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HP gain expected

Does anyone know if tests have been conducted on the results of an open exhaust with very low backpressure. I see guys saying that the 4.0 needs backpressure and suffers if it has none. I think thats crap. Im just looking for actual fact data. A dyno test with and without exhaust would be the answer.
 
the 4.0 needs some back pressure to have good horsepower on acceleration.. take the cat and muffler off and drive it, and you will know what i mean.
 
It's not about backpressure, it's about exhuast gas scavanging and air velocity. People misuse that term all the time. You can't put 4" pipe on a 4.0 and just cover up half the tail pipe to get some backpressure. The piping need to be the right size all the way back--to big, air goes to slow and your low RPM power sucks--to small, high end gets restricted. Smooth bends, high flow muffler, and the right size piping--2 1/4 to 2 3/4 I would guess for a 4.0/4.6 I'd guess. Maybe Dyno will comment.
 
I can't tell you what amount of H/P is gained, but I can tell ou that when I added a Flow Master 40 series to the otherwise stock exaust on my 2000 4.0 that there was a very noticable difference of more power at freeway crusing, and full throttle passing. On the intake side I have a a 3" pipe into the cowl area under the windshield, and a K&N filter in the stock air box. I'm shure the whole set up has some sort of ram air effect on the intake that helps the extra power feeling than what it was before.
 
cmotsvt said:
So running an open header and short pipe would be ok then? Aside from sounding horrible.
i'm telling you, if you want power from this, you could get a $100 dynomax catback and replace the cat with a straight pipe, but it wouldn't be street legal. with no muffler, it's really too loud and when you floor it it feels like a 4-popper. i woulda said exhaust scavaging, but i knew what you were trying to say, and tried to keep it simple. those two terms are used interchangably so much....
 
Backpressure is always bad for any kind of power production. Exhaust velocity is what is important for torque. There are a million modified 4.0Ls out there, if you think you have a better idea than all those owners, you're wrong. It's been tested, and trued; stick with a hi-flow cat, a hi-flow muffler, mandrel bent pipes, and keep it in the 2.25"-2.5" range. End of story.
 
scoobyxj said:
I can't tell you what amount of H/P is gained, but I can tell ou that when I added a Flow Master 40 series to the otherwise stock exaust on my 2000 4.0 that there was a very noticable difference of more power at freeway crusing, and full throttle passing.

Exactly. I call :bs: when people say exhaust work on a 4.0 doesn't do anything. The difference on mine was noticeable on acceleration and the jeep doesn't feel like i'm jake braking whenever I let off on the gas. The stock system is too restrictive. Of course my opinion is kind of biased since I did over the whole exhaust system from the head back and before I did that, the muffler was rotting inside, the cat was plugged and the pipe after the header was all dented in from impacts.
 
Cjmartz2k said:
It's not about backpressure, it's about exhuast gas scavanging and air velocity. People misuse that term all the time. You can't put 4" pipe on a 4.0 and just cover up half the tail pipe to get some backpressure. The piping need to be the right size all the way back--to big, air goes to slow and your low RPM power sucks--to small, high end gets restricted. Smooth bends, high flow muffler, and the right size piping--2 1/4 to 2 3/4 I would guess for a 4.0/4.6 I'd guess. Maybe Dyno will comment.

I couldn't have said it better myself. Here's how I would size the exhaust:

4.0 stock or external bolt-on performance mods only: 2.25"
4.0 highly modified, ported HO head, performance cam: 2.50"
4.5/4.6/4.7 stroker: 2.50"
4.8/4.9/5.0 stroker: 2.75"

The optimum pipe size will vary depending on displacement, cam used, and desired rpm range but this guide will get you in the ballpark.
 
I don't know exactly how it works so I won't argue with you guys. :gag:

But I do know one thing is that mine full straight through and I've noticed only a difference in power when I'm in the high RPM range starting from 3000 RPM. It's all (2"1/4) except were the cat was (2"1/2) and a 3" tip.

I did have it running without everything from the mount for the cat and back (long story, back wheel ripped it off) and it did feel torquier but didn't see full throttle because I fear the cops hearing me across town. :laugh3:

So I don't know whats going on but I hear that the stock exhaust is very restrictive.
 
Thanks for straightening it out guys, I was getting sick of hearing guys say that "the XJ needs backpressure". I feel good hearing from others that thats not right.
 
cmotsvt said:
So running an open header and short pipe would be ok then? Aside from sounding horrible.
"Okay"?

Define okay. It'll make a tad more horsepower above 5,000 RPM, and you'll lose most of your torque below 4,000 RPM.

Is that your idea of "okay"?
 
cmotsvt said:
So running an open header and short pipe would be ok then? Aside from sounding horrible.

Like Eagle said...No. Open pipes are for full throttle only. You could tailor a straight pipe to make power at a certain RPM BUT it would be in that narrow range only. I'm more familiar with bikes so I'll use them for an example. See all those bikes running around with straight drags? Sounds good, eh? Not making the power it sounds like tho. Add a simple little bolt thru the pipe (preferabbly with a washer welded to it and the scavenging goes up (usually mistakenly called backpressure) and the power does too. Lots of tests of bikes with opens and mufflers out there to prove the point. After all a motor is a motor. Also if you open the exhaust up with a freer flowing setup you should also improve the incoming flow. One don't work as well without the other. It's a system not a collection of independent parts.

Sarge
 
cmotsvt said:
Thanks for straightening it out guys, I was getting sick of hearing guys say that "the XJ needs backpressure". I feel good hearing from others that thats not right.

Sorry but you missed the point. Freer flowing yes, open no. If you wish to use the term backpressure then do so but it does need some (scavenging actually). The motor uses the previous exhaust pulse to help the next one exit the motor. Too open, no help, loss of power EXCEPT at WOT. You run at 5000 rpm or so all the time?

Sarge
 
Ok, I'm going back to my first post saying that I would like anyone with a dyno tester available to do runs with exhaust and without. Im a numbers guy.
Im still not convinced that scavenging benefits apply much further than the 6-1 pipe.
If anyone can point me to a dyno study of this question I would be very grateful.
 
The only reason that I see for back-pressure or "scavaging" is for turbo applications. I know that turbo egines really need a certain amount of
back-pressure to run properly.

Like I said before I'm not that experienced in this area. Iv'e seen very short exhaustes on very powerfull cars (not stock) and usually all the same configuration, short wide pipes that are routed as straight as possible.

I'm just saying you guys are contradicting what a lot of modders do with their exhaust systems. Also I just found out why my exhaust does what it does, it gets a boost at 3000 RPM because thats the point where the gases flow fast enough to beat the back-pressure the pipe is creating.
 
John(XJ)Jeep said:
Like I said before I'm not that experienced in this area. Iv'e seen very short exhaustes on very powerfull cars (not stock) and usually all the same configuration, short wide pipes that are routed as straight as possible.

Just cause they do it don't make it right. Short wide pipes, basically open pipes will make power but not as much as a correct system. The big pipes will make lots of power at a very high rpm for a short range.

John(XJ)Jeep said:
I'm just saying you guys are contradicting what a lot of modders do with their exhaust systems. Also I just found out why my exhaust does what it does, it gets a boost at 3000 RPM because thats the point where the gases flow fast enough to beat the back-pressure the pipe is creating.

It's NOT beating the back pressure. It's finally hit the spot where the scavenging is working the best. It also has to do with the intake and the cam.

Never seen dyno sheets on auto motors comparing pipes but have on bikes and a good flowing system with a low restriction muffler whops the crap out of open pipes, every time EXCEPT at WFOT.

Go ahead and create whatever system you wish and believe what ya want to.

Sarge
 
John(XJ)Jeep said:
The only reason that I see for back-pressure or "scavaging" is for turbo applications. I know that turbo egines really need a certain amount of
back-pressure to run properly.

Like I said before I'm not that experienced in this area. Iv'e seen very short exhaustes on very powerfull cars (not stock) and usually all the same configuration, short wide pipes that are routed as straight as possible.

I'm just saying you guys are contradicting what a lot of modders do with their exhaust systems. Also I just found out why my exhaust does what it does, it gets a boost at 3000 RPM because thats the point where the gases flow fast enough to beat the back-pressure the pipe is creating.

Which usually are set up to run at WOT (or near that) on a drag strip or race track.
 
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