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Cat-back 2.5" versus 3.0" tubing

rf_ross

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So in the old days, the thought process was to use big diameter tubes for all-out top-end horsepower and to use smaller diameter tubes for improved exhaust flow and more low-end grunt. Is that still true today?

I'm asking this question because I'm building my '18 GT as a touring car versus a track, autocross or drag car. It's going to be street driven 99.9% of the time but I may drag race it a couple of times just to see what it runs.

I'm down to three different cat-back exhausts: 1) Ford Performance Touring with 2.5" diameter tubes; 2) Krona with 3.0" tubes and; 3) the Thermal with 3.0" tubes. So my old school thinking tells me that I'd be happier from a primarily street performance perspective with the Ford Performance exhaust than the others but with so many choices I'm just not sure.

Does anyone have any thoughts or data on this topic?
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Jay-rod427

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Unless making monster F/I power 2.5" is more than ample from cat back concerns. The restriction is in the cat section. The cats themselves, and the neckdown. Unless eliminating those the cat back portion is irrelevant.
 

BmacIL

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Unless making monster F/I power 2.5" is more than ample from cat back concerns. The restriction is in the cat section. The cats themselves, and the neckdown. Unless eliminating those the cat back portion is irrelevant.
This.

The only other thing that changes is that 3" will tend to have a bit deeper & louder exhaust note, all else being the same, although it's not that much.
 

Kahboom

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3" would be less restrictive and be much more ideal for nitrous or boost, even an all out motor build. Consider what your future goals are with the car.
 

Jayme

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i plan on revving mine to 8K+ with an all motor build. im building a new exhaust system soon with a short x pipe and 3" race mufflers with turndowns. In my thinking im not losing anything by going 3" at that RPM.
 

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rf_ross

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From a strictly power perspective, my current line of thinking is adding an exhaust, cold air intake, and a tune and that's really it given what I want the car to be. Now I suppose I could go crazy down the road and add a supercharger but at this point, I'm not thinking that far ahead.

Thanks for everyone's input so far.
 

texasboy21

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You wont have any noticeable difference in power or powerband with a 2.5" or 3" exhaust, so why not go with a 3"? More flow and will work better should you decide to go with additional flow mods in the future.
 

yote41

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3" is def. Louder and more volume in sound. I think it even sounds like its breathing easier
 

SVT-DADDY

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As Jay Rod said the front part of the exhaust is 2.5....even 2.25 in some places. Ultimately that's going to be your restriction. I don't think you'll see any difference in power between a 2.5 and 3" system, like others have said its mostly sound.

I have a 3" cat back and kinda wish I had a 2.5 now that I am taking the cats off the car. 2.5 is just so much easier to work with and so many more options.

In short get the one that sounds best to you.
 

Grintch

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You wont have any noticeable difference in power or powerband with a 2.5" or 3" exhaust, so why not go with a 3"? More flow and will work better should you decide to go with additional flow mods in the future.
So if there is no noticible difference between a 2.5 and 3" exhaust (which I agree with), why recommend a 3"?

It doesn't matter, just like the 4, 5, or 6" tips don't matter. Buy based on quality, sound, and price.
 

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texasboy21

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So if there is no noticible difference between a 2.5 and 3" exhaust (which I agree with), why recommend a 3"?
Per my post, "3" will work better should you decide to go with additional flow mods in the future." :D
 

SVT-DADDY

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If I had to do it over again:

NA- I'd weld in a set of GT500 mufflers and do a resonator delete.

FI-Cat delete, magnaflow TruX resonator, GT500 mufflers.

Instead I am 2k deep on a 57% custom, all 3" Magnaflow/MBRP, mash up system. lol
 
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rf_ross

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If I had to do it over again:

NA- I'd weld in a set of GT500 mufflers and do a resonator delete.

FI-Cat delete, magnaflow TruX resonator, GT500 mufflers.

Instead I am 2k deep on a 57% custom, all 3" Magnaflow/MBRP, mash up system. lol
Yeah, project dollar/scope creep. I had been contemplating widening my rear factory foundry wheels then powdercoating all four and throwing on some new tires. But when I started pricing that project out I realized that I could purchase brand-new, better fitting and lighter wheels for less money than I was going to sink into the wheel widening project. So it is easy to go down a path and get so deep you can't get out!
 

SVT-DADDY

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Yeah, project dollar/scope creep. I had been contemplating widening my rear factory foundry wheels then powdercoating all four and throwing on some new tires. But when I started pricing that project out I realized that I could purchase brand-new, better fitting and lighter wheels for less money than I was going to sink into the wheel widening project. So it is easy to go down a path and get so deep you can't get out!
Thats exactly how I ended up with my CF5V's...started with foundry's
 

Rick#7

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FWIW I remember the pushrod 5.0 saw gains across the board with 3" pipes vs 2.5" once the whp numbers topped 300, frankly I'm amazed the coyote makes the power it does with such small pipe diameter!

In any case, as others have already said the tubing size of the cat-back portion is secondary to the cats themselves and their crimped pipes as far as hp goes. Personally I think there's enough restriction in the factory resonator and mufflers to justify replacing those with aftermarket, but even 2.5" pipe size aftermarket mufflers will outflow the OEM cat section of the exhaust, so unless you plan to replace the whole thing just get whatever system sounds best to you.
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