1985-1998 [Subscribe to Daily Digest] |
![]() | [Main 9000 Bulletin Board | BBFAQ |
Prev by Date | Next by Date | Post Followup ]
Member Login / Signup - Members see fewer ads. - Latest Member Gallery Photos
My brain hurts! Posted by Ari [Email] ![]() ![]() In Reply to: Ari-Whats Wrong ??????????, Ron J, Wed, 5 Jan 2005 08:25:04 Members do not see ads below this line. - Help Keep This Site Online - Signup |
I need to disagree with you on some points-
At idle, there is minimal air-fuel mixture in the cylinder - throttle closed. This won't ignite sufficiently until it is properly compressed. It will also burn very quickly, because there isn't as much of it. And yes, things are moving very slowly. So you want to ignite the mixture when the piston is near the top of the stroke. Advance, retard, whatever - call that a reference point.
As the engine speeds up, not only is there more air/fuel mixture, the piston speeds are faster. Flame fronts have a set speed (as a function of pressure, I think), so it takes longer for all the fuel to burn. So you want to set the spark off SOONER than at idle - because it'll take time to burn that extra fuel, AND the piston is moving faster, so you need that burning done sooner. If you don't fire sooner, the fuel mixture will still be burning as the piston is well decended, and the exhaust valve starting to open.
So you kick off the spark a longer time before TDC, which is sooner than at idle. I call that advancing the timing - more degrees before TDC.
That physics doesn't change with a turbocharged engine. All that matters is that you have what amounts to a variable compression ratio - as boost comes up, you've got more compression -some from the piston, and some from the turbo.
Under boost, you want to retard the compression, yes. If the plug fires too soon, the flame-front moves out too soon before TDC, and that advancing flame front squeezes the unburnt fuel between it and the rising piston. That causes detonation.
It's possible that I'm screwing something up here - I am doing a few things at once. I'd check it on a car, but my last turbocharged distributor car is gone. I'm going to stick by a vacuum retard. Beyond that, I'm going to bow out of this one with some shred of dignity left.
posted by 192.249....
No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.
![]() |
![]() |
![]() |
![]() |
![]() |