I am almost losing sleep over this question, but I fell like a jackazz saying it.

My understanding is at WOT the kpa values are above 90 at any given RPM. On a non-computer car, the spark advance is per the RPMs, higher the RPMs more advance WOT or NOT. What I don't understand is why there is MAX advance at around 50 kpa, and not under WOT kpa ranges. Am I missing something here?

 

Mechanical distributors advance /retard the engine timing in two ways which is what electronic distributors emulate only better. Weighted cams with spring return provide centrifugal timing advance as RPM increases and retard as RPM decreases. A small vacuum canister provides timing advance as vacuum increases, retards as pressure increases (vacuum decreases). With the butterfiles closed, an engine draws high amounts of vacuum at idle, more at decelleration overun so the vacuum advance is adding timing, typical is 20 - 25 deg. at idle while the timing can never go lower than the base mechanical timing. As the butterflies open, exposing the engine to atmospheric pressure, and vacuum decreases (MAP increases), timing is retarded and at WOT, only the centrigugal weights control timing. With a base timing of 6 deg. the maximum timing cam be 51 deg. for a V-8 (45 + 6). HTH

 

I'm getting a little farther trying to figure this out. I've been google searchin and reading for a couple hours. This is what I have come up with.

Due to the inefficency of the engine at low load and engine speed, more spark advanced is needed to make up for slow velocities and self EGR. At WOT and higher RPMs turbulance/swirl and cylinder fill reduce the amount of spark advance needed because the the quicker combustion from the turbulance/swirl, combustion chamber design, etc...

 

A good breathing engine will be over 90....

A mechanic distributor, is well, primitive. 1/2 of EFI's advantage is being able to control the timing in a way inpossible with a mechanical distributor.

An engine is doing it's absolute *best* at peak torque. That's the sweetest spot in the whole RPM range, for the engine to make the most of the air and fuel, it's offered.. Some engines taper off slowly from there, some drop like rock, it's all about any one given combo how it does.

Also, the combustion process is only cascaded by the spark event. It generally (sorta kinda) takes a reality fixed amount of time, for it to occur. So at higher RPM it has to be started early, since it takes more degrees to equal a given amount of time. But, then again, *we* have to look at how effeicent the engine is over peak torque to see how much the timing needs to change.

There are guys that have spent their entire live looking at this stuff, and still have questions about what's really going on, so don't lose too much sleep over it.

 

Well my engine breaths well

Thats why I keep thinking about the advance, because the piston is moving a lot faster so the spark would have to fire earlier to peak at the sweet spot ATDC.

From what I can find/see, there is only one SA table in my bin. Stock has the advance at only 16 degrees (80+ Kpa), that seems kind of lame.

I guess its, guess and check for knocks and nothing more