2.5" vs 3" Exhaust and Muffler Options

[CV355]

Our Mach 1 is going for startup next week, but has to hit the exhaust shop first.  I'm debating between 2.5" and 3".  I am also looking at Borla ProXS or XR1s for a more unique sound.  Pipes will be rear-exit.  I have a set of 2.5 Magnaflows and they just weren't all that awe-inspiring.  I've put 800hp through 2x 2.5 before, so this is less about performance and more about sound.  We're guessing the Mach 1 will be around 700-750hp max when all is said and done, street use only.  A few exhaust guys have said you can get 3" under a '69 Mustang but it is tight at the rear axle.

Has anyone run dual 3" on an FE?  Was it obnoxiously loud?  My wife runs Borla ATAKs (5.0 Coyote), and I ran ATAKs on a '14 GT500, so that's about where I would like to be noise-level for the street.  We're not obnoxious drivers either. 

Looking for feedback before we start welding things up

[cjshaker]

I have the Borla ProXS on my Mach 1. Yes you can get a 3" exhaust on them, but it is tight. It's really tight if you have staggered shocks with the 4spd factory setup. It takes some very careful routing to get them through with the staggered shocks!

Here's a short idle video of my car with the ProXS and 2 1/2" exhaust. I'm a big fan of the ProXS. I think they have a great sound, aren't obnoxiously loud, don't drone at speed and they flow better than most all others I've seen in tests.

https://www.youtube.com/watch?v=PDiG2OSZ0bw

Here's a short in car video to give you an idea of what they sound like at normal acceleration and when you get on it. They will let you know when the loud pedal is smashed, but they still aren't obnoxious or ear splitting. At cruise, which doesn't come through real well on the video, they do have a really nice mellow tone, but they are louder than some people would like. In other words, you can't whisper to the person beside you..lol

https://youtu.be/KhXVsk_Tos4?t=32

I use them on both my Mach 1 and my '70 F350. The Mach has a 497 hp 427. I love them.

[fe468stroker]

I have the 3" Magna Flow from the headers out the rear valence cutouts.  This is a '68 without the staggered shocks.  It is very tight but doable for an experienced exhaust shop.  To me the Magna Flow 3" ovals are too loud but are non restrictive.  Like you said it is a matter of what sound you like.

[CV355]

I have the 3" Magna Flow from the headers out the rear valence cutouts.  This is a '68 without the staggered shocks.  It is very tight but doable for an experienced exhaust shop.  To me the Magna Flow 3" ovals are too loud but are non restrictive.  Like you said it is a matter of what sound you like.

Loudness is definitely subjective.  "Too loud" was when I had a supercharged mod motor, cammed, with headers, catless X pipe and SLP Loudmouths.  It was ear-splittingly loud and after two months I had to pull the SLPs and put on a Roush/Magnaflow side exhaust system.  I really liked that combo on a mod motor. 

We did insulate the Mach 1 with SecondSkin Damplifier Pro and LuxuryLiner MLV.  It sounds like a recording studio in the car now.  Hopefully that makes a nice loud exhaust a bit more tolerable in the cabin.

I have the Borla ProXS on my Mach 1. Yes you can get a 3" exhaust on them, but it is tight. It's really tight if you have staggered shocks with the 4spd factory setup. It takes some very careful routing to get them through with the staggered shocks!

Here's a short idle video of my car with the ProXS and 2 1/2" exhaust. I'm a big fan of the ProXS. I think they have a great sound, aren't obnoxiously loud, don't drone at speed and they flow better than most all others I've seen in tests.

https://www.youtube.com/watch?v=PDiG2OSZ0bw

Here's a short in car video to give you an idea of what they sound like at normal acceleration and when you get on it. They will let you know when the loud pedal is smashed, but they still aren't obnoxious or ear splitting. At cruise, which doesn't come through real well on the video, they do have a really nice mellow tone, but they are louder than some people would like. In other words, you can't whisper to the person beside you..lol

https://youtu.be/KhXVsk_Tos4?t=32

I use them on both my Mach 1 and my '70 F350. The Mach has a 497 hp 427. I love them.

That sounds awesome.  Decision made

[cammerfe]

I built an FE for my '63 Effie ICB. Started with a '64 PI engine and did the usual things. Used a set of '64 iron 427 exhaust manifolds and had a complete exhaust system built from three inch 321. Used an X-pipe and had Bassani make me a pair of mufflers with a three foot body. Nipples added to the length. Going up and over the axle was a bit tight. Enough that after several weeks of driving the truck I could just see where the  underside of the up-and-over had had slight contact with the axle tubes on each side. The mark was enough to feel and see but I doubt that the pipes were indented by as much as an eighth of an inch.

The sound was very mellow. Had a bit of authority to it.

KS

[JERICOGTX]

Not sure why anyone would want 3" exhaust on a street car. If you have a way to dump the exhaust at the track, why have it so loud it's horrible to drive?

[chilly460]

https://www.youtube.com/watch?v=_PVXvHkr-Vs

2.5" is down 20hp/20ftlb along the curve compared to 3.0" on a 600hp combo, would think a 700hp setup would be hurting it more. 

Maybe just me, but to reach 700hp would seem to mean optimizing heads (ported vs cast), compression, cam duration/lift...right to a high level.  Then seems odd to go back and de-optimize the motor with small exhaust when there absolutely is an alternative with minimal drawback beyond being tougher to fit (but possible). 

Now, if this was a street/strip combo where the exhaust is getting dropped at the track, that's a different scenario but I read this as street only.   

[gregb]

I have the 3 inch Magnaflow on my 69 Mach.  It’s pretty loud. Loud enough that I put some Car Chemistry inserts in the ends of the tailpipes to tone it down some.   Sounds good, the x-pipe that the setup has gives it a different tone at WOT than having a set of duals with a crossover.   I have nowhere near the power level you have planned…..(I can dream though….)

[427LX]

Run 3 inch to your mufflers and 2 1/2 tailpipes.   I have run every muffler there was on my 1985 Mustang LX with 5.0  351W and now 427W. I run 3 inch with H pipe combo and 2 1/2 inch short pipes that go over the axle and exit right behind rear tires. This kept the drone down.
Best mufflers so far as of 4 months ago are the Flowmaster Pro Series Hush type 3 inch core. Quiet but will handle 600+ HP. They have a 6 inch OD case and I fit them under my Mustang with enough room not to drag on speed bumps. The Magnaflow 3 inch round mufflers with a 5 inch OD case are good too. The original GM Corvair Turbo mufflers 1965 I believe with 2 1/2 inch pipe size were awesome back in the early 70's with good flow and a great sound. Used those back in the 70's.

[1968galaxie]

Regarding exhaust tube sizing and fitment over rear axle.

I once recall while attending a drag race at the old Calgary drag strip. A 1970 chevelle (10" slicks) creeps up to the starting line.
I could not hear his burn out from the stands. We laughed and thought it would be a 454 LS5 deal that would run 13's.
Surprised us all when it ran 9's (at 3500 ft track). Whisper quiet all the way down the track. WTF?

Went to inspect later in the pits. He had 2.5" exhaust - but it was split into 4 pipes, 4 mufflers, all 4 pipes exiting over the rear axle.
Cost a few $$ I suspect - but what a sleeper!

[CV355]

Run 3 inch to your mufflers and 2 1/2 tailpipes.   I have run every muffler there was on my 1985 Mustang LX with 5.0  351W and now 427W. I run 3 inch with H pipe combo and 2 1/2 inch short pipes that go over the axle and exit right behind rear tires. This kept the drone down.
Best mufflers so far as of 4 months ago are the Flowmaster Pro Series Hush type 3 inch core. Quiet but will handle 600+ HP. They have a 6 inch OD case and I fit them under my Mustang with enough room not to drag on speed bumps. The Magnaflow 3 inch round mufflers with a 5 inch OD case are good too. The original GM Corvair Turbo mufflers 1965 I believe with 2 1/2 inch pipe size were awesome back in the early 70's with good flow and a great sound. Used those back in the 70's.

I'll talk to the exhaust shop about that.  Not a bad idea, sort of checks all boxes. 

Regarding exhaust tube sizing and fitment over rear axle.

I once recall while attending a drag race at the old Calgary drag strip. A 1970 chevelle (10" slicks) creeps up to the starting line.
I could not hear his burn out from the stands. We laughed and thought it would be a 454 LS5 deal that would run 13's.
Surprised us all when it ran 9's (at 3500 ft track). Whisper quiet all the way down the track. WTF?

Went to inspect later in the pits. He had 2.5" exhaust - but it was split into 4 pipes, 4 mufflers, all 4 pipes exiting over the rear axle.
Cost a few $$ I suspect - but what a sleeper!

Not sure why anyone would want 3" exhaust on a street car. If you have a way to dump the exhaust at the track, why have it so loud it's horrible to drive?

I just want to be different, that's really what it comes down to.  I've run straight pipes several times in the past, but I think those days are behind me.  I'm just looking for a unique sound that can quiet down when I need it to. 

https://www.youtube.com/watch?v=_PVXvHkr-Vs

2.5" is down 20hp/20ftlb along the curve compared to 3.0" on a 600hp combo, would think a 700hp setup would be hurting it more. 

Maybe just me, but to reach 700hp would seem to mean optimizing heads (ported vs cast), compression, cam duration/lift...right to a high level.  Then seems odd to go back and de-optimize the motor with small exhaust when there absolutely is an alternative with minimal drawback beyond being tougher to fit (but possible). 

Now, if this was a street/strip combo where the exhaust is getting dropped at the track, that's a different scenario but I read this as street only.   

Correct- mine will be street only.  And I'm cheating to hit 700-750, it'll have 4-5psi.  It'll only ever see that on the dyno.  When I eventually do get back on the 1320, it'll be in a dragster.

[TwoFifty]

FWIW,
We ran 3" from the headers back to the mufflers, then mandrel bent 2.5" over the axle on our 69 Mach just as 427LX has suggested.  We are running Dynomax UltraFlo mufflers which I really like.  Might be a good option for you to consider. 

This is behind a 500hp street CJ.  I can try uploading a video if that sounds like it might be helpful for you in making your decision.

-Jim

[fryedaddy]

watch the engine masters episode where they compare 2 1/2 to 3 inch exhaust.after watching it you may want to run one 3 inch pipe with a y and 1 straight through muffler

[Sand hauler]

We run 3" true duals on my son's 75 highboy with the Jones exhaust single chamber mufflers

[JamesonRacing]

Would second that on the 3" main with 2-1/2" tailpipes.  I have 3-1/2" pipes from my headers to the Dr. Gas X, then 3" to a Spintech resonator (added later), to 3" Dynomax Ultraflows, then 2-1/2" over the axle and out the back.  Sounds wicked when thrashing it, very tolerable at idle and cruise.

Car has run low 11's with that setup on drag radials.