Yes, old thread but common question
There is a lot of MYTH-conceptions about exhaust systems, lol
Scavenging exhaust systems have been around since late 1950's in production vehicles, earlier in racing vehicles
Scavenging exhaust system is a way to get extra power using an engine's exhaust flow, in a specific RPM band, and its free, just needs to be designed correctly
In a multi-cylinder engine, like a V6, there are 3 exhaust ports/pipes that flow into one larger pipe, the collector
In physics when you have flow from a smaller pipe into a larger pipe you will have a pressure drop at the transition to the larger pipe
How much of a pressure drop is decided by the Velocity in the smaller pipe and the size of the larger pipe
The pressure drop is the whole point of scavenging exhaust systems, in the case of a V6 in a static example, the pressure drop extends back on the other 2 smaller pipes, so when their exhaust valve opens exhaust is PULLED out of that cylinder leaving extra power on the crank because it doesn't have to PUSH as hard to get the exhaust out
Free scavenged power
Velocity is created by the size of the smaller "header" pipe and the displacement of the cylinder(engine size), and the size of the larger "collector" pipe
This is beyond my pay grade, lol, but the designs are out there for engine size and RPM band
Most vehicle makers use mid-band scavenging exhaust manifolds, 2,500rpm or so
Off the shelf "headers" can lower that to 2,000rpm or so, i.e. off the line low end power
"Racing headers" use 3,000rpm or so, if not higher
Its all in the design
The MTYH of "back pressure", no 4-stroke engine runs better with exhaust back pressure
The MYTH came from people putting on larger diameter "header" pipes to get "free flow" exhaust
Then going for a drive and "WTF!!!!!, I lost power????"
They lost the power from the factory scavenging exhaust system manifold, but not knowing that they surmise "this engine must need back pressure", they are wrong of course but the MYTH persists
Working Cat converters and mufflers have no effect on engine power, racing vehicles don't use them because of weight and cost, lol
"Cat back" exhaust can go bigger and will give you a lower exhaust tone but won't effect engine power one way or another, just sounds better if you like the lower tone
Straight pipe from "header" back is the same, larger pipe lower tone, no effect on power
Dual exhaust can give you the opportunity to add to the scavenged power from the "headers", "H" or X" pipes are added between the Dual exhaust pipes, since exhaust pulses in each pipe are opposite it will drop the pressure in each pipe, not much but not 0 either
Using the correct "header" is what gives you extra free power in the RPM band you want
What about dragsters or racing engines with just one pipe for each exhaust port exiting out into the air
Look at their diameter and length, this is also calculated to get some extra scavenged power at a specific RPM band
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