Exhaust question for off road only xjs

[FlexdXJ]

Not trying too start a pissing contest on who is right or wrong .If an engine that was designed with back pressure ,doesn't need back pressure then why did you contradict your self with this

"The reason people complain about loosing low and mid range torque is because they have installed an exhaust setup that is too big for the operating range the engine is making power in. They end up with a system tuned for a peak of 6000rpm and the engine peaks at 4700"

is this not loss of back press? and we are talking a bought an in line 6 cylinder no a v12 in a Indy car . Jeep engineered this engine with back pressure involved so if you take it away what do you gain?

pressure is created by flow through restriction ex. catalytic converter,exhaust tubing bends,muffler . so if you take that away you get crap except attention from your buddies saying that sound like ass and THAY WILL ALL LAPH AT YOU .

WHen you get into strokers and high performance engines getting more air in/out of it does help. Why do you think diesels gain so much power when they get intake and exhaust mods.The factory crap is choking them. Like i said before though, a muffler made my brothers civic a completely different car and it was just the OEM one from the parts store.

 

[techno1154]

I am not sure wether I agree with the term "back pressure" or not. And then again it may be the correct term,....
When I changed the cat back to a high flow cat and Rusty's Flow Master series 50 with 2.5" pipe all the way back, I experienced the classic slow off the line symtoms along with the very aggravating drone at crusing speeds. I called Flow Master and spoke at length with an engineer about my XJ and and what I did not like about it. He researched the 4.0 engine, RPM range, displacement and length of exhaust pipe. What was his conclusion?

It was OK to have 2.5" pipe up to the inlet to the FM. From the FM to the tip need to be 2.25" max. Yes, he did give reasons. What were the reasons?

Based on the cubic inch of the 4.0, the normal crusing RPM based on the gear ratio and tire size and the length of the exhaust pipe the exhaust gases was getting too cold to flow properly through the 2.5" pipe. His exact words were "The exhaust gases was loosing velosity". His advice was:

1) Get a higher cubic engine.
2) Regear so I could keep the engine at a higher RPM.
3) Reduce the size of the pipe from the FM (Flow Master) out back.

Me, not intending to change the engine to match the exhaust and at the same time loved to cruise at 80 MPH below 3,000 RPM, I opted to reduce the pipe that ran along side the fuel tank. According to the butt dyno, the off the line get up and go improved and the droning went away. Here is where I am forced to agree on the term"back pressure'. By reducing the size of the pipe even if it is just the last 24" of it would increase the back pressure.

His other advice? Get a configurable tip and play around with the plates until I get what I want.

 

[simsfamxj]

. In this case my turm back pressure was rong sould of used exhust flow. Now i see .my bad

 

[casual]

I run a stock Renix header with a 2.5" collector welded on, my stock one cracked. A 2.5" custom down pipe, 2.5" piping, stock cat into a 40 series. Its dumped right before the axle.

I'll be getting a high flow magnaflow cat here shortly.

A buddy runs a stock header, stock down pipe, stock cat with a dump welded straight to the cat. Its loud but it sounds amazing! Plus it doesn't stink lol. Its dumper right where the transfer would be.

 

[Hoooper]

lots of missuse of the word "backpressure" here and lack of use of "exhaust velocity". no engine needs backpressure, thats a fact. ALL engines need exhaust velocity. High exhaust velocity leads to high Tq. Removing all the exhaust equipment and going to a large pipe creates low exhaust velocity.

1) research the difference between backpressure and exhaust velocity
2) resume pissing contest

 

[usmclovesXJ]

So i was wondering what you guys where doing with your exhaust when you use the cherokees only off road. Do you take the cat out and run just a muffler? what do you do with the o2 sensor after the cat? Any input would be great. Thank you

Hey check out www.rustysoffroad.com its got a great mufflers flowmaster 40 or 50 and a cherrybomb muffler (non glass pack).
anyways keep the the cat or go high flow. you will be greatful. personaly i dont like the sound of stright pipes. i like a deep deep rumble. but its all personal chocies. but to be safe and to keep the fuzz off your tail:roflmao:keep the cat.

 

[crimsondragon]

Did anybody hear of Bernoulli's Principle? As velocity increase, pressure decreases and vice versa. What's a prime example of Bernoulli's Principle? Carburetors. The venturi is exactly where pressure decreases and velocity increases. It's this pressure drop that causes fuel to flow out of the fuel vent. The same can be applied to exhaust systems. You ever see the rear of a jet engine? Ever noticed the cone? That's what we call a divergent cone. It's this cone that causes an overwhelming expulsion of exhaust. Now because the combustion chamber in the jet engine has higher pressure than the rear of the engine, guess what it creates? Thrust. By enlarging the diameter of the exhaust after the cat, you basically cause the exhaust to decrease velocity and increase PRESSURE.

 

[Hoooper]

this is not really true as it is based on physics of frictionless surfaces, which an exhaust pipe is most certainly not. There is a proper exhaust pipe size depending on your mass flow and equivalent tube length which will result in the most power. It really is that simple. Adding crap to the exhaust if it is properly sized (cats, mufflers, etc) will decrease power.

 

[usmclovesXJ]

well then

 

[Darky]

Did anybody hear of Bernoulli's Principle? As velocity increase, pressure decreases and vice versa. What's a prime example of Bernoulli's Principle? Carburetors. The venturi is exactly where pressure decreases and velocity increases. It's this pressure drop that causes fuel to flow out of the fuel vent. The same can be applied to exhaust systems. You ever see the rear of a jet engine? Ever noticed the cone? That's what we call a divergent cone. It's this cone that causes an overwhelming expulsion of exhaust. Now because the combustion chamber in the jet engine has higher pressure than the rear of the engine, guess what it creates? Thrust. By enlarging the diameter of the exhaust after the cat, you basically cause the exhaust to decrease velocity and increase PRESSURE.

Not sure Bernoulli's applies to XJ exhaust...

 

[techno1154]

Adding crap to the exhaust if it is properly sized (cats, mufflers, etc) will decrease power.

The XJ or at least the 1996 that I have, the pipe size off the manifold/header through the first turn under the engine is 2". It changed to 2.5" under the engine to the cat. Coming out of the cat, it go back to 2" through the OEM silencer and continuing all the way out back. Judging from the comments from those in the know, it seem the XJ exhaust is well optimised from the factory for the cubic inch and RPM of the 4.0L. The only real improvement seem to come when a good designed header is added. Mos aftermarket headers also seem to be only 2" at the collector.

When I installed the Rusty's 2.5" cat back I thought it help out in the HP department since everyone say that it did. Well, here in Miami, going through the stop lights, the butt dyno did not desern a change :eyes: . High humidity and a full tank of gas would make the assumed 5 hp gain un-measuresable. What got me was the anoying drone of the exhaust at the RPM I usually drive wether it was at low speed in the city or the normal cruise speed on the highway. Reducing the pipe out back reduced the drone to bearable levels.

 

[simsfamxj]

Headers do so much because "if thay are of quality and designed corect" the equal length tubes creat a vacuum for the next cylinder helping to scaveng burnet air/fuel mixture from the cylinder . With this effectit alows more clean air/fuel to enter the cylinder creating more POWER. Also changes hp and torque curves. If i rember right. PISSSSSSSING ON THE TREE MORE BEERS PLEASE.

 

[nblanton]

I got to this thread due to a cat that failed on my recent trip to Radisson, Quebec last week. After 380 miles of sh*t two lane roads my cat decided to fall apart and clog the exhaust. Couldn't get the engine to rev over 2000rpm without blowing exhaust out of the manifold gasket and header collector joints. That is hella backpressure!!! Sounded like the most sick and wounded animal you could ever imagine. I managed to get the cat removed and a straight pipe jammed in its place for the time being. I will say, the XJ is much more alive now than at any time since I picked it up. The mileage jumped up about an extra 1 mpg to around 21 mpg now. So, in my case no cat vs. 330K blown cat is better. I plan on replacing the cat and muffler soon, probably going to go with hi-flow cat and a Magnaflow. Will report back on the seat-of-the-pants and gas pump difference when I can.

 

[CherBear]

Nothing was changed except the car gaining a muffler and it drove like a completely different vehicle. An OEM muffler no doubt adds back pressure. I'd say that had a lot to do with it. Now, can you back up what you've said?

Correlation does not imply causality. I am sure a stock civic did run like crap with no muffler! So does a stock jeep!