How free flowing should the XJ exhaust be?

[copbait]

Pretty soon (in about a week) I'm gunna have an actual dyno sheet that I can use to stop this debate once and for all! Those who think that cats aren't a performance bottleneck beware :firedevil lol.

i hope you'll be doing back to back runs, one with cat and one without, same weather conditions, engine temp, etc. eagerly awaiting your results

 

[PurpleCherokee]

Well I can't do back to back since it's already installed. But I do have a new Marshal motor so I know that its specs should be really close to stock. So if I can dig up the stock torque curve I'll just compare to that. I'm lookin forward to it too. Cause my ass-meter reports quite a gain lol. So we'll see if I can see it on paper. But I already know what people are gunna say, since I'm runnin a Clifford header... you got all that power from the header! I'm pretty sure even Jack Clifford himself (RIP) would tell you that the header doesn't add that much power lol.

 

[gradon]

So you got a new motor, put the clifford on(w/ stock downpipe, or does it come with one?) originally had the cat on, and now you've removed it and feel the difference? Or are you comparing your old motor w/ cat with the new motor w/ clifford and no cat? You can't use the factory dyno graphs either as some motors off the assembly line might be +/- some hp and ft/lbs. If that Clifford made no difference, then you wouldn't have gotten it--of course it's better than the factory header. Anyhow, I'm neutral on this one, just make sure the evidence reflects the one variable.

 

[seanyb505]

Im interested in the results, but you kinda need to run a cat as a control, you cant just say, I put in a new motor, header and probly a couple other things and look at all the gain. Whether taking it off yields power or not, your results will be ignored because of improper testing.

 

[copbait]

It is illegal to remove if it's in good working order under 80k miles ($10k fine sounds about right too), but don't you just need a straight piece of 2.25" pipe or gut the OE cat (and maybe combine the two)?


OK now for the dumb question, will a cat thief get fined in the same manner if caught in the act of stealing a working cat besides the theft charge?

technically it's not illegal for offroad use. and yea it should be very simple to just weld a straight pipe with flanges. many shops just won't do it though if they see the intended use. you could probably just measure it up yourself and bring them a simple design on paper and not have any problems. might even give that a try later this week...

 

[copbait]

Im interested in the results, but you kinda need to run a cat as a control, you cant just say, I put in a new motor, header and probly a couple other things and look at all the gain. Whether taking it off yields power or not, your results will be ignored because of improper testing.

yea pretty much, even if the runs are done with only one variable changing between them (cats), they will still be based on a different motor and other parts it sounds like. so the results won't be an indication of gains or no gains with a stock setup.

but it would still be very interesting to see a run done with the cat and another with a straight pipe, even if with a different motor and headers, just for s&g. who doesn't enjoy seeing dynos

 

[seanyb505]

who doesn't enjoy seeing dynos

I dont. I hate em....jk. But in all seriousness if you want your results to be taken seriously, make five runs with the Jeep with the cat on. Throw out the high and low, and average the rest. With the same Jeep take the cat off and do the same five tests, throw out the high and low again and average the remaining three. This should all be done on the same day, preferrably with no change in weather. I think that would be the most reliable test.

 

[PurpleCherokee]

I just spoke with Marshall, and as I suspected, there's no power difference whatsoever in their motors vs. the stock motor. Only the normal minute variances from motor to motor just like from the factory. So comparing a stock dyno sheet to my modified Marshall motor would be just as accurate as if it was a modified stock motor. The question is, how much power did I pick up from the exhaust setup. The ONLY other mod is intake. Which I think we can all agree doesn't add more than 5HP TOPS. It'd be really nice if someone else could dyno their jeep w/a straight through muffler set-up (or no muffler) but WITH a cat. However, the MAIN argument that I'm gunna end once and for all is the MYTH that a free-flowing exhaust system kills power.

 

[wolfpackjeeper]

Free flowing exhausts Move around the powerband. And a 2.25in free flowing exhaust is going to make more power on a cherokee than a 3.25 in exhaust, due to flow velocity dropping off. Every time this debate comes up people start comparing if a cat is restrictive, then it moves to how big of an exhaust, then it moves somewhere else. Length, diameter, smoothness, obstructions all matter. Making one dyno run is not going to settle or refute anything. It will just add one datum point to be considered.

2 settle this would require the following pulls:

1 run bone stock
1 run stock with removed cat
1 run larger diameter with cat
1 run larger diameter with removed cat

this is the minimum number needed and more would probably be better, and considering dyno runs in Raleigh cost 75 bucks for a 3 pull average, I will not be dropping the money to do it. I like the exhaust on my stroker, and I like the exhaust on my stocker, both are very quite and serve me well.

 

[JohnX]

Free flowing exhausts Move around the powerband. And a 2.25in free flowing exhaust is going to make more power on a cherokee than a 3.25 in exhaust, due to flow velocity dropping off. Every time this debate comes up people start comparing if a cat is restrictive, then it moves to how big of an exhaust, then it moves somewhere else. Length, diameter, smoothness, obstructions all matter. Making one dyno run is not going to settle or refute anything. It will just add one datum point to be considered.

2 settle this would require the following pulls:

3 run bone stock
3 run stock with removed cat
3 run larger diameter with cat
3 run larger diameter with removed cat

this is the minimum number needed and more would probably be better, and considering dyno runs in Raleigh cost 75 bucks for a 3 pull average, I will not be dropping the money to do it. I like the exhaust on my stroker, and I like the exhaust on my stocker, both are very quite and serve me well.

Fixed it for you....one run leaves way to much room for errors. You need at least a few to average, because the dyno has a slight amount of "tolerance" in it.

 

[wolfpackjeeper]

ok, by 1 run I ment 3, sorry. I consider 1 run to be the average of 3 dyno pulls. Average it, then move to the next series.

 

[PurpleCherokee]

I'm doin three runs. And it will say quite a bit about exhaust configuration. It does WAY more than just "move" the powerband around and that's the main thing I'm gunna prove when I get my dyno results.

 

[copbait]

i would be happy just seeing the first two configurations on the dyno. the theory of using a larger diameter pipe on a stock six cylinder has been widely debunked.

 

[GrimmJeeper]

i considered removing the cat, but i couldn't even find a damn shop to fab up a straight pipe with 2 flanges for me, they all refused. i don't think there's a law against doing that necessarily, they just didn't have the stones. i suspect there are licenses at risk in addition to the fines, but maybe they were just being lame. this was in cali.

they refused because ITS ILLEGAL TO REMOVE SMOG EQUIPMENT, ESPECIALLY IN CALIFORNIA. :twak: :twak: :twak:

 

[PurpleCherokee]

I've said this before and I'll say it again... I'm so glad I don't live in CA

 

[stephenspann27]

On my '95 Z-28 my dad and I took the heat shield off cut two slots on teh bottom of the cat and pealed it back, placed and welded in a straight pipe, hamered back down the window we made, welded it up,a nd replaced the heat shield... you can't tell by looking at the cat anything has been done. If you put a pipe straight through them it supposidly pops back less under decerlation.
Now the reason we did this is because the cat fell apart inside and was rattling.. I though tI had major engine problems..

From a drivers prospective.. the major drawback is the odor.... I can't stand the way car's smell without cats.. and I smell it all the time.. in the drive through and stuff..

 

[copbait]

they refused because ITS ILLEGAL TO REMOVE SMOG EQUIPMENT, ESPECIALLY IN CALIFORNIA. :twak: :twak: :twak:

actually i only asked them to make the pipe section, so just measure and weld. no removing anything. they wouldn't even do that, bums!

 

[PurpleCherokee]

On my '95 Z-28 my dad and I took the heat shield off cut two slots on teh bottom of the cat and pealed it back, placed and welded in a straight pipe, hamered back down the window we made, welded it up,a nd replaced the heat shield... you can't tell by looking at the cat anything has been done. If you put a pipe straight through them it supposidly pops back less under decerlation.
Now the reason we did this is because the cat fell apart inside and was rattling.. I though tI had major engine problems..

From a drivers prospective.. the major drawback is the odor.... I can't stand the way car's smell without cats.. and I smell it all the time.. in the drive through and stuff..

I wondered why my jeep stopped poppin! They only time I can get it to pop is if I accelerate hard in second up to around 70, let go, let it go rich for a while then tap the pedal and BAM... blue flames out each pipe :firedevil

 

[wolfpackjeeper]

I am sorry dude, but 3 runs or 20 runs, the only thing it will tell you is the power and tq of your truck as an average. It says nothing about removing or not removing your cat. There is no control to your experiment and no test group. You have a new motor, a long tube header and a different intake. It may give you a number that you are happy with but it will not definitively prove anything at all.

 

[seanyb505]

I am sorry dude, but 3 runs or 20 runs, the only thing it will tell you is the power and tq of your truck as an average. It says nothing about removing or not removing your cat. There is no control to your experiment and no test group. You have a new motor, a long tube header and a different intake. It may give you a number that you are happy with but it will not definitively prove anything at all.

x2, you need to run a control, and it cant be someone elses Jeep, its a different engine and a different dyno. You arent solving anything by only running your configuration.