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Long story short; I racked the pipes at a cop while he was doin a traffic stop and I wind up having to get my exhaust toned down. I got a bullet muffler mid mounted, sounds much sexier now. Anyways, I think I may have gained some mpg and I also noticed a gain of low end torque back. Why is it that low end torque is associated with a bit of back pressure?
 

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It isn't. Low end torque is associated with exhaust gas velocity. Your motor will expel the same quantity of exhaust gas per rpm regardless of what size exhaust you have. With smaller pipes, at low rpms the gases move faster, which creates a low pressure that helps suck the gases out of the engine when the exhaust valve opens. If you actually had backpressure in your exhaust system, it would perform like crap. At high rpms with too small a pipe, it is possible for the exhaust gases to build up so that they are under slight pressure, which quickly kills the power curve above that rpm. At low rpms with an oversized exhaust system, the pressure in the exhaust pipe is the same as atmospheric pressure. With a properly sized system, it is lower than atmospheric pressure (AKA under vacuum, though not anywhere near as much as the intake). With a system that is too small, it will be at a higher pressure than the atmosphere.

As for the MPG gain, having the exhaust under vacuum and working efficiently means that the engine doesn't have to work as hard to push the gases out of the cylinder, so that frees up that power to actually move the car, so you can notice a slight mileage increase.
 

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What did u have for a setup before?
 
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