Back-pressure question
07-12-2006, 10:55 AM
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Ok so first point...stop referring to reducing backpressure as bad thing, it never is it's a bad misnomer created by people who have no idea what they are talking about. If some article refers to backpresure reduction as the cause of power you can effectively discount it because the author has no idea what he is talking about.
Backpressure is resistance to flow, and it's reduction is always a good thing. The greater resistance to flow the more exhaust is left in the cylinder chambers in the exaust stroke, and the less fresh oxygen rich air can enter on the intake stroke reducing overall power. Effectively the lower backpressure helps with cylinder scavanging
The problem with a larger diameter exhaust isn't in the reduction of backpressure(which is good), but along with it you get a reduction in exaust gas velocity(which is bad). Without going into too much detail, exhaust velocity also impacts cylinder scavanging, the faster the velocity the better the scavanging.
So exaust size for NA is a balancing act between reducing backpressure(good) and reducing exhaust velocity(bad). There's always a crossover point where, the benefits from reduced backpressure are outweighed by negatives of reduced exhaust velocity.
As yourself the following question? Has anyone every lost power from replacing a cat with a testpipe?(provided the ECU didn't go into limp mode of course). I would highly doubt it. Why cat reduction is all about backpressure reduction and has no negative impact on exhaust velocity(might raise it some in fact). This is because the cat reduction keeps the same diameter exaust pipe. If lack of backpressure were to cause power loses then one would expect cat reductions to cause significant power loss, but they don't.
Backpressure is resistance to flow, and it's reduction is always a good thing. The greater resistance to flow the more exhaust is left in the cylinder chambers in the exaust stroke, and the less fresh oxygen rich air can enter on the intake stroke reducing overall power. Effectively the lower backpressure helps with cylinder scavanging
The problem with a larger diameter exhaust isn't in the reduction of backpressure(which is good), but along with it you get a reduction in exaust gas velocity(which is bad). Without going into too much detail, exhaust velocity also impacts cylinder scavanging, the faster the velocity the better the scavanging.
So exaust size for NA is a balancing act between reducing backpressure(good) and reducing exhaust velocity(bad). There's always a crossover point where, the benefits from reduced backpressure are outweighed by negatives of reduced exhaust velocity.
As yourself the following question? Has anyone every lost power from replacing a cat with a testpipe?(provided the ECU didn't go into limp mode of course). I would highly doubt it. Why cat reduction is all about backpressure reduction and has no negative impact on exhaust velocity(might raise it some in fact). This is because the cat reduction keeps the same diameter exaust pipe. If lack of backpressure were to cause power loses then one would expect cat reductions to cause significant power loss, but they don't.
Well said.
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