ALB- thank you for your input. Basically, what I'm reading is exhaust must free flow unrestricted allowing the chamber to ready for new air/fuel mixture stroke. The best way is to remove the exhaust pipe, but that would be really noisy. So, the pipe size, length and muffler structure/baffling are very important. And each of these depends on the motor size for the chamber gas output must at best exit with least amount of head pressure (obstructions). Thanx again.
Engine size dictates how much air is being pumped through the exhaust. A small engine produces less volume than a large engine therefore you need a smaller exhaust on a smaller engine.
Exhaust is a compromise between large pipes for flow volume (HP) and small pipes for flow velocity (torque).
Scavenging is when you have exhaust pipes merged together such that as one exhaust pulse enters the merge it creates a vacuum pulse on the adjacent pipe that travels back up to another cylinder as the exhaust valve is open and helps to pull the exhaust gasses out of that cylinder. This suction clears out the gasses in the cylinder better than the gasses expanding out the exhaust port plus the piston pushing it out can do on their own. Less exhaust gas means more room for the next fuel/air charge, that's why a merged header gets better output from the engine than the stock non-merged pea-shooter muffler or the sebring-style. Higher exhaust velocity from a smaller pipe creates a stronger vacuum pulse than low velocity exhaust in a bigger pipe, hence why smaller pipes provide better torque. The compromise comes from the fact that a small pipe simply can't flow enough volume the engine needs at higher RPM's, restricting the HP.
When people swap to a big diameter exhaust and notice the loss of torque, they say the solution is to add more "backpressure" in the exhaust. The actual problem is their exhaust diameter is too big so they've lost the velocity needed for good low rpm scavenging and torque. Backpressure is just an unwanted side effect of the real solution to the issue.
Pipe length controls the RPM at which the exhaust most benefits the engine, long pipes for low rpm, short pipes for high rpm. The reason is exhaust pulses take time to travel down the pipe and it takes the same time for the vacuum pulse to travel back up the pipe. However, this time is only dependent on the length of pipe and the speed of sound which is why the maximum torque benefit (scavenging) only happens at a specific RPM (and weaker effects at multiples of that RPM due to resonance). If you have equal length pipes going into a merge, they will all hit at the same RPM so you will have a very strong but narrow torque peak there. Unequal length pipes will hit at different RPMs so the peak will be lower and spread out. Not necessarily a bad thing as wider but lower torque curve can be nicer to drive. Due to the design of these engines, choices in pipe length can be rather limited, so in most cases, it's packaging that dictates this, not desired performance.
Muffler design varies the way the end of the pipe "looks" to the exhaust pulses. It could look like a restriction, open air, or just more pipe. The first is bad, the other two are just more ways to set the total pipe length and thus tune the exhaust. Ever see a car with what look like a second set of mufflers halfway between the engine and muffler? Those are resonator boxes. It's just an empty box that makes the exhaust act like the pipe just ended at open air at that point, effectively tuning the length of the pipe to be much shorter than it physically is.
If I can help you I most certainly will. As to whether it's here or a pm, whatever you're more comfortable with.
