2.5" Exhaust vs. 3" Exhaust

fivelitrecobra

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I've scoured the net trying to find any solid info on whether there would be a significant gain in HP/TQ going from a 2.5" X/H-pipe and exhaust to a 3" X/H-pipe and exhaust on our cars, but came up empty. Does anyone have any real world experience on the dyno or reliable information between the two? I recently watched an Engine Masters episode using an LS3 and they picked up +/- 20hp and 20tq going to the bigger exhaust. I'm in the process of trying to make my setup as efficient as possible, if there is 15whp/tq or more to be had, I may pull the trigger on a full offroad 3".
 

GodStang

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There is tons and tons on this but long story short depends on what power you are at as to whether you will see gains. There is a point where 2.5" is better. As you get past that point 3" will be better. As you go further past that point the gains on 3" will be substantially more.
 

Livernois Motorsports

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2.5in or 3in depends on what you are looking for an the modifications done to the vehicle because to big of an exhaust can also harm the hp & tq since NA motors need back pressure. You want to pair the vehicle with the right size exhaust
 

geoffmt

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There is tons and tons on this but long story short depends on what power you are at as to whether you will see gains. There is a point where 2.5" is better. As you get past that point 3" will be better. As you go further past that point the gains on 3" will be substantially more.

I am wondering what that point is myself, I would guess under 500rwhp a 2.5 would be better. But with 281-331 cu and not the 376-500+ cu that most tests are done on it throws things off a bit. Probably the drive ability of riding a cr125 with a turbo compared to a cr500. I also wonder what the best turbo muffler is. Spintech is the only muffler I found that is available in oval tubing unless I fab up a couple. Would love to see a Dyno on small cu turbo motor with different mufflers (chambered and straight through design)and size of tubing


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oldstv

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Unless you are racing the car I wouldn't do 3". You most likely will loose low end torque and stop light to stop light might not be as you remember it. And as already mentioned, without the hp you could loose power across the band.
 

Molon Lave

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If I remember correctly kenne bell’s site in the tech section for our cars has info on this. I want to say that 600 hp plus is when it’s advantageous to go 3 inch. Check it out for yourself, I don’t want to be a liar.
 

badjuicedgt

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I have stock manifolds and 3" catless x pipe and bassani 3" catback and my horspower when we pushed it hard on the dyno was over 650 horse but I was running very lean so we dialed it back to a safe range where it's not running lean its still making 605 horse but torque seems low to me. I lost my job and then went into respiratory failure and finally diagnosed with copd at 39 now have to be oxygen at least 20 hours a day. Son after loosing my job the new fuel system was put on hold, atleast until I find another dealer that needs a buyer since it's what i have been doing or the last 20 years even while I was working as a full time firefighter and paramedic because we worked 24 hours on shift and 48 hours off, it was great the money and the time until the paramedic service I worked for found out I was an ase certified mechanic on diesels and gas engines the would pull me off my shift to work on ambulances. At that time I had a 99 cobra and who ever had it before me beat the living piss out off it and had 4 different sizes of exhaust and that car had no power sounded like a ricer, where my 03 cobra will blow your ear drums out very quit at idle and at normal driving but once you get on it you can hear it a mile away.
 

gimmie11s

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2.5in or 3in depends on what you are looking for an the modifications done to the vehicle because to big of an exhaust can also harm the hp & tq since NA motors need back pressure. You want to pair the vehicle with the right size exhaust

The ol "na motors need backpressure" argument. LOL.. gotta love it.
 

badmpg03

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Heard from my tuner that once you get past into the midpipe it doesn't matter much. Most of gases have expanded and cooled off some by the x/catback.
I would get bigger primary headers first
fyi Im running 3"to 2.5"x pipe and 2.5" catback with 800whp
 

Revvv

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Heard from my tuner that once you get past into the midpipe it doesn't matter much. Most of gases have expanded and cooled off some by the x/catback.
I would get bigger primary headers first
fyi Im running 3"to 2.5"x pipe and 2.5" catback with 800whp
Exhaust tuning is real, and it has nothing to do with sound. You want the exhaust pulses to act as a siphon and pull the other gasses. If the exhaust is too small, everything is choked. If it is too big the gasses tumble, and then they create an invisible restriction.



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TAF

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2 1/2” will handle above 600 RWHP depending upon the size of the primaries (long tube header diameters) and other less restrictions (High Flow CatS or Cat delete). Above 650-700+RWHP...dual 3” will be helpful (depending on same upstream “restrictions” stated before) and will help evacuate from such a high velocity output. Oh...and...”backpressure” is a “wives tale”...
 

c6zhombre

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I'm over 700 with factory manifolds and 2.5" mid and catback. Mid has cats too. I doubt I'll ever actually add long tubes and full 3".....but all this talk of it being a bottle neck past 650 has me wondering what it might add. Lol.

The factory manifolds aren't works of art, but they're damn impressive performers IMO
 

gimmie11s

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I'm over 700 with factory manifolds and 2.5" mid and catback. Mid has cats too. I doubt I'll ever actually add long tubes and full 3".....but all this talk of it being a bottle neck past 650 has me wondering what it might add. Lol.

The factory manifolds aren't works of art, but they're damn impressive performers IMO

You’d probably gain 50hp from full 3” exhaust.


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Bdubbs

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I'm running arh 1 3/4" header's, 3" o/r x pipe, reduced to 2 1/2" catback exhaust. Works great, not lacking any low end torque here.

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roy_1031

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I'm over 700 with factory manifolds and 2.5" mid and catback. Mid has cats too. I doubt I'll ever actually add long tubes and full 3".....but all this talk of it being a bottle neck past 650 has me wondering what it might add. Lol.

The factory manifolds aren't works of art, but they're damn impressive performers IMO

You should add a 3” exhaust and re dyno to see how much you pick up.

Side note, when I had my 3” exhaust it seemed to drone less than the 2.5”. It also seemed quieter until you stepped on it, it cruised quieter. So getting rid of the drone by going with a 3” exhaust was worth it alone.


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GodStang

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I think a lot of people confuse back pressure with Scavenging.

2.5" is recommend to about 650rwhp by many companies. Of course everything varies and depending how far past that your power will determine your gains.

I use to have tons of info on this but over the years I have lost it. I know some of the bigger power guys gained a lot at the track when swapping from 2.5" to full 3". I am sure somewhere I have the info on my car before and after my switch.
 

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