2.5 or 3" exhaust

73' mach 1 · Mar 1, 2021

Wondering what others are using here. The motor is a 408c using FPA headers which have a 3" collector. The motor is EFI and around 500ish FWHP in theory. Cam specs are as follows.

Camshaft is a hydraulic roller and the specs are adv dur 286/294 @.050 224/232 with a lift of .586/.609

I currently have 2.5" pypes street pro mufflers which I can either just replace with 3" or use reducers.

The real question is what to expect going one way or the other? Would 3" be too much and not sound right? Should I just reduce at the collector and go 2.5" the rest of the way?

Going to be using an H pipe also.

turtle5353 · Mar 1, 2021

I am running 2.5" stainless on my 408C with about 600hp. I talked to my exhaust shop about 3" and he said it would sound a little deeper but on the street you're not going to see any noticeable gain in power. He also said it very difficult to make the tight bends around the rearend with 3" I am very happy with the sound of mine and I am using an X pipe. If you really want some nice sound install a set of electric cutouts just behind the headers. It really fun on cruise nights and shows rolling up with open headers.

tony-muscle · Mar 1, 2021

It will make a noticeable effect only if you have outgrown the flow potential of the dual 2.5". This is a good write up about all the theory:

https://www.musclecardiy.com/performance/exhaust-system-performance-math/

Based on this information 2.5" may be borderline for 500 hp, but I think you will only notice the difference at the top end. Below and at peak torque shouldn't see much difference. One thing is theory and the other is practice. As Kevin says, 3" exhaust would be challenging in our cars, specially going over the axle and can't even think if you had staggered shocks. Side exhaust may work well.

I have exhaust cutoffs. One day I want to dyno with and without the cutoffs open to understand if there is a difference.

c9zx · Mar 1, 2021

I took generic "Mustang" 3 inch Flowmaster exhaust pipes and Summit generic H pipe to the best custom exhaust shop around. It was challenging and took 4+ hours for him to do. The staggered shocks added additional difficulty. The outcome was good but not cheap. The power numbers for the engine are 529 HP/496 TQ. Chuck

Kilgon · Mar 1, 2021

I have a 460 and am running 3" all the way back. Have H pipe and am using MagnaFlow . One word to sum it up - loud - but sounds good. As mentioned above I would be surprise if you could tell any noticeable difference between the 2.5 and 3" unless you were racing or had it on a dyno. I have asportsroof but the bends over the axle are still a pain. The other negative part of going 3" is it is all custom and will likely need to be done at a shop unless you buy enough parts to piece it together.