Distributor advance and dwell curves/settings

[Johnfm]

Quite a difference between standard and sprint advacne isn't there!

Note too that your manual is more detailed, in that it specifies that static must be added.

[Johnfm]

Won't be able to get to a rolling road for some time.

So, after mucho reading, and Rohan and Peter's comments, I will set it as per Rohan/Manual (ie get it to 24 degs by 2500 rpm - and then see how agressive I can set the curve from 0 to 2500 rpm without pinking.

[Johnfm]

Well, having a fiddle with the distributor I was surprised how much static timing affected idle speed.

At 40deg or so it must have been idling at close on 2000 rpm.

What criteria determines optimum static timing?

I assume it was initially determined based on the mechanical advance available and the desired max advance (24 degrees needed at 2500rpm plus minus 12deg of mech advance = 12 deg static advance).

But with electronically settable advance, I can set static at any optimum figure and define an advance curve - so why not have more static advance?

[rgh0]

Optimum advance is determined by engine speed and combustion burn rate. Burn rate is determined by combustion pressure and turbulence in the combustion chamber. turbulence is determined by engine speed and gas flow.

At idle the engine will tolerate a higher advance as it has very low burn rate as the combustion pressure is low with the throttles closed and turbulence is low as there is little gas flow and slow engine speed. However when you open the throttles the engine suddenly needs much less advance as it comes off idle or it pings. The idle advance rate is set more based on what is required at low speed with open throttles. With a modern electronic ignition you can feed in more parameters and you could use more advance at idle and immediately reduce it when the throttle were opened. It does not do anything for performance so people dont worry about it for an Elan . Maybe modern cars do this sort of thing as it would reduce emissions and fuel use in the city test cycle where idle time is a factor

cheers
Rohan

[Johnfm]

Cheers for that.

With just a rpm -v- advance curve, it is pretty blunt.

As you suggest, with a modern engine or more complex engine management you would tweaking a lot more.

For the moment, I am quite happy that it runs ok. Just need to get it to a rolling road guy and get it all properly tuned.