Ignition Timing Theory

[racebmw]

I am trying to understand why ignition advance values typically stop increasing at somewhere between 3,000 and 4,000 RPM.

If the timing is correct at say 38 degrees advance at 4,000 how can that still be the correct advance at 8,000 RPM?

The time it takes the piston to travel from BDC to TDC at 8000 is half that at 4,000 therefore the ignition advance should be different for these two engine speeds. Yet every map I have looked at shows max advance happening somewhere between 3,000 and 4,000. Shouldn't the timing be advanced more at 8,000 than at 4,000?

Has anyone experimented with this on a dyno?

My reasoning is that the time it takes to generate the spark is fixed, the time for the fuel to ignite and start to burn is fixed so if the piston is moving faster don't we have to light the fire sooner?

Or does the cam timing effectively place a limit on how early you can fire the plug? Maybe that's the limiting factor....

Therefore we can just end our ignition maps at 4,000.

Thoughts, comments?

Jeff

PS I might have the car on a dyno next week and will experiment a bit.

[capri_turbo]

from the tuning books that I have read for the Ford OHC (Pinto), and going back over my college notes, the combustion chamber shape and position of spark plug will limit the amount of advance that an engine can use, also, compression ratio and fuel octane rating will also affect the amount of advance required. On overrun conditions, you can use much greater advance figures, but under load, there comes a limit where the physical design of the engine restricts what advance you can run.

My modified Pinto can be given way too much advance at 3000 RPM without any pinking... the result is a harsh sounding engine, and whilst there is no audible pinking, it's doing it no good at all, so the timing had to be dropped back at this point until the engine sounded right, and it noticably drives better with slightly less advance.

Cam timing, lead, lag and overlap will also affect what ignition advance you can run, and no 2 engines are exactly the same.

I'm sure someone will be able to expand on this.

[merlin]

you need to be very carfull with advance over 4000 RPM as if the piston does not have enuf time to get out of the way quick enuf it just melts and then your in a hold biger world of hurt simple speeking.