You dont need back pressure. if you do why doy ou think putting headers ON a car frees up more HP instead of takes it away? your pretty much saying you need exhaust fumes to make your car run better. Only way POSSIABLY i can see it as even remotly useful is if your running exstreamly rich and the unburnt fuel gets put back in the chamber. But taht seems like it would even fuck up your air/fuel mixture.
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Driving definitions: Understeer is when you hit the wall with the front of the car and Oversteer is when you hit the wall with the rear of the car. Horsepower is how fast you hit the wall and Torque is how far you take the wall with you.