How free flowing should the XJ exhaust be?

[seanyb505]

Good argument, but you need a baseline before you start saying my exhaust system makes x hp. Even the guy with the 351 ran a basline, its what his numbers were based on. I think thats the main opposition, not that backpressure is good, but that you cant assume your stock engine (which isnt stock) makes 190 flywheel hp. And since you have the air intake too, you have to factor that in. All you can say is "my Jeep makes x hp", not "headers and no cat give you x hp."

 

[wolfpackjeeper]

my experience has been that my stroker runs stronger with a cat on it. I have a map adjuster and played with it both times, it just likes the cat more.

Camaro was the same way with its exhaust. Put my OR ypipe on and no muffler or cats, ran crappy. The next day when I had time to finish it it ran well. Some of that is it not being tuned for no mufflers or cats. However, map and air/fuel were both adjusted after the Long tube install on a dyno, so the tune should not have been off by much.

this thread is now devolving into the backpressure is good/bad debate. It is undoubtably bad, but removing a cat to reduce it is just as bad, has to do with messing with the "tuned section of exhaust" coment above. If you could build a smooth setup exhaust and then tune your air/fuel with a megasquirt or standalone setup(I plan to on my stroker) then you will see gains.

However in the long run, you have to realize that spending $500+/- or so, on a complete custom exhaust, to gain 10 horsepower is a really, really expensive way to do it. It is a 6cyl Jeep, not a big inch drag car. There are plenty of places I will be spending money first to gain performance. Hell, I own a tube bender and a mig setup and I dont consider the performance gains worth my fabrication time.

But in the end it is your time and your money, so if it makes you happy go for it.

 

[BBeach]

What he said ^^. I too like how people start tossing this backpressure debate thinking we believe you need backpressure. Look through the posts (at least mine) before you come to that conclusion. I'm just arguing that it's stupid to remove something that removes the pollution from the air (even if it puts out more CO2 with the cat).

 

[seanyb505]

Did anyone click the hatchbacks link? The guy who was quoted in post 80 on here went on to say, "dont remove the cat, get a high-flow one. it causes all sorts of problems, and it will hurt your emissions." Just thought I would throw that into the mix.

 

[PurpleCherokee]

I'm still waitin on somebody to admit that catless is a better performance option. Period. And as far as "spending $500+/- or so, on a complete custom exhaust, to gain 10 horsepower is a really, really expensive way to do it. It is a 6cyl Jeep, not a big inch drag car", wouldn't it be really stupid NOT to put the best performing exhaust system possible on your project? What's the point of building an engine and doin a buch of mediocre mods?

 

[BBeach]

I'm still waitin on somebody to admit that catless is a better performance option. Period. And as far as "spending $500+/- or so, on a complete custom exhaust, to gain 10 horsepower is a really, really expensive way to do it. It is a 6cyl Jeep, not a big inch drag car", wouldn't it be really stupid NOT to put the best performing exhaust system possible on your project? What's the point of building an engine and doin a buch of mediocre mods?

Im pretty sure you can gather my opinion for my 66st post. What are you planning on doing with the nearly 1" pipe bend on the downpipe? A fluids system can almost be thought of as a chain and that'd be its weakest link...especially for the reduction of backpressure. Maybe you should get some nice headers, then do a straight pipe for a little bit, then a turndown if thats what floats your boat. As far as why wouldn't you put the best performing exhaust, that'd be a quote a bit of money, i'd be fine with a cheap APN header, custom bent downpipe, high flow cat, keep the magnaflow (basically straight through), and then a custom tailpipe. You can do my setup for under $500 and Id rate it at 10-15, possibly 20hp depending on how bad my/your old setup is.

But let me finalize my point. Modern cats, especially a high flow cat, dont create much "power-robbing" backpressure as people say they do. Spend under $100 to be legal, safe, environmentally friendly, and respectable (at least my opinion) and keep a cat on, even in the case you lose less than 1hp. On a side note, I think that no cats (or at least large/multiple) high flow cats on 500hp cars would be benefical....not on a 200cfm jeep.

 

[wolfpackjeeper]

Catless is not a better performance option, it is illegal, stinks, and gives the hobby a bad rep(read:we hate the environment so they close national parks and trails), and from my experience it is a worthless mod. period.

if the difference between "absolute best" and "gets the job done well" is 5hp, then yes, it would be stupid. Especially since you can spend $250 or less and have a great exhaust.

The exhaust system I will be running will cost me about $200:
APN header - $120
07 or newer downpipe and cat - free from a buddy, I cut the dent out and rewelded it.
super turbo muffler by dynomax - $44, sounds good and works well
stock tailpipe is reused
random paint, hangers, ect - $30

and this is only being done because I lost an injector and went lean, the vibrations cracked my manifold, and the rest of the system is welded together.

This is on my 4.7, with a 2000 intake, lt1 injectors, ac delete, bored tb, ect
my dd can stay bone stock, it works so I wont fuck with it till it breaks.

 

[alex22]

I was talking to the owner of a street performance shop today. When I asked him what he thought of removing the catalytic converters the first thing he said is that every engine & exhaust system is different. After that he said that removing the cat was benificial in the 70's and 80's because of the pellet style cat was like a "patatoe in the pipe." The new high flow honeycomb cats have a minimal amount of restriction. On most cars it is not noticable.
For example, They did back to back runs with a 2005 mustang. First test with the cat: 201 rwhp and the second with the cat removed: 201 rwhp. He also added that they are geting 600rwhp out of another mustang, with cats.
The only thing removing the cat really does is make the vehicle louder. He did not have any data on cherokee's.
When I redo my exhaust it will have a high flow cat, as of now I have a rusty stock exhaust.

~Alex

 

[BBeach]

I was talking to the owner of a street performance shop today. When I asked him what he thought of removing the catalytic converters the first thing he said is that every engine & exhaust system is different. After that he said that removing the cat was benificial in the 70's and 80's because of the pellet style cat was like a "patatoe in the pipe." The new high flow honeycomb cats have a minimal amount of restriction. On most cars it is not noticable.
For example, They did back to back runs with a 2005 mustang. First test with the cat: 201 rwhp and the second with the cat removed: 201 rwhp. He also added that they are geting 600rwhp out of another mustang, with cats.
The only thing removing the cat really does is make the vehicle louder. He did not have any data on cherokee's.
When I redo my exhaust it will have a high flow cat, as of now I have a rusty stock exhaust.

~Alex

See, I dont think that a jeep with 200cfm or whatever can produce enough of a flow problem for a honeycombed cat to even cause a problem, nevermind with a nice aftermarket high flow kit. Speaking of high flow cats, any opinions on which is a good model? I think that 5-90 recommended a larger displacement v8 cat that is a direct fit.

 

[alex22]

Whenever I finish my stroker build IF I have the time I will make a quick chunk of pipe and make a run or two with no cat and an O2 simulator. Don't hold you're breath for the results though. I might have it done in a few months.
If it was to happen I would do a few runs with my cat and make sure all is well then remove the cat, incert pipe and simulator then make 2 runs then back to the cat for one run. makeing it as conclusive as possible.

~Alex

 

[wolfpackjeeper]

Any of the high flow ones will work fine, the reason to get the larger v8 one would be to get a stock cat that still flowed the same. On that note it would have to be from a ypipe setup, because if you get one from a car with duals it will flow enough for a 4 cyl.

 

[alex22]

Shouldnt the I6 high flow cats be larger than the V8 because it has all 6 cylinders sending air into the exhaust system vs. the V8 engines sending only 4 cylinders into each side. Or is the logic that a V8 cat would have less stuff in it makeing it flow more air?
~Alex

 

[wolfpackjeeper]

no you got it. but a lot of stock v8's have a ypipe that joins the cylinder banks together. If they have the cat after that union it would have to service all 8 cylinders

 

[BBeach]

I just remember 5-90 saying that at some point a while ago.There's gotta be quite a few single outlet exhausts (obviously not a vette or something) that'll require a lot of flow.

 

[wolfpackjeeper]

yea, just getting one that is rated for 30in more displacement then your motor should be fine.

Think we can get this thread renamed to "The exhaust thread that wouldn't die"

 

[alex22]

yea, just getting one that is rated for 30in more displacement then your motor should be fine.

30 inches larger, why stop there?:dunce:

~Alex

 

[gradon]

I haven't searched yet, but what voltage do you set the simulator to?

 

[copbait]

the backpressure/whatever debate can go on forever for all i care, i just want to know how much gain there is, if any, which is why controlled dyno testing is the only way to produce results that mean anything (to me anyways).

theory is great, but results i still await...

 

[PurpleCherokee]

I haven't searched yet, but what voltage do you set the simulator to?

You don't have to set a voltage on the sim. As long as you get the right one, it'll work.

 

[gradon]

I figured you could make a voltage regulator(like my $5 MAP) and set the voltage(dunno if it's 0-1v like the main o2 sensor). I guess someone is already marketing a ready to go simulator and probably sells it for $30-50, huh?