Quote:
Originally Posted by Vettezuki
This I understand.
BRUTAL,
It seems back pressure is non desirable in any case. The big diameter exhaust problem typical on small import engines (when guys just put on big exhaust) is not because of reduced back pressure, but because of reduced exhaust velocity. It think you said more or less the same thing referring to scavenging.
TC,
If the intersection of low BP (close to zero as possible) and high exhuast velocity in typical operating range is most desirable, then why do most OEM engines have that low amount of BP at cruise/off idle? I guess that's just the most optimum position they can engineer for the total system, but it has nothing to do with "utilizing" back pressure.
My guess that back pressure in small amounts may be desirable because it may increase cylinder pressure and therefore torque seems to be wrong. There is never a benefit to back pressure (talking just about the efficiency of a NA engine). BP is just a consequence of other design elements in the system, it is not intentionally designed into the system.
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Scavenging is the adjustable part of the issue. You want no Back Pressure and you use scavenging to pull the exhaust gas out of the cylinder. The amount of scavenging is adjusted from a little to a lot depending on the flow of the intake charge and exhaust port. You adjust the scavenging ( like adding a 30 inch pipe to lenghten the collector to give more mid range torque) to fit the engine.
I don't like the term Back Pressure.
Throttle Crazy--"Actually "resistance to flow" would be considered the cause and back pressure would be the result"---This is as close to correct as we are going to get.
You want no "Back Pressure".