dave@ihu1f.UUCP (07/01/83)
I have never really understood the difference torque and horsepower,
until I read net.auto. Thanks for the information.
Now, can anyone explain to me how engine timing works in the same detail?
How do the initial setting, mechanical advance, and vacuum advance
all interact?
What compromises are made for pollution controls?
Thank you,
Dave Miller
8-367-3791
ihuxu!davewookie@alice.UUCP (07/11/83)
Engine timing is simply when the ignition spark occurs to get the fire going in the cylinder. Since it takes some amount of time to start the fire and since we would like maximum pressure to begin just after the piston reaches top dead center on the compression stroke, we must ignite the mixture sometime before top dead center (TDC). The next variable is the speed of the engine. Assuming the fire takes about the same time to get going then it may be deduced that as the engine runs faster the spark must fire sooner in the compression stroke. This is accomplished by the mechanical advance which is a variation on a flyweight governor. The farther out the weights move due to centrifugal force the more the timing is advanced (ignition occurs earlier in the compression stroke). The timing curve may be adjusted by changing the springs and weights in the distributor. Bringing the advance in sooner results in a peppier engine. The vacuum advance takes into account differing amounts of fuel and fuel mixture in the cylinder. While you are just cruising along not demanding much power from the engine, the mixture is leaner and there is less of it entering the cylinders. This type of mixture burns more slowly and so the timing is advanced even further for maximum efficiency. The vacuum in the engine gives a good indication of the demands on the engine and so can be used to control this additional advance. The simple setup is then a mechanical advance and a simple manifold vacuum advance. (In high performance work like racing the vacuum advance is not used since we want all or nothing from the engine most of the time) Pollution comes in to complicate the game. Advancing the timing increases pollution and thus we now do not advance the timing as much (check the initial timing settings for cars from the sixties and note how the timing is more retarded into the seventies. This results in a less efficient engine that doesn't pollute as much but burns more gas to get the same power. The vacuum advance used now used what is termed ported vacuum. By monitoring the vacuum just above the throttle plate we see no vacuum when the throttle is closed, high vacuum as the throttle is opened and then low vacuum as the throttle becomes wide open. Using this ported vacuum then provides no vacuum advance at idle and thus reduces pollution at idle and really doesn't affect the engine much since we aren't demanding anything at idle. Setting the initial advance gives a small amount of initial timing which gets the fire going early enough at very slow idle speeds like 600 RPM (before the centrifugal advance comes in). The initial advance will of course affect the total ignition advance at high speed when all the mechanical advance is in. In my race car the distributor is set up to get all the mechanical advance in at 3000 RPM and so we set the timing at about 3200 RPM at 38 degrees before TDC. More advance than this and the fire is started too soon and will try to push the piston back down on the compression stroke (it's trying to compress an explosion!) This results in pistons with holes in them or maybe bent connecting rods so we don't want to overdo the advance. If I set my timing at 37 degrees then the engine is less efficient and in the race car I have an overheating problem on warm days. You peobably won't see this on the street!) The type of fuel also affects how much timing may be used. High octane fuels burn more slowly (to prevent detonation) and so require more timing advance to get the fire burning. Air/fuel ratio also affects the burning speed and thus timing. I hope this gives you some idea of what's going on. If you have any questions don't hesitate to ask! Keith Bauer White Tiger Racing