Exhaust 2.5" or 3" - Ford Mustang Forums : Corral.net Mustang Forum
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post #1 of 30 Old 05-16-2017, 11:00 PM Thread Starter
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Exhaust 2.5" or 3"

I bought a car recently that has a 331 that was dyno'd at 349hp/352 ft-lbs to the wheels. Getting under the car last night I found the Flowmasters welded into the OEM pipes from the midpipe back. The rest of the exhaust is BBK 1 5/8" long tubes and a 2.5" BBK H-pipe.

I will be replacing the cobbed together restrictive cat-back with a mandrel bend cat-back. I can go with a 2.5" or 3" setup.

At what point does experience tell us that that 1 3/4" long tubes and 3" exhaust are the right thing to do? If I redo my exhaust in a year or two I will go with 1 3/4" to 3" if it would benefit the car. If that is the case I will run the 3" cat-back now. Otherwise I will run a 2.5" cat-back

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post #2 of 30 Old 05-17-2017, 12:35 AM
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2.5 is plenty big enough.
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post #3 of 30 Old 05-17-2017, 08:33 AM
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Corral will say smaller is better. Better do 2in!
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post #4 of 30 Old 05-17-2017, 09:40 AM
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As a rule of thumb I think a 2.5 inch dual exhaust setup is good till about 500hp(at the motor) before it starts to become a restriction. Take it for what it is worth... In my opinion it comes down to money, if your current stuff isn't all crusty and rusty keep it and be happy with the 2.5in setup. Otherwise take the opportunity to put the big 3inch setup on and don't look back.
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post #5 of 30 Old 05-17-2017, 09:50 AM Thread Starter
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post #6 of 30 Old 05-17-2017, 12:39 PM
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Quote:
Originally Posted by BK_5.0 View Post
As a rule of thumb I think a 2.5 inch dual exhaust setup is good till about 500hp(at the motor) before it starts to become a restriction. Take it for what it is worth... In my opinion it comes down to money, if your current stuff isn't all crusty and rusty keep it and be happy with the 2.5in setup. Otherwise take the opportunity to put the big 3inch setup on and don't look back.
So 2.5 is not a restriction under 500hp?

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post #7 of 30 Old 05-17-2017, 02:47 PM
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I had the same decision with my 331 with 1 3/4 long tubes and went ahead and built a 3 inch system with a x pipe and race bullets and I am very happy with it

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post #8 of 30 Old 05-17-2017, 03:04 PM
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Originally Posted by 1995gtCPE View Post
So 2.5 is not a restriction under 500hp?
The 14' GT500 made 662 HP with cast iron exhaust manifolds and 2.5" exhaust.....

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post #9 of 30 Old 05-17-2017, 03:25 PM
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Originally Posted by Saleen414 View Post
The 14' GT500 made 662 HP with cast iron exhaust manifolds and 2.5" exhaust.....
Congrats.

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post #10 of 30 Old 05-17-2017, 03:37 PM
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Quote:
Originally Posted by Saleen414 View Post
The 14' GT500 made 662 HP with cast iron exhaust manifolds and 2.5" exhaust.....
and the 2.5" exhaust hurt performance a bunch, on a quick glance MM&FF picked up 38 rwhp with just a 3" catback


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post #11 of 30 Old 05-17-2017, 03:43 PM
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it's not going to hurt power by going with 3" you just may not gain much, the big thing it it will be a little heavier and if doing tail pipes a tighter fit

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post #12 of 30 Old 05-17-2017, 03:52 PM
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Search?

3" exhaust or not



2.5 or 3" exhaust for 351w


Notice the video posted in both links showing a gain of 14HP on a 600+ HP motor (2 1/2" to 3") using an exaggerated 20' truck-sized exhaust system.

Notice a certain poster trolls in every thread, doesn't give a hoot if the guy will spend $1000+ for 3-4 HP. Don't be a sucker for internet machismo.

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post #13 of 30 Old 05-17-2017, 04:44 PM
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Hotrod just did a back to back dyno comparison, 3" was better everywhere
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post #14 of 30 Old 05-17-2017, 04:46 PM Thread Starter
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I tried searching. Got mixed results.
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post #15 of 30 Old 05-17-2017, 04:50 PM
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The 3" will be louder. I doubt that at 350ish rwhp that you could tell the difference between a 3" and a good 2.5" exhaust, except for the noise and the lighter wallet.

1989 Saleen 414 358cid SBF 2.8L Kenne Bell S/C
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Best 1/8 mile time 5.384 @ 130.75 mph with a 1.29 60'
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post #16 of 30 Old 05-17-2017, 05:30 PM
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Originally Posted by Saleen414 View Post
The 3" will be louder. I doubt that at 350ish rwhp that you could tell the difference between a 3" and a good 2.5" exhaust, except for the noise and the lighter wallet.
This is a less abrasive version of my post.

I agree.

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post #17 of 30 Old 05-17-2017, 05:30 PM
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You are probably good at 2.5 inches with duals if the pipe isn't all kinked up. While it will be a tiny bit better bigger, the weight and cost will go up. The power return on the investment at your power level would never pay you back, especially since it would add weight.

Don't think for a second the pipes will suddenly just choke off at a certain power level and be perfectly fine below that level. It is a gradual back pressure increase with more power.

I can watch this on my data log. My downpipe pressure into dual 3" and dual Borla Pro XS was about 1-2 lbs at 800 HP. At 1200-1300 HP it is about 2-3 psi. It doesn't suddenly skyrocket.


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post #18 of 30 Old 05-17-2017, 06:54 PM
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Shortcut here, shortcut there.... Next post, "hey Corral why is my combo down on power?..." Get what you can afford. If 2.5 is what you can swing, then get it and not worry about it. Like the old saying goes, "Buy once, cry once". Everyone has different ideas of what they want and im good with that. I want the most HP for my dollar. I bought what i could afford and thats that. Some want good MPG, some want a quiet v8, some want a 427 stroked windsor to make 500hp cause anything smaller than that is not streetable. Folks post here asking for opinions. Theres mine.
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post #19 of 30 Old 05-17-2017, 07:45 PM Thread Starter
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I am going to change my cat-back. I had MAC exhaust years ago and I plan to run it again. 2.5" v. 3" is a $45 difference.
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post #20 of 30 Old 05-18-2017, 12:23 AM
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Do a comparison of the 2 (its only a $45 difference!)and let us know if you can FEEL the diff!
I think I know the answer.........
6sally6
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post #21 of 30 Old 05-18-2017, 02:30 AM
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1995 has it right...if you're buying new, you spend as much money to go slower if you buy parts meant to flow less air.


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post #22 of 30 Old 05-18-2017, 08:17 AM
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Do a comparison of the 2 (its only a $45 difference!)and let us know if you can FEEL the diff!
I think I know the answer.........
6sally6
OP, there you go. The above post insinuates that you will not be able to feel a power difference with 3.0 vs 2.5, AND you will save $45! Never mind that your heads, cam, intake stroker is making 302 power. You dont need the extra bit of power cause you will never FEEL it!

Seriously, get what aligns with your goals and finances.

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post #23 of 30 Old 05-18-2017, 09:46 AM
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Originally Posted by 1995gtCPE View Post
OP, there you go. The above post insinuates that you will not be able to feel a power difference with 3.0 vs 2.5, AND you will save $45! Never mind that your heads, cam, intake stroker is making 302 power. You dont need the extra bit of power cause you will never FEEL it!

Seriously, get what aligns with your goals and finances.
Truth spoken here....

....if the OP will listen to this advice is a whole other topic...



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post #24 of 30 Old 05-18-2017, 11:12 AM
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Originally Posted by Saleen414 View Post
The 14' GT500 made 662 HP with cast iron exhaust manifolds and 2.5" exhaust.....
would the 5.8L have made more with a larger exhaust? My 2010 5.4L made 673 to the tire with cast iron and 3" system.

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post #25 of 30 Old 05-18-2017, 11:34 AM
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would the 5.8L have made more with a larger exhaust? My 2010 5.4L made 673 to the tire with cast iron and 3" system.
Yes it probably would. I was just stating that as delivered in stock form it cranked out 662 hp with cast iron manifolds and 2.5 exhaust. I never insinuated it could not be improved upon. It just was not a huge issue at that power level as far as Ford was concerned. I am sure NVH, emissions and other factors went into it as well. If the end goal of the engine was around 700 HP and above, then maybe the exhaust system would have been different.

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post #26 of 30 Old 05-18-2017, 12:13 PM Thread Starter
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For $45 bucks I am going with the 3" cat-back. In a year or so I will complete the change/upgrade by going to 1 3/4 long tube headers and a 3" mid-pipe.

My current exhaust will get the job done, as it does now, but I want to change it. There isn't a ton to do mechanically for more power in the combo short of a custom cam and perhaps I will do that in a couple years. For now the once chrome headers look like a steaming pile of poo and the H-pipe is crusty. So I do the cat-back now and the mid-pipe next year and the headers (ceramic) after that. Don't have to do it all at once.
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post #27 of 30 Old 05-19-2017, 11:33 AM
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the main pieces of the puzzle in exhaust sizing needs are inlet flow capability and valve events. HP is not a design criteria except in magazines.

If the motor responds successfully with higher ratio exh rockers or with higher flowing and larger cross-section headers and exhaust system, then the motor combination needs more exhaust activity in the valve events.

If the motor responds successfully with lower ratio exh rockers or with lower flowing and smaller cross-section headers and exhaust system, then the motor combination needs less exhaust activity in the valve events.

In either of the scenarios HP vs exhaust exh size is not a player. The engine's air movement and valve events (that time the air movement to piston position/crank rotation) are the players.

I dont care if your supercharged motor does make 1000 hp on 2" exhaust system with 15psi, or your old carbed 289 ford makes 475 hp on a 3 1/2" exhaust system and ported early heads. Its not relavent. the combination defines the need.

Your better assumption would be to say:
If I have xxxxx inlet capability, then I need between 60% and 80% on the exhaust side (if naturally aspirated). This is not the head flow. this is the total flow seat to muffler outlet (or collector)
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post #28 of 30 Old 05-19-2017, 11:59 AM
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For $45 bucks I am going with the 3" cat-back. In a year or so I will complete the change/upgrade by going to 1 3/4 long tube headers and a 3" mid-pipe.

My current exhaust will get the job done, as it does now, but I want to change it. There isn't a ton to do mechanically for more power in the combo short of a custom cam and perhaps I will do that in a couple years. For now the once chrome headers look like a steaming pile of poo and the H-pipe is crusty. So I do the cat-back now and the mid-pipe next year and the headers (ceramic) after that. Don't have to do it all at once.

Now that your goals are clearer, there's no reason not to replace the cat back with a 3" version.

As far as the H-pipe and headers, buddy is correct in that it's not a simple formula. There are a few great threads about headers on a couple of the more technical forums out there that better flush out what buddy is raising, though at your power level you're talking about minimal torque curve shifts that may or may not be noticable or measurable. "It depends"
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post #29 of 30 Old 05-19-2017, 12:45 PM Thread Starter
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Originally Posted by buddy rawls View Post
the main pieces of the puzzle in exhaust sizing needs are inlet flow capability and valve events. HP is not a design criteria except in magazines.

If the motor responds successfully with higher ratio exh rockers or with higher flowing and larger cross-section headers and exhaust system, then the motor combination needs more exhaust activity in the valve events.

If the motor responds successfully with lower ratio exh rockers or with lower flowing and smaller cross-section headers and exhaust system, then the motor combination needs less exhaust activity in the valve events.

In either of the scenarios HP vs exhaust exh size is not a player. The engine's air movement and valve events (that time the air movement to piston position/crank rotation) are the players.

I dont care if your supercharged motor does make 1000 hp on 2" exhaust system with 15psi, or your old carbed 289 ford makes 475 hp on a 3 1/2" exhaust system and ported early heads. Its not relavent. the combination defines the need.

Your better assumption would be to say:
If I have xxxxx inlet capability, then I need between 60% and 80% on the exhaust side (if naturally aspirated). This is not the head flow. this is the total flow seat to muffler outlet (or collector)
Any suggestion on how one might accurately estimate the inlet capability of any given setup? Assuming you know the cam specs.
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post #30 of 30 Old 05-19-2017, 03:58 PM
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PipeMax is a good program good sizing your whole inlet and outlet system. It's cheap as those kinds of programs go and has been developed over many years by Larry Meaux - look him up on Google. It will size your headers, X or H pipe, tell you lengths for various pieces of the system and recommend a total length for the whole system. I bought it years ago for $25, not sure what it is today.
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