I still maintain that the result of more back pressure is loss of power at any rpm.

The effects of different sized headers determines the rpm where the highest torque is achieved.

The headers and cats determine the rpms at which the scavenging is maximized.

A good scavenging clears the cylinders for a fresh charge. The only way back pressure would help scavenging is if it had positive results in improving the velocity at which the exhaust gases flowed.

I not very smart and I am asking that someone explain this to me.

 

I'm not saying that backpressrure is good for torque, What I am saying is that a decrease in gas velocity in the low rpm range is..


You wont be able to get good scavenging througout the rpm range... The best you can do is to tune your exhaust to scavenge at the rpm range you spend most of the time in.

 

Although, I realize that back pressure, per se does have an effect of torque at different levels of rpm, I don't believe that you can carry on a complete discussion without taking into consideration the type of cam you are running, including the roll that overlap has on torque.