I am having a hard time understanding how Backpressure is directly related to Torque and HP. Torque means , but on some "performance" exhausts, I realized that Hp was increased drastically but Torque decreased. Is there any advantage to this?

 

My vote is that there is no such thing as backpressure. Merely exhaust gas velocity. Bigger pipe doesn't make you lose torque because of lack of pressure, it makes the exhaust pulses slow down and hit each other.

 

HP is derived from torque. You don't have more of one and less of the other at a specific spot...

what you usually see when people use larger exhaust piping, is that the larger piping is designed to be optimal at a higher rpm range than smaller piping, thus more torque is produced at a higher RPM... which translates into more HP (since HP = Torque*RPM/(5252) ). However, the offset is that you could lose torque/HP at a lower spot in the RPM range by using larger piping which isnt optimal for lower rpm use vs a smaller diameter piping.

 

The torque band is just getting moved around, when you start playing with exhaust size...

 

back pressure in ANY case is bad.. bad bad bad. puts strain on your motor and causes it to exit its exhaust gasses LESS effeciently. proper FLOW is more important. exhausts are suposed to be like vacumes, low pressure at the end which will physically suck gasses out instead of having the motor push them all the way out. larger diameter exhaust = less vacume effect b/c pressures at header and and end of exhaust become more and more equil. a 5" exhaust on a NA 1.6L is just as bad as a 12' exhaust on the same motor b/c both pressures will be basicly the same.