exhaust modding...

[fast_Z28_guy]

so doing some exhaust investigation while changing oil today. i found out that the exhaust from the engine back to the catalytic converter is a nice mandrel bent 2.5" pipe.

HOWEVER...

the exit pipe from the cat is 2.25"

and then after the muffler, it goes into some fairly constrictively bent "over the axle" tubing and out the back.

so i was thinking...cut the exhaust just before the cat, run a short 2.5" pipe to a standard catback exhaust that is 2.5"

minimal restriction......good flow....and i'll be able to put it all back on if emmissions comes into play in the future.

anyone done it??

or how bout just gutted the cat?? any noticable gain? there should be. i know you can pick up 5-10hp on a single cat setup on most V8 sports cars...so it's logical similar gains can be had from this.

 

[laxpug19]

what does the term "straight pipe" mean?

 

[Bounty__Hunter]

Not worth the trouble, most late-model catalytic converters don't restrict enough to bother removing. I would leave it alone, or go with a performance cat-back.

I see your Z in your sig, what jeep are you referring to?

 

[fast_Z28_guy]

"what does the term "straight pipe" mean?"

basically any unrestricted piece of exhaust tubing...ie...no cats, mufflers or anything.


i know cats these days are not terribly restrictive, but i don't like the idea of the exhaust going from a nice 2.5" pipe...down to a 2.25" pipe.

kindda makes it pointless to get a 2.5" catback setup if there is 8" of "restrictive" (i know...not really..but you see my point) smaller diameter pipe.

it was just a thought

it's like air intakes. you can run the fattest air filter, and widest inlet tube...but if the throttle body is the size of a midgets sphincter...you will only see a certain amount of gain.

 

[laxpug19]

Are you saying if i was to get a cold air intake it wouldnt make that much of a distance? you think i should get a whole new throttle body or just a spacer?

 

[lethaldose]

my 01 has 2 precats right at the header so i removed the big cat down wind right before the exhaust flange put in a section of 2.5" pipe felt like it breathed easier but nothing really noticeable besides louder haha

 

[fast_Z28_guy]

hmmm..

are you sure those are "pre-cats"??

the 03 has something similar but to me they looked like flex joints to allow for engine twist without stressing the exhaust mounts.

but i could be wrong.


"Are you saying if i was to get a cold air intake it wouldnt make that much of a distance? you think i should get a whole new throttle body or just a spacer?"

um...you in the right thread?? we're talking about exhaust...not intake

but to answer your question...a CAI will make a noticable difference. besides i picked up a consistent 1.5-2mpg increase in economy both city and highway

 

[laxpug19]

it's like air intakes. you can run the fattest air filter, and widest inlet tube...but if the throttle body is the size of a midgets sphincter...you will only see a certain amount of gain.

are you in the right set of mind? YOU SAID IT!!

 

[fast_Z28_guy]

sorry bout that...wasnt reading far enough back...was confused as to the context of the question.

as i said...a CAI will help because of how restrictive the stock air box is, as well as because it will draw in cold air.
cold air= denser air = more oxygen to burn per given volume = more power

is the stock TB restrictive...probably a little. i'm new to the jeep world so i don't know how much. but i'm sure there is a little power to be gained from going a little bigger.

i would say a good CAI setup is more important than swapping TBs if you are just starting modding.

basic performance upgrades start with:
easier air in.....easier exhaust out

 

[fast_Z28_guy]

back to my original discussion...

still wondering if it's worth swapping out the cat for a piece of 2.5" pipe.

it will replace the stock cat and the pipe that is 2.25" behind the cat. the pipe prior to the cat is 2.5"...and the reduction in tubing diameter is nearly a 20% reduction in volume...that's alot!!

 

[98XJ]

"what does the term "straight pipe" mean?"

basically any unrestricted piece of exhaust tubing...ie...no cats, mufflers or anything.


i know cats these days are not terribly restrictive, but i don't like the idea of the exhaust going from a nice 2.5" pipe...down to a 2.25" pipe.

kindda makes it pointless to get a 2.5" catback setup if there is 8" of "restrictive" (i know...not really..but you see my point) smaller diameter pipe.

it was just a thought

it's like air intakes. you can run the fattest air filter, and widest inlet tube...but if the throttle body is the size of a midgets sphincter...you will only see a certain amount of gain.

As exhaust goes from the engine to the tip of the tailpipe it does cool down a very lot. From high school chem we know volume and temperature are proportional to each other. So as that temperature decreases the volume also decreases as well as you'd like to keep the velocities at a certain rate. In any case, I always thought the downpipe was 2.25" but I could be wrong (98xj idk about newer ones). I think keeping it 2.5" the whole way may be slightly beneficial though. And as far as throttle bodies, they come 60mm necked down to 55mm whereas the intake manifold opening is 62mm. You can have them bored out and get a few extra ponies with this as well. A spacer isnt a big deal but if you can get it for very cheap go for it. As far as very small throttle body openings having no power...thats not 100% true. IIRC, most SAE cars have a 20-22mm or so throttle body but using velocity stacks they can make it well worth it, putting out more ponies than a jeep 2.5 will. 8) In any case, its all a balancing act.