[Grady Clay]

Scott,

One of the interesting features of this is the function of spark timing for idle stabilization and taking off from a stop.

I’ll be the first to admit that I paid way too little attention to this 40 years ago. We tend to think of the ignition advancing from some number at idle, through various rates-of-change to some fixed maximum. At idle and below there are important functions.

There is the situation at idle where the engine misses once, the idle drops and the system slightly advances the timing to bring the idle back to norm. This same feature is involved with advancing the timing when you start from a stop and the idle goes low and/or the vacuum under the throttle changes.

The vacuum advance & retard system lends itself to much tinkering to help our 911s be more ‘drivable’.

YES! For best performance there is an ‘ideal’ advance curve for full throttle performance. Our typical driving isn’t that. This means there is always a compromise depending on the various driving circumstances. When Porsche built our cars new, they had to deal with all these issues, including cost and eventually emissions. Even our carbureted & MFI race cars have to start, run at ‘idle’ and take off from a standing start. Can you imagine a 917 or 911R stalling on the start of the LeMans 24 hour?

I can tell first hand that our old Porsche race engines perform better at part (30%) throttle than full throttle when ‘off cam’ and during transition of power. Part of that is ignition timing. This is why Porsche uses ‘drive-by-wire’ to control throttle position, spark timing, mixture, turbo vanes and more.

For this discussion we need to address all the issues that CAN be done with our old mechanical systems. This is also a good basis for understanding how to apply computer based electronic systems.

Best,
Grady