Back pressure!!! HOW LOW CAN I GO????

[kenny schmitt]

I have a 96 xj and my current mods are RE 5.5 with 33 x-terrains, fram air hog filter, centerforce clutch and no cat with dynomax muff. i want to remove the tailpipe and or the muff but i dont wnt to experiment. who has done this or know what will happen

 

[outlander]

you will be really annoying to everyone around you.









Seriously it will be loud and sound like shit without atleast a muffler.You can punch out the cat but if its not clogged I wouldn't mess with it.I removed my tailpipe and notice more fumes while driving even with the windows up.Oh and if the EPA asks you why you did any of this,you dont know me:exclamati

 

[JNickel101]

Getting rid of backpressure is going to rob you of low end torque as well....

Flame on.....

if you want an awesome sounding muffler, get a twister - its an 18 inch long, 5 inch diameter tube muffler with a corkscrew inside it. sounds awesome.

get a high-flow cat. removing your cat doesnt do shit, unless its an old clogged or busted up cat....

 

[5-90]

Getting rid of backpressure is going to rob you of low end torque as well....

Flame on.....

if you want an awesome sounding muffler, get a twister - its an 18 inch long, 5 inch diameter tube muffler with a corkscrew inside it. sounds awesome.

get a high-flow cat. removing your cat doesnt do shit, unless its an old clogged or busted up cat....

While I didn't do any dyno pulls while I had the exhaust off, I recall having the exhaust off from the cat back on my 1987 years ago, while I was getting a flange welded up. I don't think I changed the way I drove - but it sure did feel like I pulled a lot harder off the lights.

It may be a faulty memory - but it tends to agree with what I've been reading in Taylor, Ricardo, et al - which can be summed up thusly: "Back pressure is spinach."

Granted, since most exhaust systems aren't designed or tuned very well for the engines they're on, or the applications they're used in, they may be trying to use backpressure to mimic some effects of exhaust tuning (I don't know how) but an increase in backpressure would increase pumping losses, increasing parasitic drag. I'd have to dig out my books to substantiate that a little more.

Don't suppose you have a source you can cite, do you? I'd like to check it against my own, and correlate the information (I may be missing something, after all...)

 

[hotrod07]

Getting rid of backpressure is going to rob you of low end torque as well....

Flame on.....

if you want an awesome sounding muffler, get a twister - its an 18 inch long, 5 inch diameter tube muffler with a corkscrew inside it. sounds awesome.

get a high-flow cat. removing your cat doesnt do shit, unless its an old clogged or busted up cat....

i removed mine and it made quite a bit of difference in the way it sounded.

my system has 2.5in to a flowmaster then out the back.
befor it was the same 2.5in to a cat,to the same flowy out the back and it made mine sound more throaty

the cat was perfectly fine when removed

 

[Lowrange2]

Someone stole my kitty.

 

[5-90]

i removed mine and it made quite a bit of difference in the way it sounded.

my system has 2.5in to a flowmaster then out the back.
befor it was the same 2.5in to a cat,to the same flowy out the back and it made mine sound more throaty

the cat was perfectly fine when removed

It would sound different - even though it's not designed as such, I think the catalytic converter case acts as a resonator (mainly because of the increase/decrease in cross-sectional flow area) to reduce noise. Apart from that, it shouldn't matter.

 

[amschnellsten]

removing the cat does make a difference, minimal percentage and you will not feel the difference but it does. There is not much restriction to a cat (hehe) but they are not "high flow".

 

[harwood39]

my exhaust is missing from cat back, it sounds like ass but does pull harder than before(cat is also hallow)

 

[broqenjeep]

after destroying 2mufflers in a year from rocks I put 2.25 in from cat back lost back pressure lost low end torque and loud!!! cant hear cell phone or stereo even with windows up

 

[trippled]

after destroying 2mufflers in a year from rocks I put 2.25 in from cat back lost back pressure lost low end torque and loud!!! cant hear cell phone or stereo even with windows up

x2. mine sets car alarms off. but im broke so no muff for me...

 

[jeeperjohn]

When you lose backpressure you change the cylinder scavenging effect. This can cause some of the unburned fuel and air to be pulled out of the cylinder before the exhaust valve closes causing loss of torque and heating up your pipes more than necessary. Having an effective exhaust is not as simple as removing restrictions. There are small diameter exhausts on many high powered engines because tuning is everything. I agree with what 5-90 said about the stock exhaust being a poor compromise. The engineers who design the exhaust for stock vehicles have to take in to account the cost of manufacture, clearance of front suspension upon full up travel, and numerous smog and noise requirements. The best performance comes from a system tuned for the specific application.

 

[dfreeman616]

so does anyone know what kind of effect removing the cat has on fuel economy?

 

[89xj]

so does anyone know what kind of effect removing the cat has on fuel economy?

not enough to get rid of it.

why do some feel the cat is such an enemy for power.

 

[IslanderOffRoad]

not enough to get rid of it.

why do some feel the cat is such an enemy for power.

because they're ignorant, or really old and stuck in their ways.

if you think the cat is the problem, odds are its not. if your cat really is clogged, BUY A NEW CAT.

My cat was clogged. I replaced it with a brand new Magnaflow high flow cat, installed by a local shop for $150 out the door. The difference is amazing.

 

[5-90]

When you lose backpressure you change the cylinder scavenging effect. This can cause some of the unburned fuel and air to be pulled out of the cylinder before the exhaust valve closes causing loss of torque and heating up your pipes more than necessary. Having an effective exhaust is not as simple as removing restrictions. There are small diameter exhausts on many high powered engines because tuning is everything. I agree with what 5-90 said about the stock exhaust being a poor compromise. The engineers who design the exhaust for stock vehicles have to take in to account the cost of manufacture, clearance of front suspension upon full up travel, and numerous smog and noise requirements. The best performance comes from a system tuned for the specific application.

Yah - I'm familiar with scavenging, and the resultant role of exhaust pressure waves and suchlike in improving scavenging (which is why tuned, equal-length headers tend to allow an engine to make more power - improved scavenging = larger fuel-air charge per cycle.) However, I'm having some difficulty understanding how backpressure can actually help scavenging - seems to me it would either cause the pressure waves in the exhaust system to "pile up" on top of one another until there's no "room" between them, or it would cause a reduction in exhaust charge speed to the point where you don't have efficient scavenging anymore. Have you a source? I'd like to read it in original, if I may (it's not that I'm overtly doubting you - it just seems to run counter to what I've learned so far. So, I may need to learn something else...)