Air cooled and rich mixture at idle

[piggdekk]

Hello,
this is not a question striclty related to 911's, but still something I cannot quite understand and happens with air cooled 911's as well.
I happen to have mostly 2 valves air cooled toys (1971 911T, ducati 900SS i.e., sportster '97) and all of them run better if I set the idle mixture quite rich (CO about 3-4%) compared to what seems to work on water cooled engines. Why is that? I understand that 911 with carburators need a rich mixture to compensate for the off idle condition, but also my ducati with fuel injection works better when I have a richer mixture at idle. So it looks like air cooled engines beneficiate for running richer at idle, can anyone explain me why?
thanks
luca

[psalt]

luca,

My experience is that this has more to do with the combustion chamber design than air cooling. A hemi head engine with a wide VIA, like the 911 or Jaguar XK, Alfa, Lotus, etc. has a tall, open, large volume combustion chamber and needs domed pistons to get a decent compression ratio. This combination results in a lousy combustion chamber shape at TDC and poor cylinder filing at idle. A pent roof head with a narrow VIA and a shallow, low volume combustion chamber will run smoothly on a much leaner mixture. Under load at high rpm this is largely irrelevent, but the ignition timing requirements are very different.

Paul

[piggdekk]

thanks Paul,
I cannot find any good reason related to air cooling either, so combustion chamber shape may well be the reason. However, both the ducati and the my sportster have got a very compact combustion chamber (bathtube the ducati, hemi with lots of squish the Harley), and still they run better if rich at idle.
Could have something to do with head temperature? Maybe in air cooled engines head temperatures varies much more compared with water cooled.
thanks
luca

[psalt]

luca,

I'm sure there are other variables involved comparing different engines. If you really wanted to research this issue, I would start with surface to volume area, chamber volume to cylinder displacement, and intake manifold design. At part or closed throttle, there is a very messy mix of pulsing gases in the manifold, including exhaust, resulting in poor cylinder filling and low VE, which varies with cam timing, port length and velocity. Many cylinders with more firing pulses per rev, compact CC's, mild cam timing, tiny ports, long runners and a tiny throttle are all good for idling, but this is not a very interesting goal. It's what happens under load at WOT that matters.

Paul