ptuomov wrote: ↑Tue Nov 03, 2020 1:21 pm
adam728 wrote: ↑Tue Nov 03, 2020 12:49 pm
I'm trying to picture how a cold engine, with the same airflow and ignition timing as a hot engine, runs 200 rpm faster cold vs hot just by richening things up.
First, it doesn’t have the same air flow, it has the same size hole flowing more air mass when cold and less air mass when warm.
Second, the fully warmed idle is tuned to 0.95-1.00 lambda even with the oxygen sensor closed loop disabled.
The combination of more air from the same size hole and more fuel per gram of air because of warmup enrichment makes it want to run faster when cold.
So, that would effectively make the IAC orifice a masshole?
Sorry, you teed it up perfectly.
Belgian 1979 has summed up things quite nicely.
I worked on cold driveability for around 10 years using a chassis dyno, 2 gas IR, volt meters, scopes, and obd scanners, oh, and a sensitive built-in butt dyno.
It's pretty much all the same: give the engine the fuel it wants when it needs it, keep the idle up to keep it stable, and if it'll take more advance, use it.
You run yer hot rod at stoich at warm idle, which is around 0.6-0.8% CO which is not enough fuel for a performance cammed engine. The reason the idle speeds up when you add fuel when the engine is warm is that the idle CO is now around 1.5-2.5% CO or maybe a little more. Back in the days where one turned idle mixture screws, 2% CO was kinda universal for making an engine happy at idle and could be achieved with feel and one's ears.
Unless you have an unmeasured intake leak when cold (post MAF), I'm not sure how you will achieve increased idle speed without an increased IAC command.
If this thread hasn't done much, it sure has brought out a lot of practical, hands on info. Hopefully it'll be put to good use.