Exhaust Tubing: 2.5" or 3.0" for a N/A car

[Jon_Purdy]

Hello everyone. I'm looking at getting a better mid-pipe for my N/A car. Which would be better for me, 2.5" or 3.0" tubing? I believe the Magnaflow is 3.0", the Bassani and Kooks are both 2.5". I would assume that a larger diameter tube would be better if I had a S/C or am I wrong? Also, with the addition of high-flow cats, will I need to get a custom tune?

Regards,
Jon

 

[Royb]

2.5 is stock and will be fine for n/a. it's hit or miss on the tune with just a cat change, I'd try it and if it runs bad then go for a dyno tune.

 

[Bingo]

Anyone know at what power level 2.5" starts becoming a restriction?

 

[TexStang]

+1...

 

[ChevyKiller]

'restriction' is subjective word IMO. I don't think the 2.5 'becomes' a restriction at any specific power point. I think a more accurate question would be at what point would you 'need' more flow and on that I'm not sure for N/A if you would ever 'need' to go bigger.

 

[Bingo]

'restriction' is subjective word IMO. I don't think the 2.5 'becomes' a restriction at any specific power point. I think a more accurate question would be at what point would you 'need' more flow and on that I'm not sure for N/A if you would ever 'need' to go bigger.

It's like when a turbo is too small for the boost being made - it still makes more power as you increase the boost, but for each PSI increase, the marginal power gain decreases. That's what I mean by "restriction". At what point, with 2.5" piping, do the marginal gains start rapidly decreasing? The power level should, I imagine, be quite a bit higher than any point attainable without the aid of forced induction or nitrous.

 

[ChevyKiller]

I understand but my thinking is that with the amount of cubes we are talking about with the 3V in n/a form, I don't think you could reach a power level where the 2.5 would be restrictive. I could be wrong, but I would think it would be surpassing the 400 hp mark.

 

[TexStang]

what about FI, where does it start becoming the restriction? Kind of like the throttle body isnt really a restriction around 450rwhp, but as you up the boost it becomes a bigger and bigger restriction

 

[Bingo]

I understand but my thinking is that with the amount of cubes we are talking about with the 3V in n/a form, I don't think you could reach a power level where the 2.5 would be restrictive. I could be wrong, but I would think it would be surpassing the 400 hp mark.

Right, but I was just curious as to what it would take - I figured it would take more power than an n/a engine could offer.

what about FI, where does it start becoming the restriction? Kind of like the throttle body isnt really a restriction around 450rwhp, but as you up the boost it becomes a bigger and bigger restriction

Exactly.

 

[ChevyKiller]

Right, but I was just curious as to what it would take - I figured it would take more power than an n/a engine could offer.

Exactly.

Gotcha. Like I said, I think it would be easily surpassing 400 and maybe even the 550 ballpark. Does the GT500 5.4 have a 2.5" tube?

 

[Bingo]

Gotcha. Like I said, I think it would be easily surpassing 400 and maybe even the 550 ballpark. Does the GT500 5.4 have a 2.5" tube?

Pretty sure the exhaust is the same on the GT500 and the GT. I've seen that thread on ModularFords where TurboHorsepower put out 700HP+ with their twin-turbo setup...I asked if the exhaust was stock diameter, but never got an answer. Since they didn't mention otherwise, my guess is they left it stock. I'm sure 2.5" is good well past 1000HP, but I just wonder if a car, at 750HP+ would benefit with 3" pipes.

 

[marcspaz]

The 5.4L GT500 has 1 3/4 header primary tubes down to a 3 in cat and h-pipe, then reduces to a 2.5 in cat back exhaust...

Most of the major exhaust part companies make a 3 in cat back exhaust to make the complete system 3 in...I have seen claims of 30 HP gain on the GT500 with a 3 in cat back system. I would expect to get 15 to 20 in the "real" world.

 

[n0nleft13]

I understand but my thinking is that with the amount of cubes we are talking about with the 3V in n/a form, I don't think you could reach a power level where the 2.5 would be restrictive. I could be wrong, but I would think it would be surpassing the 400 hp mark.

Last year when I was asking Jim Jr about a kooks header system up he recommended 2.5" for my setup, their Stage 2 Saleen setup. I can't recall at what levels he recommended going up to 3". I think it was around 500 rwhp but I'm not sure, right now I'm at 466. Due to my budget the headers had to sit on the back burner so I won't be able to do the install for a while.

 

[marcspaz]

If you want to race and want max HP, just get cut-outs for the track and use the 2.5 in exhaust. When you are on the street, it doesn't make enough diff at 500 HP to go from 2.5 to 3.

The best way to judge is based on having the correct back-pressure for your specific system. Once you get to max airflow based on your engine without valve slap, you're done. You can't really say some gen number as a cutoff point.