Stock /2.5inch vs 3inch

[Venom55]

I was talking with one of my friend about making him a custom cat-back like i did on my car but he was arguing me that

Stock 2 1/4 vs 2.5inch wont make any difference at all. Taking into consideration both setup has 3inch catless OBX downpipe and straight thru Dynomax Ultraflow muffler

Is it a waste on money going from stock to 2.5 or hes better 3inch like im trying to convince him?

[Jn2]

even 2.5" will make a difference in flow...to be honest, i would go to 3" unless i was going into the 300whp+ range....

[afinley]

Assuming a 2 bar input pressure, and a 7ft system length, the system flows like this:
2.25" - 111m3hr
2.5" - 147m3hr +32%
3" - 237m3hr +113%

and resists like this:
2.25" - .61bar
2.5" - .35bar -42%
3" - .13bar -78%

Of course, that all depends on the muffler, but you get the ballpark idea.

[Venom55]

Understandable, but he was arguing that with a stage Vtune, 3inch downpipe and stock catback with straight thru muffler was enough

SO basically, I should tell him to go 2.5inch all the way back at least, cuz on a 300hp car i dont see the point of having 3inch all the way except for the downpipe.

I've had a low 12 sec@110+ DSM with 3inch downpipe and 2.5 all the way back and it was sufficient, so i guess 2.5 should cut it for him

[sootman73]

Originally Posted by Venom55

Understandable, but he was arguing that with a stage Vtune, 3inch downpipe and stock catback with straight thru muffler was enough

SO basically, I should tell him to go 2.5inch all the way back at least, cuz on a 300hp car i dont see the point of having 3inch all the way except for the downpipe.

I've had a low 12 sec@110+ DSM with 3inch downpipe and 2.5 all the way back and it was sufficient, so i guess 2.5 should cut it for him

Well if you have a 3 inch downpipe why go down to 2.5 in? or if you stick with 2.5" why put a 3" down pipe on? flow is restricted to the smallest orifice in a system. having a 3" downpipe and 2.5" the rest of the length is pointless. you can only claim the performance gains of a 2.5" system then. never go down in size to the back of the vehicle. always go up or stay the same.

[pacotaco]

^no...


on stock turbo @stage 3 there is a 3-4hp difference when running 2.5in vs 3in exhaust the 2.5in being more powerful. just go with a 2.5in unless you decide that you want to go to a larger turbine turbo down the road. also you will decrease size in your tunnel so you will run into less fitment issues and clearance issues.

[Venom55]

Originally Posted by sootman73

Well if you have a 3 inch downpipe why go down to 2.5 in? or if you stick with 2.5" why put a 3" down pipe on? flow is restricted to the smallest orifice in a system. having a 3" downpipe and 2.5" the rest of the length is pointless. you can only claim the performance gains of a 2.5" system then. never go down in size to the back of the vehicle. always go up or stay the same.

OBX downpipe goes from 3 to 2.5 after the flex/fake cat. Alot of downpipe are like that also.

[ERP]

Originally Posted by afinley

Assuming a 2 bar input pressure, and a 7ft system length, the system flows like this:
2.25" - 111m3hr
2.5" - 147m3hr +32%
3" - 237m3hr +113%

and resists like this:
2.25" - .61bar
2.5" - .35bar -42%
3" - .13bar -78%

You've got to be careful with simplified flow calculations, the input pressure and flow velocity will decrease as pipe size increases since the engine can not produce and endlessly increasing mass flow. So you will not see the gains listed. Otherwise, why stop at 3" pipe diameters.

I like 3" downpipes because they allow for a 3" catalytic converter then scaling down to 2.5" after the cat. The cat is a bigger restriction than 2.5" and the gases are cooling down as they travel further from the engine and require less space to maintain the same flow characteristics. It's best to save money by using 2.5" pipe after the cat.