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Re: exhaust wrap

> On Wed, 17 Jun 1998, Jim Adney wrote:
> +
> + In a normally aspirated engine all the power in the exhaust is going
> + to be wasted anyway, so no one really cares HOW you waste it.  Of
> + course wasting it in such a way that it makes the engine overheat
> + would be be counterproductive.
> 	It won't *all* be wasted.  (If it were, why would we even have
> headers?)

To reduce back pressure, and in real headers to tune the power peak 
to a useful RPM.  This has nothing to do with temp.

> Keeping the exhaust gases hot lowers their drag in a tube,
> increasing the velocity at the collector. 

I think you will find this hard to demonstrate at the pressures found 
in an automotive exhaust.  There is an effect in a vacuum (molecular 
flow region, say less than 1 Torr, but this goes away near 
atmospheric pressure.

> Higher velocity at the
> collector increases the scavenging effect, thus drawing more intake
> charge into the cylinders.  Not a whole hell of a lot of effect, but
> if it shaves a tenth of a second from your quarter-mile ET, it's
> potentially worthwhile..

Your last "if" is a big one.  It is true that higher velocities
generally create more scavenging, but if this is done with higher
temps, then the consequent higher pressure will negate your effect. 
Scavenging is actually the result of lowering the pressure at the
exhaust valve, after all.  The higher velocity only helps if
everything else stays the same.  In this case it doesn't and
everything becomes more complicated. 

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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