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Originally Posted by Hose92SVX
I'm a little confused as to how a smaller pipe helps torque. I understand that less backpressure is good and I know on newer sportbikes they come with butterfly valves in the exhaust that open and close as the rpm changes. I've always thought the reason smaller pipes help torque is because as the gases reach the rear of the exhaust pipe they start to cool off and slow down and keeping a smaller pipe keeps velocity higher which helps keep the gases from slowing down and creating turbalance in the pipe. Turbalance in the pipe would cause backpressure and that's what you try to avoid. Is that correct or am I still not getting the picture. I know at laundry mats where the dryers exhaust pipe are real long they purposely keep the diameter small so that the velocity of the air keeps the lint airborne. Is this the same prinicple with car exhaust? Thanks for any info that clears this up for me. Ya'll might have answered this above but to be honest you guys are talking above my head.
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I stress, put in simple basic terms, where an engine has a fixed amount of valve overlap, in the low/medium RPM range, a portion of the charge entering via the inlet, will be lost via the exhaust, while both inlet and exhaust valves are open simultaneously for a period. Therefore some restriction/back pressure in the exhaust system can reduce this affect, but to the detriment of VE (cylinder filling) at higher RPM. However this downside, is partly offset as the advantage of overlap takes effect in the higher RPM range. Compromise is the name of the game. You can not have your cake and eat it.
