240zstroked Posted October 9, 2010 Share Posted October 9, 2010 (edited) Ok Ive read some of the post about exhaust size in realation to hp in turbo cars.My question is how big of a exhaust pipe should be used on a na 3.1L stroker motor without loosing power.Understaning backpressure is inportant factor.I want to run oval tubing for ground clearance issues.Is 3inch or 31/2 inches to large for 3.1L stroker or going v8 with these size tubing run better. Edited October 9, 2010 by rickdench Quote Link to comment Share on other sites More sharing options...
johnc Posted October 9, 2010 Share Posted October 9, 2010 Ok Ive read some of the post about exhaust size in realation to hp in turbo cars.My question is how big of a exhaust pipe should be used on a na 3.1L stroker motor without loosing power.Understaning backpressure is inportant factor.I want to run oval tubing for ground clearance issues.Is 3inch or 31/2 inches to large for 3.1L stroker or going v8 with these size tubing run better. No such thing as backpressure. There's just pressure, its neither backward nor forward. Without going into a lengthy explanation (which I've done in other threads on this site) a 3" exhaust is fine on your car if your header primaries are properly sized and the header has good merge collectors. Quote Link to comment Share on other sites More sharing options...
Daemione Posted November 3, 2010 Share Posted November 3, 2010 Something more useful to research would be exhaust flow requirements, and maintaining exhaust velocity. Engine size/displacement is only 1 variable - desired power band & how many cfm your particular intake/head/exhaust manifold can flow are important in exhaust sizing. 2.5" piping can move a LOT of exhaust. I haven't done the math, but unless you're running a 8k+ rpm engine with a race only cam, I'd be amazed if any n/a engine this size would find 3" beneficial. Honda's may be generally despised for whatever reason in the Z community, but check out the tech articles on http://team-integra.net. Lots of good information there. Quote Link to comment Share on other sites More sharing options...
Leon Posted November 3, 2010 Share Posted November 3, 2010 Read the sticky. Quote Link to comment Share on other sites More sharing options...
johnc Posted November 3, 2010 Share Posted November 3, 2010 2.5" piping can move a LOT of exhaust An perfect 2.5" straight pipe can flow very well. A typical 2.5" exhaust system flows more like a 2" straight pipe. Crush bends, poor welds with drop through, muffler shop "Y" pipes, etc. all conspire to reduce the theoretical 2.5" flow to something around a theoretical 2" pipe, or worse. Quote Link to comment Share on other sites More sharing options...
Daemione Posted November 4, 2010 Share Posted November 4, 2010 An perfect 2.5" straight pipe can flow very well. A typical 2.5" exhaust system flows more like a 2" straight pipe. Crush bends, poor welds with drop through, muffler shop "Y" pipes, etc. all conspire to reduce the theoretical 2.5" flow to something around a theoretical 2" pipe, or worse. Yeah, that's true. I think I'd rather have a true 2" exhaust with no restrictions than a "mostly" 2.5" with disruptions & diameter changes. At least then, even if the high end was limited, it'd have consistent exhaust velocity to provide good low rpm power and pumping loss reduction for better mpg. The good news it that most of these cars are old enough to be legally flexible in catalytic converter choice. The best designed header & exhaust system in the world is going to be wasted when it has to pinch down through a 2" circa-1970's cat. Quote Link to comment Share on other sites More sharing options...
Leon Posted November 4, 2010 Share Posted November 4, 2010 Yeah, that's true. I think I'd rather have a true 2" exhaust with no restrictions than a "mostly" 2.5" with disruptions & diameter changes. At least then, even if the high end was limited, it'd have consistent exhaust velocity to provide good low rpm power and pumping loss reduction for better mpg. The good news it that most of these cars are old enough to be legally flexible in catalytic converter choice. The best designed header & exhaust system in the world is going to be wasted when it has to pinch down through a 2" circa-1970's cat. I'd rather have the "mostly" 2.5 inch exhaust because that will give you less pumping losses than a true 2 inch, given it's not completely cobbled up. It's simple fluid dynamics. A 3 inch exhaust pipe won't hurt anything, a 5 inch won't hurt either (except for ground clearance). Once again, read the sticky I linked to earlier. BTW, higher exhaust velocity = higher frictional/pumping losses. Quote Link to comment Share on other sites More sharing options...
piston Posted December 13, 2010 Share Posted December 13, 2010 (edited) Match exhaust flow with how much your air your engine drawing in. Edited December 13, 2010 by piston Quote Link to comment Share on other sites More sharing options...
Leon Posted December 13, 2010 Share Posted December 13, 2010 Match exhaust flow with how much your air your engine drawing in. Ok... Maybe as a baseline, but not a final design. You need to design the collectors for resonance at your design rpm with your valve timing. The pipe must be big enough as to not rob horsepower. You can't just make a blanket statement like that and expect everything to be sized perfectly. Quote Link to comment Share on other sites More sharing options...
johnc Posted December 13, 2010 Share Posted December 13, 2010 Match exhaust flow with how much your air your engine drawing in. FYI... the match is not 1 to 1. Remember, you're expelling combustion gasses (air and fuel) which have increased volume and energy compared to the the air that came in the intake. Quote Link to comment Share on other sites More sharing options...
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