Draft: Ignition timing & distributors 101

[Llama]

Draft techie article. More pics to come. Article is written with most of my experience with British stuff, so there are probably some things that won't apply to Jap stuff.

Important terms

TDC: Top dead centre, where the piston is at the top of the cylinder

Advance: Causing the spark to ignite earlier, measured in degrees before top dead centre (BTDC)

Retard: Opposite of above

Why is timing important

In an internal combustion engine, peak power is generated when the maximum pressure of combustion occurs just after the piston reaches TDC. Because the fuel/air mixture takes a while to burn (the fancy word is deflagrate), it takes a very short but important amount of time for peak pressure to be reached. Thus the spark has to be carefully timed to create peak pressure in the ideal situation.

Why timing varies as engine conditions change

As the car increases it's RPM, the piston moves faster, and so timing must be advanced to ensure peak pressure is reached as the piston approaches TDC, as the fuel/air mixture will take the same amount of time to burn regardless.

At low throttle positions (cruising/idle speed), there is a lower amount of fuel/air mixture entering the cylinder than compared to WOT. Upon reaching TDC, the pressure without combustion in the cylinder will be LESS than atmospheric pressure by about half (ie, ~0.5 bar). Thus as the piston approaches the end of the compression stroke, there will be less fuel/air and lower pressure than compared to at WOT. Because of the lower pressure and less mixture, it will take longer to burn, thus timing is advanced more by the vacuum advance to insure maximum pressure is reached as the piston reaches approaches TDC.

What happens with bad timing

If your timing is too advanced, you may cause a pressure so high, so that along with burning normally, it also detonates in a pressure wave explosion (also known as engine knock). This usually starts in the edges of the cylinder away from the spark plug. It causes extremely high pressure that can damage the internals of the piston. If timing is WAY too advanced, the pressure of combustion will try drive the piston backwards as it has not reached TDC, and the car will not run at all.

Timing too retarded means the piston has already passed its optimal point. Power output will be reduced, as more of the energy from combustion will turn into heat, potentially causing overheating issues/burnt valves etc.

How is timing controlled

-Vacuum advance

A usually rubber diaphragm is hooked up to a vacuum line that will run to a port on the manifold or carburettor (more on this later). As the diaphragm is exposed to vacuum (idle/low throttle), it pulls a spring connected to the base plate of the dizzy, advancing it.

-Mechanical advance

Mechanical advance is controlled by weights on springs attached to the distributor shaft. As the shaft spins faster with higher RPM, centrifugal/centripetal/james bond force pulls these weights out and advances the timing

-Computers/black magic

Modern computer controlled systems essentially replicate the vacuum and mechanical advance with a angle sensor on the cam/crankshaft and a throttle-rpm-timing lookup table.

Boost & timing

When you compress air (with a turbo or supercharger), it's temperature rises. Higher temperature fuel/air mixture is more susceptible to pre-ignition, thus intercoolers are used to reduce temps to normal. Higher pressure means faster flame front (inverse of what happens at low throttle), so timing will thus be retarded to prevent detonation/backfiring.

Ported vs manifold vacuum

(big can'o'worms right here. Don't kill me please)

Manifold vacuum is taken off the intake manifold (surprising, isn't it?), where it gets high vacuum at idle, and then as you open the throttle vacuum in the manifold decreases (and therefore timing is retarded) until you reach WOT, where there should be no vacuum and no extra advance from the vacuum advance unit. Manifold vacuum gives you the most efficiency at idle, but on a lot of cars & carburettor combinations the engine will stumble on take-off. There are a few rather complicated reasons for this, I'll explain the main ones here, but you can always PM me for more info.

Vacuum signal from the lines can change instantaneously with the butterfly valve (throttle plate) position. So as you quickly move from idle to WOT the advance changes quickly. However the faster air from WOT takes time to move through the carbs and into the manifold. The delay in the faster air means that you have a moment where spark timing is too advanced, and not enough fuel is drawn from from the venturi. The combination of the two is enough to make a car stumble a bit on idle. Modern fuel injection systems and even some carbs have mechanisms to compensate for this by spurting extra fuel if the throttle plate is moved fast enough.

Ported vacuum is where the vacuum line is hooked up on the atmosphere side of the butterfly. This gives a similar effect to manifold vacuum EXCEPT that you get no advance at idle.The retarded timing at idle creates higher temperatures in the cylinder that means you get less unburned fuel exiting with the exhaust, which is good for an emissions point of view. Of course higher temperatures are not always a good thing, especially if you are running boost, but if your car was designed with ported vacuum in mind, then you shouldn't be having overheating problems. It does mean however you will be drawing more fuel at idle, so economy will decrease somewhat.

There is no 'best' option for ported vs manifold, it depends on the car and what set up you have. Unless you are upgrading significant portions of the engine (ie lumpy cam, ported head etc) or tuning for race spec, stick with what was standard.

Maybe I'll do a writeup on carbs sometime too...

P.S. Don't get ported vacuum confused with venturi vacuum, which is created by the venturi effect inside the carbs and is different from what we are talking about here.

Peace out homies.

[kpr]

boost= higher cylinder pressure, basicly same deal as what you've written re: low throttle vs wot on n/a

most decent intercooled forced induction setups, will have air inlet temps no higher than n/a setup

[Llama]

Ah yes, I had forgotten about intercoolers. Updated & Thanks.

[Llama]

Professors brain picked, textbooks consulted.

Now updating with extra on manifold vs ported vacuum

[EDIT]Updated.