Is 2.25" Exhaust Suffecient

[Bullhead]

I have 2.25" exhaust tubing that's in good shape but I was considering 2.5". By my research and calculations 2.25" is plenty unless over going over 200hp (approximate). Let me know what you guys are running and if back pressure is in good standing where your at. I am running 1 cat into the muffler.

[MaxBell]

I'm running 2.5" in my M50 build, but it's built up a bit.

You need to be really careful about oversizing your exhaust. The stock M20 came with 1 5/8", which flowed plenty well for their applications.

[Bullhead]

I'm running 2.5" in my M50 build, but it's built up a bit.

You need to be really careful about oversizing your exhaust. The stock M20 came with 1 5/8", which flowed plenty well for their applications.

I completely agree, I see too many putting huge exhaust on cars that don't produce the HP numbers to support it. Backpressure in the correct amount is a good thing. If you don't have the correct amount of backpressure your pistons don't get the push of resistance against the header they need to travel back around. I have told some to look at it this way, if you were to push someone while standing flat footed with nothing at your back it is hard to push them away, but if you were standing with your back to a wall then pushing them is easier. If the combustion event happens and there is low backpressure per flow and hp numbers, the piston doesn't get that extra push after tdc to rotate back into the next event. You can also use how a rifle works, the shorter the barrel the less velocity with the same charge and ammunition, but the less it kicks as a result of decreased back pressure. It is especially bad with the diesel truck crowd, I tuned on an older 96 12 valve and was running 4" exhaust and making around 475hp and 850fp of torque. I had friends making no where near that running big 5" exhaust and it was totally unnecessary. I totally understand the theory and use of backpressure I was just trying to see where you guys were regarding both, per my numbers 2 .25" flows plenty imo but I am no expert on these cars so I was just double checking.

[digger]

I completely agree, I see too many putting huge exhaust on cars that don't produce the HP numbers to support it. Backpressure in the correct amount is a good thing. If you don't have the correct amount of backpressure your pistons don't get the push of resistance against the header they need to travel back around. I have told some to look at it this way, if you were to push someone while standing flat footed with nothing at your back it is hard to push them away, but if you were standing with your back to a wall then pushing them is easier. If the combustion event happens and there is low backpressure per flow and hp numbers, the piston doesn't get that extra push after tdc to rotate back into the next event. You can also use how a rifle works, the shorter the barrel the less velocity with the same charge and ammunition, but the less it kicks as a result of decreased back pressure. It is especially bad with the diesel truck crowd, I tuned on an older 96 12 valve and was running 4" exhaust and making around 475hp and 850fp of torque. I had friends making no where near that running big 5" exhaust and it was totally unnecessary. I totally understand the theory and use of backpressure I was just trying to see where you guys were regarding both, per my numbers 2 .25" flows plenty imo but I am no expert on these cars so I was just double checking.

sounds like a complete load of BS

however a single 2.25" is enough for basically stock but personally i cant see a 2.5" hurting anything and it gives you room to move

i always thought stock was dual 1-7/8" or 1-3/4" but its been a while since ive picked one up

[Bullhead]

sounds like a complete load of BS

however a single 2.25" is enough for basically stock but personally i cant see a 2.5" hurting anything and it gives you room to move

i always thought stock was dual 1-7/8" or 1-3/4" but its been a while since ive picked one up

Roger that on the info of the stock exhaust.
My examples may have not been sufficient but they are based from years of experience on exhaust and I am not counting my own. I should have inserted velocity instead of back pressure I guess. Back pressure itself isn't the foundation of the principle but it is incorporated into the equation to achieve the best velocity vs. volume. Let's say you were to run 1 3/4 primary headers on your stock 2.7 and then had a cut out that ran wide open. Sure you would have no backpressure but your exhaust pulses could possibly suffer without the correct path to have the maximum scavenging effect achieved through velocity. I am not the best at describing things in letter form. Let me see if I can round something up where someone can essay better than me (which isn't hard to do lol)

[Bullhead]

Here is a good article, and yes I didn't tell what I was trying to get across the right way. So all in all I guess I blew it but oh well. I have to learn to write better lol. I was trying to state that No backpressure can be bad for your power band and hp numbers I just didn't get it out right.

http://www.gomog.com/allmorgan/exhaustbackpressure.html

Here's another decent article: http://www.nsxprime.com/nsx-faq/exhaust-theory/

I remember that when I went to a 2" primary on my Camaro I actually lost .5 in the 1/8th versus the 1 3/4 Supercomps. And I could feel that my torque down low had fell off too. I knew this would happen but hoped the hp numbers on the top end would compensate and pick up the slack but it did not.

[digger]

the first two sentences in the link contradict what you said so to say you didn't get it across the right way is an understatement lol

primary header pipe, collectors etc are very different things to the tailpipe.

[Bullhead]

the first two sentences in the link contradict what you said so to say you didn't get it across the right way is an understatement lol

primary header pipe, collectors etc are very different things to the tailpipe.

Yeah I suck at explanations, I was just trying to contribute, but I did find what I was trying to say to help clear things up so I didn't look like a total jackazz. At least my research skills are decent lmbo.

[Bullhead]

I was trying to say that if velocity was correct you usually have a small amount of BP and that helped the pistons and compression stroke etc.. I will get this forum stuff down. If I would have just said in my native tongue:
If the dang holes too big that crap will flow but it won't flow fast and explosions inside the engine won't be fast either. So this will result in some slow sheite.

[digger]

exhaust back pressure doesn't help the pistons go down on the compression stroke

[nando]

single 2.25" is slightly smaller effectively than the stock exhaust.

backpressure is not what you want. velocity is - and basically past the merge, it doesn't matter what size the exhaust is. it can only be too small (or too heavy), not too large.

[Bullhead]

single 2.25" is slightly smaller effectively than the stock exhaust.

backpressure is not what you want. velocity is - and basically past the merge, it doesn't matter what size the exhaust is. it can only be too small (or too heavy), not too large.

I have an E car Nando, it came with a single from the factory right? I may just snatch me up a factory 325i exhaust next time I'm at the jy.

[MR 325]

For an Eta I'd say single 2.25" is perfect. For an i car or 2.7i hybrid I'd go single 2.5".

[Bullhead]

For an Eta I'd say single 2.25" is perfect. For an i car or 2.7i hybrid I'd go single 2.5".

Thanks, I am prob going to do the 2.7i deal so I may just go ahead and buy some 2.5" pipe.