stroke (down). In practice, when speaking of ignition and top dead center, we mean the first one.

- number 1 cylinder. On a Chevy, the front cylinder on the driver's side of the engine. Thsi is the cylinder whose spark you use to drive your timing light so your flywheel mark reads correct on your timing tag. The number one cylinder for any GM V8 will be at top dead center when the key in the crankshaft is at 45* clockwise from vertical, at 1:30 on the clock when looking at the front of the engine.

Timing values

The values for timing that I see most often recommended for pre-pollution GM V8s, and what I have found to work best on my engine (re-cammed 350/260hp crate, with Edelbrock 1406, dual-plane manifold, headers, duals, Flowmaster 40s) are as follows:

Base timing: 16* BTDC

Mechanical advance: 20* additional

Total timing: 36* BTDC

Vacuum advance: 15* additional. Note Vacuum advance canisters are normally specified in camshaft degrees, and the camshaft rotates half as fast as the crankshaft, so the recommended value here is 7.5 camshaft degrees. This is lower than the typical vacuum advance cans used in the emissions-controls era. If you want to replace your emissions-control-era vacuum advance can on your HEI distributor to be better suited for non-emissions ignition timing, use the AR23/VC1853 vacuum advance can.

Cruise timing: 51* BTDC

Why all the complication?

So why do we change the ignition timing all around, depending on engine rpm and throttle position and mixture and everything else? The issue is combustion chemistry. It all comes down to getting the most horsepower and torque we can out of the burning fuel, so the way fuel burns is what is driving the seeming complication of ignition timing. "Ignition timing". Ignition of what? The fuel. When do you ignite it? At the best time to get the most push on the face of the piston as it moves downward on the power stroke. That's what it's all about, and that goes to the issue of the actual chemistry of burning gasoline.

Let's start with mechanical advance.

When the throttle is wide open, and the engine is accelerating, the manifold vacuum is at zero, so the manifold is at atmospheric. If you have an 8:1 compression ratio, you are pulling in atmospheric-pressure mixture and compressing it to one-eighth the size. Your cylinder pressures at TDC are eight atmospheres absolute, or seven atmospheres above normal pressure. (I will leave out the complications of dynamic compression ratios and intake runner length and such here; assume a volumetric efficiency of 1.0 for this discussion. You guys who know what I said there can figure this out on your own.)

There is an optimum time in milliseconds of advance for that cylinder pressure. That is, there is a certain amount of time it takes that mixture to burn down to the piston face and start pushing. At that point the piston should be starting down. So, the reason you need base timing to be before TDC is because it takes some time for the fire to burn from the spark plug to the piston face. That optimum time is about 2 milliseconds, so this is all happening very quickly.

The problem is that that time is in milliseconds, not degrees of rotation. As the engine turns faster and faster, it turns more degrees during those milliseconds of time. At 1500 rpm, the crankshaft rotates about 18 degrees in 2 milliseconds, while at 3000 rpm, the crankshaft rotates about 36 degrees in 2 milliseconds. That 18 degrees at 1500 rpm is close to our recommended base timing, while the 36 degrees at 3000 rpm is our recommended total timing. So the burn time in seconds isn't changing, but the measurement of the burn time in degrees of rotation is changing. The mechanical advance keeps adding degrees of advance to the timing to try to get to that same value in milliseconds to maintain optimal timing as the engine speeds up.

OK, so what's vacuum advance?

When the throttle is shut, you might be pulling 15" of vacuum, say. One atmosphere is 30" of mercury, so the manifold has one-half atmosphere of mixture in it. The cylinders pull in half as many molecules of air and fuel as they did with the throttle wide open. You now compress that at your 8:1 compression ratio and you have four atmospheres of cylinder pressure absolute, which is three above normal atmospheric, not seven as before with WOT.

That's a huge difference in cylinder pressure, and the fuel mixture will not burn at the same speed. That thinner mixture will burn slower. The molecules are farther apart, and the combustion process is slower. That means that, in order for the front of the combustion pressure wave to hit the piston at the right time, you have to ignite the charge sooner -- i.e., you need more advance. Said again, instead of 2 milliseconds, that burning charge now needs 3 or more milliseconds to burn from the spark plug to the piston face. Instead of 18 degrees of advance at 1500, we need more like 30 to 35. So the vacuum advance canister pulls the guts in the distributor about 7.5 camshaft degrees out of place from its relaxed position, which adds 15 degrees of crankshaft advance, and moves our recommended timing from 16* BTDC base timing to 31* BTDC.

Now to ported vs. manifold vacuum.

Manifold vacuum is the signal from the manifold that tells how thin the mixture in the cylinder will be at TDC, and how much in advance of TDC the charge needs to be ignited for the pressure to reach the piston face at the optimum time. There is no difference in the combustion chemistry between idle and just-off-idle, and no reason for the timing to be different.

Ported vacuum purposefully ignites the charge late at idle. The idea is to dump a still-burning charge into the manifold so that added air from a smog pump together with the still-burning charge will burn up some of the pollution before it exhausts the vehicle. This change with ported vacuum only happens at idle, so it reduces pollution from idling cars, such as those stuck on LA freeways at rush hour. That's the purpose. Normal vacuum advance is immediately engaged as soon as the throttle is opened even a bit, because otherwise power and mileage would suck big time.

Running on ported vacuum will require a higher idle throttle setting, because not as much of the energy of the charge is applied to the piston. This reduces gas mileage. It will also heat up the heads more, because the charge is still burning when it heads out the exhaust runners, resulting in higher engine temperatures. Some systems have a special temperature sensor on the engine to switch from ported to manifold vacuum if temps climb too high for this reason. Ported vacuum may add an off-idle hesitation, as the vacuum advance has to suddenly re-adjust the distributor when the throttle is depressed, and that takes a fraction of a second to happen.

A compensation for ported vacuum that some people make is to run a richer mixture, because a richer mixture will burn faster than a lean one, reducing the effect of igniting the charge late. This is why when someone moves the vacuum advance from ported to manifold without adjusting the idle mixture and idle throttle setting, the engine might initially run worse. But lean out the idle mixture and reduce the idle throttle setting, and the engine will idle better, have no off-idle hesitation, will run cooler, and get better gas mileage than on ported vacuum.

Setting the timing.

Sometimes you really need a picture. Additional information:

Finding Top Dead Center
https://www.youtube.com/watch?v=xg6eAWWvrPA

Using a timing light and adjusting the distributor are the center portion of this video. Don't forget to disconnect the hose to the vacuum advance first.
https://www.youtube.com/watch?v=UYGU...youtu.be&t=247

If you have an ECM, you need to disconnect the tan wire first, which he does at the beginning of this video but yours can be there or near the steering column:
https://www.youtube.com/watch?v=Cj5_Hk9Pgrw

Which Top Dead Center?

Another thing. I mentioned above that there are two top dead centers per engine cycle. Mechanically from the point of view of the bottom end of the engine, they are the same. It's the top end of the engine -- the camshaft and the valves -- that make them different. This is because the camshaft rotates half as fast as the crankshaft.

So how do you tell them apart?

Really simple. At top dead center between the exhaust and the intake stroke, both valves are open. This is because we want the intake mixture to get started into the cylinder to push the last bit of exhaust gasses out for just a bit. So both valves are still somewhat open. On the top dead center between the compression and power strokes, both valves are tightly shut, because we are squeezing that charge before igniting it.

So, set the engine to top dead center by aligning the timing mark of the flywheel with the "0" mark on the timing tag. Take an air hose and put it into the #1 spark plug hole and put a rag around it to cover the opening. Now hit the air. If the rag stays put, the air is going out the open valves, and you are between the exhaust and intake strokes. If the rag pops up off the engine, the valves are closed, and you are between the compression and power strokes.

At top dead center between the compression and the power strokes, the distributor rotor should be pointing toward the front of the car, at about the 5:00 position if you are looking down on it from the front of the car. If it is pointing to the 11:00 position, the distributor was installed with the engine at the wrong top dead center, which you already knew because you were popping flames out the carb or throttle body when trying to start it.
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Old 11-22-2015, 12:21 PM   #2
1970pelle
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Re: Ignition Timing for First-Generation GM V-8 Engines

When I start to dial in my engine after some sort of repair or just a tune up. I have a vacuum gauge mounted in the engine compartment full time. It helps when setting everything up. I can just use a vacuum gauge reading alone to set the timing, carb air/fuel mixture, rpm.
But the other tools are very help full.

Ken
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