05 exhaust diameter questions

twmattox

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I have an '05 LJ with stock exhaust that has worn out. I originally wanted a Borla system, but they won't make it for my vehicle anymore. But, my local shop will make what I want. They just asked me to tell them what to make. So, I started looking at catback systems and noticed something about diameters:

Stock manifold through converter is 2.5" and the muffler back is 2.25"
Borla catback is 2.25"
Gibson catback is 2.25"

Magnaflo catback is 2.5"
MBRP catback is 2.5"
Flowmaster catback is 2.5"
Banks catback is 2.5"

so, 2.25" or 2.5 inch. Which is "better"? I know that the larger diameter will flow better at higher RPM. I also know that my Jeep is RARELY above 3000 RPM. Rumors abound about narrower pipe and backpressure and velocity at low RPM. What is the reality of all this?
 
If it were my decision and having a local shop do the work- I would look at 2.5 " only-
  1. i want to see exhaust flow improvement
  2. I would run a longer muffler case- to be on the quieter side- let the pipe diameter have the better flow - the bigger muffler to insure no drone.
P.s. I have been know to chase the low single digit improvements- those single digits add up to doubles somewhere down the line.
 
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Kind of what I thought...but, then again, there are claims of 7-20 hp gains by going from a 60mm to 62mm throttle body. I have kind of taken the you can never be too sure so ask approach.
 
Kind of what I thought...but, then again, there are claims of 7-20 hp gains by going from a 60mm to 62mm throttle body. I have kind of taken the you can never be too sure so ask approach.

FWIW, I got a 15 HP and 5 MPG gain with this. Don't let my secret out. I've been hiding it since the 90s. 🤫
tornadoair (2017_11_20 00_38_12 UTC).jpg
 
Everyone knows that the Tornado only worked if you used Slick 50. And wasn't it replaced by the secret garage hack where you generated you own hydrogen as you drove.
 
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I bought a Hooker Blackheart CAT back for my 97. 2.5".

Deciding characteristic was it was on sale for $178 from Holley's website.

Personally I'd buy a prebuilt bolt on system before I paid a shop to custom build something that others already have templated out and can mass produce. More so finding it on sale.

-Mac
 
I did a Magnaflow stainless 2.5" from the header pipe to muffler and custom 2.5" stainless tailpipe to replace stock back when I had my 4.0. Sounded better. Felt LESS torquey. Same sizes are on my 4.6 stroker now and probably fine with the extra displacement. I would stick with a 2.25" tailpipe on a stock motor. It also has the advantage of being easier to fit.
 
2.5 " Why start out limiting potential ? If you had a V-10 , ( 488 c.i.d.) Dodge pickup would you put 2.25 dual exhaust on it ?
However , if you believe that you "NEED BACKPRESSURE " to run right why not go with a 1" system for that locomotive type torque. :rolleyes:
 
You dodged a bullet with the Borla no longer available. They had a nasty habit of cracking. Replaced three of them under warranty before I switched to Banks. Made the switch to Banks when the frame on my YJ rotted and I bought the LJ. The Borlas all cracked in the same place. Tail pipe where it exited muffler. Looked like faulty welds. Have had the Banks on my LJ since 2011. Almost 300,000 miles later and no problems.

Felt no difference in performance between 2.25" and 2.5" cat back exhaust systems. Biggest difference was felt in ditching the stock exhaust system for the cat-back system.
 
Everyone knows that the Tornado only worked if you used Slick 50. And wasn't it replaced by the secret garage hack where you generated you own hydrogen as you drove.

Not as well known are the additional 3 HP and 2 MPG gain by adding "SplitFire" spark plugs to the mix.
 
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Most aftermarket systems will be mandrel bent vs crush bends that the factory and most exhaust shops use. On a stock engine however you are probably not going to notice a difference either way.

I wish I had some dyno data to back it up, but I really did feel a low end loss of power when I went from stock system to a Magnaflow full 2.5" system from exhaust manifold back (including new cat) in my '97 4.0.

Edit: I noted similar butt dyno INCREASE in low end power when I went from a JBA header (3-2-1) to a Banks header (6-1) with my 4.6 stroker. That one I do plan to get some dyno data on.
 
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I'm not sure it will make any difference whatsoever on an engine that has more in common with a tractor than it does anything even resembling a performance engine.

You'd be talking about numbers that would be negligible at best.

You say that but a an old Chevy truck has DUAL 2.25 to a muffler then 3” out. Another tractor engine with a 4000rpm usable limit. But 5.7liters displacement vs 4.0 … I’d have to assume our volumetric efficiency is higher (more air flow) than those old crap TBI motors. I’d like to see a 4.0 tractor engine with 3” exhaust and see what happens. Also a stroker before and after. I had a 2.8 liter Datsun 240z with 2.5” collector long tube headers and knocked down to 2.25 at some point, I can’t remember and it was a torquey SOB, and a non cross flow 2 valve head.
 
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You say that but a an old Chevy truck has DUAL 2.25 to a muffler then 3” out. Another tractor engine with a 4000rpm usable limit. But 5.7liters displacement vs 4.0 … I’d have to assume our volumetric efficiency is higher (more air flow) than those old crap TBI motors. I’d like to see a 4.0 tractor engine with 3” exhaust and see what happens. Also a stroker before and after. I had a 2.8 liter Datsun 240z with 2.5” collector long tube headers and knocked down to 2.25 at some point, I can’t remember and it was a torquey SOB, and a non cross flow 2 valve head.

I will say, when looking at the exhaust it doesn’t really matter if keeping the stock manifolds (tube or cast).

We still do not have good results posted about the American Racing Headers for our Jeeps. On paper they should make a nice difference. Strikers with a ported head, even more so. Remember, a 4 liter 6 cylinder has the same volume per cylinder as a 5.3 liter v8, and a 4.6 being equal to a 6.0.
 
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