I'm kind of confused about ignition timing. I read somewhere that for higher RPM you want less advance because you have more chance of preignition. But then say on distributors that have a mechanical advance, it advances the higher the RPM. Is it just different if your boosted? Should my FD timing be less when im in higher boost and higher rpm? I'm also wondering this for my bridge ported FB because im not seeing how a locked distributor could be beneficial if you want more advance at higher RPM? Can someone explain and give some reasons?

 

Its called a vacuum advance for a reason....

 

distributors usually have both a mechanical and a vacuum advance, this lets them advance in two conditions, one is rpm and the other is at light load.

 

Here is the basic principle. There are complicating factors like combustion speed but I'm leaving those out.

At higher speed, we need to advance the spark because there is less time for the burning to occur as the engine spins faster.

At higher load, we need to retard leading spark to reduce spark knock. We also need to retard the trailing plug relative to the leading (increase split) to reduce knock.

Traditional distributors have weights to advance spark up to a certain speed. That's centrifugal advance. It also has a vacuum diaphragm to advance timing at low loads, or if you think about it the other way--to retard at higher loads. On a conventional n/a carb'd engine, the manifold vacuum is proportional to the throttle position and the load. It's all silly primitive stuff now in the world of electronic throttle, direct injection, turbos, variable valve timing, and model-based ECU controls.

The Rx-8 has the best ignition system (although the coils themselves are garbage) of any production rotary because it can individually control spark on each plug based on whatever the ECU tells it to do. The most advanced piston engines on the market now are using multiple inductive sparks per cycle and multiple direct injection sprays per cycle.

 

generally timing wants to increase with rpm, but decrease with load. fuel under a given pressure takes a certain amount of time to burn, if we increase the pressure it burns faster.

with a locked distributor we set the timing to make best power at high rpm, and just live with too much timing at idle. idle quality suffers, and there is a chance of detonation in the lower rpm range, but its simple.

with a turbo car it works the same, except that since the turbo supplies the load, it can happen at lower RPMs than the NA car

 

https://www.rx7club.com/adaptronic-e.../#post11793329

 

Thanks for the help guys!