exhaust size

what exhaust size should i use? I plan on doing every mod that I can do without pulling the lower block for the engine. I will dig in as far as a new camshaft. I will be running an Banks header to. But I am wondering what size exhaust I should run. Stock is 2.25 right. Is there any benifits from upgrading to 2.5? I take it 3 would be WAY to big.

Thanks
Chris

I heard that the 4.0 engine likes a bit of back pressure, so i am not sure if by changing the diameter of the pipe to larger one would make things better at all...

pete

from what I've read...2.25 is ideal up until you stroke it. I personally have 2.5" and I really like it (it freed up alot of top end power) but for more low end torque...you should probably stick with the 2.25.

I just put 2.5" w/a dynomax on last night. The motor is running much better.

skylo said:

I just put 2.5" w/a dynomax on last night. The motor is running much better.

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Yeah, but the dynomax has a 2.25" inlet on the muffler and a 2.5" on the outlet of the muffler with a 2.5" tailpipe. That's not going to affect the back pressure as much as a 2.5" dual exhaust all the way from the manifold/header to the tailpipe.

I want to go with a high flow 2.25" in/out cat and a 2.25" inch exhaust with a pacesetter header and downtube. I like the pace setter header because it has longer primary runners than the banks and has a longer distance between the collecter and the two three-into-one merges. This will improve scavenging somewhat but not create a counter productive decrese in back pressure. I think the design of the banks merges the two three-into-two collectors too quickly - which will create more turbulence in my opinion. Lengthening the route between the two three into one collecters before they meet the main collector will allow for smoother flow and less turbulence.

I think a full 2.5" system would be a little much for a 4.0.

2.5" exhaust, 00 style intake, cold air intake, and a map adjuster to richen it up would do quite nicely on a 4.0, prolly 30 wheel horse

go jeep has a write up w/ dyno chart for the 00 intake

It would be interesting to see dyno data for this setup run with both a 2.5 and a 2.25 inch system.

tealcherokee said:

2.5" exhaust, 00 style intake, cold air intake, and a map adjuster to richen it up would do quite nicely on a 4.0, prolly 30 wheel horse

go jeep has a write up w/ dyno chart for the 00 intake

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jeepsrock said:

I heard that the 4.0 engine likes a bit of back pressure

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All lies. Backpressure is never a good thing. Ask 5-90. He will right a damn essay on why back pressure is bad.

bzdel2441 said:

what exhaust size should i use? I plan on doing every mod that I can do without pulling the lower block for the engine. I will dig in as far as a new camshaft. I will be running an Banks header to. But I am wondering what size exhaust I should run. Stock is 2.25 right. Is there any benifits from upgrading to 2.5? I take it 3 would be WAY to big.

Thanks
Chris

Click to expand...

A 2.5" exhaust has very low restriction up to 250hp so that's the size that you need for a modified 4.0. A 3.0" exhaust would be way too big. Even a stroker only needs a 2.5" or 2.75" exhaust depending on specs.
How much exhaust backpressure is good for the 4.0 or indeed any engine? The answer is ZERO.

Dr. Dyno said:

A 2.5" exhaust has very low restriction up to 250hp so that's the size that you need for a modified 4.0. A 3.0" exhaust would be way too big. Even a stroker only needs a 2.5" or 2.75" exhaust depending on specs.
How much exhaust backpressure is good for the 4.0 or indeed any engine? The answer is ZERO.

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i swear, the doc to the rescue again!

Dr. Dyno said:

A 2.5" exhaust has very low restriction up to 250hp so that's the size that you need for a modified 4.0. A 3.0" exhaust would be way too big. Even a stroker only needs a 2.5" or 2.75" exhaust depending on specs.
How much exhaust backpressure is good for the 4.0 or indeed any engine? The answer is ZERO.

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NOT arguing, but wanting a little of your knowledge....

I had a 5.0 mustang (stock at the time) that I put a bigger H-pipe on instead of the cats.... it ran ALLOT better racing on the high rpm's, but lost power/torque down low.... WHY? I am very afraid to go 2.5 on my stock 97 4.0 as I don't want to loose that low end grunt that makes it so great to drive in town and wheeling.

What do you recomend for a stock 97. I already have the APN header.

With my 98 4.0 the only mod on my exhaust is a 5"x8"x18" Case 2 1/4" In/Out Magnaflow using all stock sized tubing It runs good. When I get enough money to buy a header I'll run 2.5" from collector to Converter then 2 1/4 out of converter and back.

Just do a bored throttle body with your bigger exhaust.

Mods need to be done on both sides of the engine. The best flowing exhaust won't work well unless you have the best flowing intake to go with it.

I have a 98 w/fed emissions; pacesetter header, stock cat, 2.5 in/out super 40 muffler and rusty's 2.5 tailpipe on the exhaust side. On the intake side; Rusty's airtube, 99+ GC intake manifold, bored to 60mm TB, new stock size (23.5lbs) injectors, and an adjustable MAP sensor set at 4.65 volts. It ran very rough at idle without the Adj, MAP, leaned it out and run beautifully. Alot more power across the power band and the throttle response is unbelieveable. I always thought I was running lean........might have been with the old injectors......but now its a torque monster. I wish there was a chassic dyno around me, so I could really dial in the adjustable MAP sensor, but it seems to be running fine now and I actually gained alot of my mileage back which makes me very happy.

Red97XJ said:

NOT arguing, but wanting a little of your knowledge....

I had a 5.0 mustang (stock at the time) that I put a bigger H-pipe on instead of the cats.... it ran ALLOT better racing on the high rpm's, but lost power/torque down low.... WHY? I am very afraid to go 2.5 on my stock 97 4.0 as I don't want to loose that low end grunt that makes it so great to drive in town and wheeling.

What do you recomend for a stock 97. I already have the APN header.

Click to expand...

If the engine's going to remain otherwise stock i.e. near the 200hp level, 2.25" is the optimum size. If you're going to add a free-flow intake, bored TB, and bored TB spacer, you'll be getting more air into the engine so upsizing the exhaust to 2.5" will help get that extra air out again without losing low rev torque.

If you are going to go to the trouble take off the cat thats about the best thing that you can do to free up the exhaust. and run it 2.5 all the way back