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exhaust pipe size?


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A 'small' pipe will keep the exhaust flowing at a high speed. A high speed will build up the exhaust inertia and the fast flowing gasses ahead in the pipe will pull the remaining gasses out of the cylinder saving some pumping action by the piston on the upward exhaust stroke. The problem is that as more and more exhaust is added it becomes a bottle neck and restricts flow. A larger pipe will help at higher RPM but now it's too big for lower speeds and exhaust speed drops and savings are lost and you loose some power.


So if you want to trade a small increase in high end power for a small loss in low speed torque you can increase the diameter 20-30%. A turbo motor would be an exception as it produces huge amounts of exhaust and needs a much bigger pipe. Now to increase the flow by say 30% you do not increase the pipe size by that amount. If you remember high school math you will also remember that doubling a pipe's diameter will increase the cross sectional area by 4 times not 2. For example if your stock exhaust pipe is 1 3/4" the cross sectional area is 2.4 sq. in. A 30% increase in area is 3.125 sq. in. To increase by 30% you would need to only increase the pipe diameter by 1/4". A 2" pipe has the cross sectional area of 3.14 sq.in. Almost perfect for what you want.

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