I hear of people putting cat-back exhausts on there jeeps. I have a 1990 Jeep Cherokee. I can get a deal on a Dynomax Cat-back 2-1/2" diameter cat-back with a turbo muffler. should I go for it? or is it not worth it? because wouldnt jeep have designed the jeeps with a 2-1/2" diameter pipe out of the cat if it was more efficient?

 

Many times, car designers will build a car on how cheap they can do it rather than for performance. As long as the 2.5" pipes are from the cat back, you're okay.

 

I dont know how much difference it would make considering you've still got the stock puny stuff coming off the manifold. Might make a little difference, but I wouldnt think too much.

 

I've heard it makes a good amount of difference because it reduces the back pressure. Doesnt it have a 2-1/2" pipe going from the manifold to the cat? Then it goes to a smaller pipe out of the cat? Thats what I think is goin on I havent really looked yet its been rainy and cold here.

 

I read somewhere that there needs to be a certain amount of back pressure for the engine to work right and that maintaining the stock size from the manifold to the cat will do that. I think you can upgrade to 2.5" all throughout, but I think you'll need to retune after that.

 

I think it all depends on the application. If you just go to a larger diameter pipe you'll lose scavenging & volumetric efficiency. In other words the exhaust pulse traveling down the pipe causes a negative pressure which helps to clear the next cylinder when that exhaust valve opens. If you increase rpms or CI with the same diameter pipe that will cause a bottle neck. If you just uncork the exhaust on a stock motor when you stab the gas pedal, it's like stepping on a wet sponge.

 

x2
nicely worded

 

confusingly worded im doin the same thing what are you talkin about?

 

I have the dynomax cat-back system on my 2.8. Not sure if it makes a difference over stock or not cause my whole system was rusted out when I bought it, so obviously I noticed a HUGE difference. It sounds pretty sweet on my Jeep so it probably sounds a lot better with a 4.0 in front of it.

 

you have a point but unless he chops off his exhaust and runs straight headers or a 3" pipe, there won't be much of a difference. As long as the OP has a cat, muffler, and nothing bigger than 2.5" pipe he should be fine and have enough backpressure. Many XJ owners have upgraded to a highflow cat and performance muffler with 2.5" piping the whole way through, and have seen gains in MPG as well as a little more power.

 

Dynomax made my 99xj run a lot stronger, if I was you I would do it

 

So are you saying its good to upgrade to the cat-back? I got lost when you said its like stepping on a wet sponge. Is that going to be good or bad?

 

i put a 2.5" hi flow cat with 2.5" pipe to a new flowmaster knock off muffler and then a 2.5" tail pipe. put new O2 sensors in and gained about 2 mpg. Did a complete tuneup after that and gained like another 2 mpg. i went from about 13mpg to 18 with all that. then i put a lift and bigger tires on and it dropped back down to like 15 or 16. the exhaust didn't give me a ton of power but it definately helped. plus its one of them supporting mods so once i starting playing with the motor the exhaust is taking care of!!