4.0l back pressure.. power loss

[hayden93]

hi, ok for the last year ive been running a cherry bomb with no cat on my xj il6 4.0L, at the time it sounded great but now its time for emissions! so the other night i pulled the old exhaust system off and set it asside and welded up a new one this time just a cat and nothing else.. its very loud right now. any way i do plan on cutting a section out and throwing in a supper 44 to guite it down (im over the loud jeep thing).

any way heres my problem since swapping over exhaust systems ive notice a substantial drop in power and torque.. i did some reserch online and it sounds like the more back pressure on the il6 the more torque but i seem to be having the opposite effect, wouldn't a cat provide more back pressure than a cherry bomb??

i really dont want to put a super 44 on it if im going to lose power over it.. i will just switch back to what i had before and deal with it being loud if it means i get my power back

both pipes are 2.25" all the way down no leaks

Thanks

 

[yossarian19]

First off,
Back pressure is a myth. It's a misunderstanding of what's taking place in an exhaust. Restriction = bad for overall HP / TQ.
The seeming contradiction, where some motors (or users) seems to like a smaller diameter system, comes about from an effect called exhaust scavenging.
As each cylinder fires & it's exhaust valve opens, the expanding gas is moving fast enough out the pipe to create a vacuum behind it. This doesn't take place 100% of the time, it's related to engine RPMs & pipe diameters. At the RPM where this is happening for a given exhaust system, the engine will have *no* back pressure, so it is really flowing as well as it ever will on a percent efficiency basis. This skews HP / Torque toward this RPM range where scavenging (vacuum behind the pressure wave) takes place.
So that being said, a couple things occur to me that may apply to your situation.
I suspect your catalytic converter (though they are really pretty good these days) is more restrictive than your straight-thru Cherry Bomb exhaust was. It's decreasing your power some. Installing a Flowmaster before or (better, I'd think) after the cat isn't going to harm anything because the flowmaster (or whatever else) is going to flow better than the cat you have already put in.
If you want to, you can go to 2.5" after the cat. Should help to minimize restriction. If you keep the factory downpipe / cat setup (or sizes, really) you will keep scavenging at a lower RPM, which will help down-low torque. This is why most people say the 4.0 loves "back pressure" - because they drive at low RPM and love the torque the 4.0 makes down low.
HTH.

 

[hayden93]

First off,
Back pressure is a myth. It's a misunderstanding of what's taking place in an exhaust. Restriction = bad for overall HP / TQ.
The seeming contradiction, where some motors (or users) seems to like a smaller diameter system, comes about from an effect called exhaust scavenging.
As each cylinder fires & it's exhaust valve opens, the expanding gas is moving fast enough out the pipe to create a vacuum behind it. This doesn't take place 100% of the time, it's related to engine RPMs & pipe diameters. At the RPM where this is happening for a given exhaust system, the engine will have *no* back pressure, so it is really flowing as well as it ever will on a percent efficiency basis. This skews HP / Torque toward this RPM range where scavenging (vacuum behind the pressure wave) takes place.
So that being said, a couple things occur to me that may apply to your situation.
I suspect your catalytic converter (though they are really pretty good these days) is more restrictive than your straight-thru Cherry Bomb exhaust was. It's decreasing your power some. Installing a Flowmaster before or (better, I'd think) after the cat isn't going to harm anything because the flowmaster (or whatever else) is going to flow better than the cat you have already put in.
If you want to, you can go to 2.5" after the cat. Should help to minimize restriction. If you keep the factory downpipe / cat setup (or sizes, really) you will keep scavenging at a lower RPM, which will help down-low torque. This is why most people say the 4.0 loves "back pressure" - because they drive at low RPM and love the torque the 4.0 makes down low.
HTH.

thank you for you in depth reply, sounds like you know what you are talking about! ok, after reading your reply, heres my issues with your explanation... when i used to tap the throttle at a stand still the jeep would pretty much lift up the whole front end and bounce around if you know what i mean.. now i dont have any of that its all sluggish and no quick throttle response, it feels like im quite frankly driving around with my a/c constantly on.. also did forget to mention that the cat im running is a magna flow so it is meant to relive more back pressure than oem cats.. still somethings just not quite adding up.. every thing your saying makes total sense, but if back pressure gives you more low end TQ shouldn't my jeep be bopping around when i tap the throttle??
once again thanks for your help, let me know if you think of anything else.:cheers:

 

[Dr. Dyno]

if back pressure gives you more low end TQ shouldn't my jeep be bopping around when i tap the throttle??

Backpressure gives you LESS TQ. It's good exhaust gas scavenging at low rpm that gives you good low rpm torque.
Since the only thing you've essentially changed was to add a cat, there's a possibility that the cat is plugged up and in need of a laxative.

 

[yossarian19]

^ What he said.
Back pressure is always bad.
A better way to say what I said before would have been this : a smaller pipe diameter will increase the speed of the exhaust gas pulses and tune the RPM at which scavenging takes place. It limits overall flow but keeps down-low torque.
A catalytic converter doesn't do that... it just limits flow and power.

 

[hayden93]

sounds good, its all making sense.. ill figure it all out. my father in laws gunna shit a brick when i tell him all the new info i know and proof ive got.. hes always told me how when i did my exhust that i was just throwing my power away, haha thanks for your guys help =)