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Old 05-28-2008, 06:40 PM   #1
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Thumbs down exhaust question

does backpressure in the exhaust really make that big a diff. with power? and is it better for fuel mileage to have more or less backpressure?

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Old 05-28-2008, 08:30 PM   #2
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This may sound sarcastic, but it is better to have the correct back pressure. As a rule less back pressure tends to move the power peak higher in the rpm range to a point where it can actually lower peak power if you have too little back pressure. And of course too much back pressure is bad too. Think "banana in the tailpipe".

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Old 05-29-2008, 08:15 AM   #3
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does backpressure in the exhaust really make that big a diff. with power? and is it better for fuel mileage to have more or less backpressure?
It's my understanding that backpressure is required to maintain proper exhaust valve functionality and over all balance of the exhaust system. Too little and the system develops backfiring (like running straight headers with no pipes or mufflers).. Too much backpressure and things get clogged (as Parrot Head's example of the banana).

Needless to say, today's smogged motors offer a lot in terms of improvement. Eliminating the catalytic converters and installing a high flow system (header, larger diameter pipes and a low restriction muffler) can make a significant improvement in terms of hp gains. However, this only solves half of the equation: you need to upgrade the intake system to compliment the exhaust improvements.

There's been a lot of discussion on this topic and the general consensus seems to indicate that such upgrading of the 4.0L is a bit of a moot point. It's very expensive and you do not get much in terms of gain. I have not seen any dyno results to support either claim.
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Old 05-29-2008, 05:51 PM   #4
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iv got a 4.0L, 04 wrangler, i got an airraid intake and a throttle body spacer on it and am thinkin about gettin a new exhaust in stainless steele and larger diamenter, mainly for better sound but more power is always welcome. i am lookin at the banks system, but it says it eliminates backpressure, but like yall were sayin, i thought back pressure was essential???? and what will it do to gas mileage?
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Old 05-30-2008, 08:39 AM   #5
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iv got a 4.0L, 04 wrangler, i got an airraid intake and a throttle body spacer on it and am thinkin about gettin a new exhaust in stainless steele and larger diamenter, mainly for better sound but more power is always welcome. i am lookin at the banks system, but it says it eliminates backpressure, but like yall were sayin, i thought back pressure was essential???? and what will it do to gas mileage?
To be honest, I'm not sure how it will impact your gas mileage. I'm inclined to think it will not decrease it because of the simple fact you have not changed the fuel delivery system (e.g. larger injectors or longer pulse duration). If anything, by making the system "breathe" better, you should increase the over all efficiency of the process and may see a small increase.

By comparison, I've built quite a few racing motors for my Mustangs and I will say there is a huge benefit to upgrading the exhaust, intake and fuel delivery systems that are proven. The old school Ford motors (302, 351W, 351C and 460) are very inefficient so they have a lot of potential for improvement.

I tend to think Jeep's 4.0L - I6 is strictly built for general purpose use only and is not a high output configuration. It's designed for long service so the internal mechanical components (crank, pistons, cam) are not maximinzing efficiency. Basically, it's a truck motor. IMO - to get more than a 10% hp gain would require extensive internal upgrading (higher compression pistons, higher lift and more duration on the cam, stroked crank). Of course, upgrading the induction and the exhaust system is a complimentary process to any build up. The fuel delivery system (injectors, pump and computer) require upgrading to support the new configuration. Doing all this will give you more hp but will degrade your gas mileage big time. I get about 5-7 mpg on my 351C but it also pumps out 425+hp. By comparison, the stock configuration was appox. 285 hp and 10-12 mpg.

Basically, it's a top down approach. IMO - the overall ratio of cost per hp increase is very high and the benefit relatively low.
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