there are three (more too) different effects that an efficient exhaust has, but a big one being low to no backpressure to restrict exhaust flow (more air+fuel = more power) and allow unrestricted escape of each cylinders exhaust gases.
But the most important things you can tune in any exhaust system is each cylinders scavenging effect (where by a hot gass flowing at a high velocity through a small tuned ex pipe will actually help draw in the intake air/fuel (cams valve lobe overlap) because as the high momentum of the gasses will draw a vacum in the cylinder) a big part of this is keeping the exhaust gas heat in the exhaust gases-hence wrapping your exhaust to prevent this)
The other part of the scavenging problem is fighting reversion (much less common on possitive displacement blowers (intake manifold pressurized)) where the exhaust gases (whether by too large a primary exhaust tube size for cylinder displacment, or wrong rpm/powerband tuning) will have too low of a velocity to scavenge the cylinder properly and will actually stop flow and start to get sucked back into the combustion chamber hindering the combustion process (try driving at WOT with your EGR open and see how much it hurts perf)
Reversion tends to happen at lower engine RPM'* (some manufacturers actually tuned this effect for various EGR'less systems) but can be helped by using a "stepped" primary tube design (small prim tube goes to a larger primary tube with a "step" that helps keep the exhaust from commin' back) and tunning it to the RPM/powerband of the engine (the distance from the cylinder port the "step" is).
scavenging is a little more complicated depending how you join the primary tubs to the header collector (per the firing order etc) and their length you can gain efficiency with the scavenging effect or loose it completely.
and since a good scavenging design at it'* tuned power range can give much better VE in the engine (it'* like a free 1-2psi of boost!) for good demonstration some of the better bracket racers/sportsman racers have gotten an extra 20-30Hp (in the tuned powerband) outta a high performance V8 JUST switching to a good set of scavenging headers (doug thorley etc)
allota physics/ research is used to go really fast when ya got a full time racing buisness (like Jegs/Summit/Heddman etc)
hope this gives ya some more fer this myth
BonneMeMN Posted: Fri Jan 28, 2005 12:08 am Post subject:
"I saw something once where it was a muffler deal that claimed to speed up the air from it'* shape. Maybe like a venturi, but it was complete BS. "
however there are several venturi type exhaust collectors (where the primaries go into a common collector pipe) where (depending on tuning) there is a tunable/replacable venturi inserted into a normal collector to help the scavenging effect (or the venturi is part of the collectors body design) this has shown on some V8'* to be an easy 5-10Hp is certain area'* of the powerband
as for a venturi muffler...eh it'* probably a restriction and doesnt do much as the exhaust gases have cooled and slowed down by the time it gets there.
PSYCHOTIC GEARHEADS paying homage to a legend, Hairy Hurst Olds
The first twin V6 94 Oldsmobile 442 462ci-4wd-2 (twin) engines
13.81 @ over 98 MPH! consistent 1.90s 60'! 4330 lbs race weight
something Sinister is coming to Chitown....