Musings about Idle Timing

[vwlover77]

So, why does the idle speed increase when the timing is advanced by rotating the distributor?

My feeble brain says that if the engine turns faster with the same throttle setting that it is running more efficiently. What is the downside that I'm not seeing?

Will the engine overheat due to low flow of cooling air at idle with an advanced timing setting?

[static]

An idle mind is the devil's playground.

[Amskeptic]

A) why does the idle speed increase when the timing
is advanced by rotating the distributor?

B) My feeble brain says that if the engine turns faster with the same throttle setting that it is running more efficiently. What is the downside that I'm not seeing?

C) Will the engine overheat due to low flow of cooling air at idle with an advanced timing setting?

A) Advancing the timing at idle speeds up the engine because it increases the "springiness" of the compression/combustion pulse against the piston just before TDC. This seems more efficient to have a faster idle speed for a given throttle position, but it is cancelled out by increased friction, and the pumping losses against the closed throttle plate. Believe it or not, the most efficient idle would be wide open throttle at 4* BTDC where you set the idle speed by leaning out the mixture.

B) VW purposely made the idle timing less efficient (particularly with the air-injection buses at 10*ATDC) so they could cram more air through (that's the move towards the WOT above which is more efficient) and then lean it out a bit. You are correct that the engine would spin faster at a given throttle position with more advanced timing. But actual efficiency is a tricky little deal with many rich and subtle variables.

C) Note that the actual quantity of air allowed into the cylinders at idle is very small, just a wisp of fuel and air. Whether or not the spark is advanced or retarded has very little effect on cooling system capacity. If you had the timing so advanced that the combustion was actually expending itself on trying to run the engine backwards, you would have serious pre-ignition heat on the piston crowns, but it would be extremely localized overheating, nothing that the cooling system could address.

[vwlover77]

Colin, you truly are a master of internal combustion! Thanks!

[Amskeptic]

Diesel engines actually do the WOT method, with fuel injection quantity as the only speed control. Half the noise of a diesel is the unfettered intake. There is no engine vacuum with a diesel. That's why they are more efficient. They have to have a vacuum pump for brake boosters and vacuum operated climate controls.
Colin