I guess Ill throw my 1½ cents in here (I need a short break from working in the garage anyways ). Otay, heres my limited-knowledge take on exhaust sizing...a fast-flowing, but unrestrictive exhaust is ideal. This may sound like a contradiction in terms, since larger pipe is less restrictive but flows slower, while smaller pipe is faster moving but restrictive. Its my opinion that most peoples exhaust pipes on their cars (street use) are way oversized, with the reason being that they watch too many shows or read too many magazines that tell them bigger is better, cuz after all, its less restrictive and can flow more than smaller pipes. For the dragstrip this is probably true, but for a car that has a street engine and sees street use, I would take an undersized pipe over an oversized one any day of the week. Simply put, too large of a pipe certainly can cause an engine to run less efficiently because of the slow-moving gases travelling through the pipe. The best analogy I can think of right now is that its similar to trying to siphon fuel from your tank...using a hose thats maybe 1/4-1/2" in diameter works far better than using one thats 1-2" in diameter. Why? It just does! lmao Just kidding...its because the larger hose is too [censored] big and the fuel would not travel fast enough to create the vacuum needed to maintain a strong flow. Another misconception, in my humble opinion, is that the terms "back pressure" and "restriction" are interchangeable. Sure, they are related but mean two different things. To me, a restriction is a flaw that occurs, whether it be by poor design, an operational malfunction, contaminant build-up, etc, and can CAUSE back pressure, and back pressure is simply the resistance of the gases to further move along the pipe, if that makes any sense (I know what I mean but am not very good at putting it into words, remember? lol). And heres where, for the first time ever, Ill hafta repectfully disagree with my buddy TOHcan...while back pressure IS usually undesirable, a certain amount of back pressure is beneficial sometimes, especially on street rides (and probably never on a strip car), and is another part of my reasoning of why oversized pipe is worse for an engine than an undersized one. Well, back to work for me...this concludes my 1½ cents worth on exhaust sizing
). Otay, heres my limited-knowledge take on exhaust sizing...a fast-flowing, but unrestrictive exhaust is ideal. This may sound like a contradiction in terms, since larger pipe is less restrictive but flows slower, while smaller pipe is faster moving but restrictive. Its my opinion that most peoples exhaust pipes on their cars (street use) are way oversized, with the reason being that they watch too many shows or read too many magazines that tell them bigger is better, cuz after all, its less restrictive and can flow more than smaller pipes. For the dragstrip this is probably true, but for a car that has a street engine and sees street use, I would take an undersized pipe over an oversized one any day of the week. Simply put, too large of a pipe certainly can cause an engine to run less efficiently because of the slow-moving gases travelling through the pipe. The best analogy I can think of right now is that its similar to trying to siphon fuel from your tank...using a hose thats maybe 1/4-1/2" in diameter works far better than using one thats 1-2" in diameter. Why? It just does! lmao Just kidding...its because the larger hose is too [censored] big and the fuel would not travel fast enough to create the vacuum needed to maintain a strong flow. Another misconception, in my humble opinion, is that the terms "back pressure" and "restriction" are interchangeable. Sure, they are related but mean two different things. To me, a restriction is a flaw that occurs, whether it be by poor design, an operational malfunction, contaminant build-up, etc, and can CAUSE back pressure, and back pressure is simply the resistance of the gases to further move along the pipe, if that makes any sense (I know what I mean but am not very good at putting it into words, remember? lol). And heres where, for the first time ever, Ill hafta repectfully disagree with my buddy TOHcan...while back pressure IS usually undesirable, a certain amount of back pressure is beneficial sometimes, especially on street rides (and probably never on a strip car), and is another part of my reasoning of why oversized pipe is worse for an engine than an undersized one. Well, back to work for me...this concludes my 1½ cents worth on exhaust sizing 
