exhaust tuning: flow vs. restriction (was: RE: engine decision's...)

["Scott F. Williams" <sfwilliams@home.com>]

> Sadly, I couldn't disagree more.

Okay, why?

> While the ultimate goal for any performance engine program should be to
> eliminate as much backpressure (restriction) as possible, its been proven
> by more teams of variously different racing organizations that some back
> pressure does help produce more power.

Alright, offer us an example.

> Hell, even in the funny car I drove years ago we made restrictive bends
> in the zoomie headers to help with this.

That's all very well. You may have made the bends to restrict the flow, but
what you were really doing was altering the timing of the exhaust shock wave
to coincide with the exhaust valves opening. By timing the wave to impact
the freshly expelled (and unburnt) fuel/air mixture while the valve is open,
you force the charge back into the cylinder and thusly make more power in
the process.

> Like anything else, finding out exactly how much is restriction is
> ultimate for your application, is the hardest part.

Incorrect. Timing the shockwave was your elusive quest. If it were just a
matter of adding restriction then you would have just plugged the existing
exhaust tubing.

> As an example, my last dyno day showed me that the Eurosport headers I
> put on the 82 Roc did in fact loose power over the dual outlet manifold.
> I felt it "seat of the pants" that it was not as good as before but the
> numbers from the dyno I saw as a result really disappointed me. Large
> diameter tube runners are best suited for high compression motors. (like
> the 11.4-1 CR that's in my 84.)

This makes perfect sense to me and it in no way contradicts the principals
that I have explained.

> If I can find a Pacesetter header (small diameter runners) ill use that
> on the FSP car at a later date. For now, the dual outlet manifold works
> best on my 9.2-1CR motor. In fact, im gonna swap the header in the 82 for
> the dual outlet manifold that's in the 84 real soon.

You'd still make better power all around if you could find a high-flowing
and less restrictive exhaust. Often you won't get one without the other, but
pipe diameter and flow are not necessarily linked. For instance, you could
take a bigger diameter pipe and then ceramic coat it to retain more heat.
This could increase the velocity of the flow but flow a greater volume at
the same time. Less restriction plus more flow equals more power.

> Another fine example was at the Pro Solo event at Atwater. On my last run
> after I launched from the start, the exhaust system pulled apart behind
> the header at the slip joint. It was REALLY loud, sounded really bitchen
> but I could feel that the torque it had wasn't there anymore.

You upset the acoustic tuning and/or the scavenging effect. The restriction
in the exhaust may have been lowered (good!) but the positive effect was
offset by the aformentioned negative conditions.

> My time
> showed it as well. No pull off the corners. When my exhaust is on and
> tight, its fairly decent power wise. Any leak at all and its a dog. Some
> restriction is a good thing, always.

NOPE! :^) Again, a certain amount of restriction is a byproduct or function
of exhaust tuning. However, it is a negative quality that should be reduced
by all practical measures. Exhaust restriction is *always* the enemy. :^)
Think about it, Shawn. If an engine is a glorified air pump, and exhaust
volume/velocity is inextricably linked to operating efficiency, then how can
a restriction of that said exhaust increase the power? It can not and it
does not.

-not even in sunny San Diego. ;^)
--
Scott F. Williams
NJ Scirocco nut
Golf GTI 16v rallycar
Mazda 323 GTX turbo assault vehicle

Check out our rally team's website!
http://www.usrallyteam.com


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