on 3/28/00 12:23 PM, Kelly Cash at [EMAIL PROTECTED] wrote:
> o Performance is better. It's an insulator, so the
> heat stays in your pipes. This increases scavenging.
I've seen this said a few different times on this list, so don't feel like
I'm singling you out, Kelly. I'm just puzzled by this statement.
Could somebody please explain to me how this makes the slightest bit of
sense? Scavenge, as I understand it, is the creation of a slight negative
pressure pulse which helps to empty the cylinder - in effect, negative
backpressure. This is performed in multi-cylinder engines by the action of
the flow of exhaust from one cylinder in effect creating a temporary low
pressure area in an adjacent pipe via a well designed and placed collector.
(I believe it's Bernoulli's principal at work - just like carburetion.)
Losing heat reduces gas volume thereby maximizing negative pressure for all
exhaust outlet pipes. Retaining heat maximizes gas volume, and, for a given
area, pressure. This leads to maximum backpressure, since area is constant
in either case, and thereby REDUCING effective scavenge, not increasing it.
Turbocharged engines, on the other hand, benefit from insulating the pipes
between the cylinder and the turbocharger, for exactly that reason. Exhaust
pressure driving the turbocharger is INCREASED, because the volume lost by
cooling is minimized, and the decreased scavenge and increased backpressure
don't matter because you're force-feeding the cylinder anyway. (On the
intake side, of course, COOLING is a good thing, since density is increased,
and along with it, volumetric efficiency.)
Am I all wet here? Have I missed something. (Wouldn't be the first, or for
that matter the thousandth, time.)
Best,
Sat Tara Khalsa
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