I need cheep power

[XJEEPER]

MMMMM....Good.

 

[Bloose]

Ok, I looked through my "research material" and did not find the writeup on exhaust but someone did an excellent job on a writeup recently giving pipe sizes and tech info on why this set-up is better than that ect...

If someone finds it could you please post it here?

Next question:

I am trying to make the best flowing exhaust with stock manifold (I can't afford a new manifold) I am after MPG NOT HP!

My thoughts are this; go from the exhaust outlet into 2.5" to the hi-flo cat, through the cat to a flowmaster40 muffler then go 2.25 out to the tailpipe.

Would this work or do I need to stay at 2.25 all the way?

If I were going to do the dual I would just make a "Y" pipe right out of the hi-flo cat and go into the dual mufflers but with any dual system on any I-6 I think that it would be too free flowing and you would loose low end torque, response and even mileage - just my opinion.

JC95

Once your past the cat I don't think it matters much if it flows too much. So if a dual look is what your after, go for it. As far as the 2.25 tail pipe that should be no problem. As the exhaust gets further from the cat it cools, as it cools it has less volume so it can flow through a smaller pipe. As far as a 2.5" pipe possibly being to too large off the manifold I don't really know. I don't think it would be a problem but without dyno numbers I'm only guessing. Personally I think the stock exhaust tube exhausts are pretty good with the excection that they like to crack. For flow though I think they're fine.

B

 

[Bloose]

Hey, what's wrong with an old soup can? A Campbell's soup can does the job very well. You can spray paint it on the outside in your favourite colour to make it look good, or even powder coat it if you feel that way inclined.

For the Jeep the soup can would be fine, but for the Ford? I don't think so. I'm thinking Aluminum tube, I'll see what we have around work. I can always use a piece of 3" conduit.

B

 

[FUNKYTEE5]

Once your past the cat I don't think it matters much if it flows too much. So if a dual look is what your after, go for it. As far as the 2.25 tail pipe that should be no problem. As the exhaust gets further from the cat it cools, as it cools it has less volume so it can flow through a smaller pipe. As far as a 2.5" pipe possibly being to too large off the manifold I don't really know. I don't think it would be a problem but without dyno numbers I'm only guessing. Personally I think the stock exhaust tube exhausts are pretty good with the excection that they like to crack. For flow though I think they're fine.

B

I just fabbed up a 2.5" mandrel bent system from header back, I had no issues with 2.5" bolting up to the header flange. I do have a Borla, but the stock flanges attach in the same manner. The only thing that I found that may be an issue is the front pinion yoke clearance under compression with under 3" or so lift. I think that 2.5" may be slightly large for stockers with no mods, where 2.25" would be ideal. Regardless though, the stock 91-96? HO header pipe sucks! It's all dented and kinked up! Not ideal for smooth flow The later 96+ 99+? Jeeps do have a much better pipes coming off of the header though. I agree with B that the stock manifolds flow well. I didn't feel much between stock/Borla, but good headers will allow for better gains when adding mods as flow increases.

FUNKYTEE5

 

[Sarge]

Once your past the cat I don't think it matters much if it flows too much.
B

Sure does. Just had a problem on my work van. Rotted out muffler and busted off tailpipe (had duals out the back). Ran like crap under about 3000RPM. Chopped the muffler off and test drove it. Same thing. Replaced the muffler and tail pipe and we're back in business. The stock setup was 2.5 back to the muffler and two 2.25 tail pipes. That was way too expensive for me so it was replaced with a single output muffler. But a short test drive with a single 2.25 output was an eye opener. Ran pretty good...on the top end, better than ever before. But the low end felt boggy. Ran a 2.5 and she's runs good low to mid.

Seen the same thing on various other cars as well. Easiest way to think of it is that any change anywhere within the entire system affects the flow thru-out the system.

Sarge - bear in mind I aint no engineer and have never played one on TV either...well OK I had the title of engineer at a previous job once but it was writing manuals.

 

[jeepcherokee95]

I think I am going to try the 2.5 from the manifold to the cat - out the cat to the muffler but go 2.25 from the muffler out to help keep the back pressure up for low to mid performance.

I am running stock 3.55 gears and 31x10.50's on a 2" lift. I am nowhere financially ready for this yet but the dream is still alive!!

Hope someone else tries this and lets me know the outcome otherwise we will have to wait till some money rolls in for me to be the guinea pig.

Thanks to all:

JC95


PS:

My shop just has the flowmaster 40's in stock...are these really too loud under cruise with a "drone" in the cabin? I want it to sound like a v-8 at idle and be able to hold a conversation in the cab at speed or cruising around town.

 

[BruteXJ]

The '99 XJ with 4.0 is not hurt by a lack of backpressure after the cat. Idle to mid is just fine, with a noticable power gain at higher RPMs.

This information is not book-tested, but my own experience after ripping my exhaust off on a rock just behind the cat early into a trail and driving it home (interstate) afterwards.

 

[xj_chris]

I'm curious why som many are talking about 2.25 for the tailpipe?
I just ordered a Cat & back kit with a 2.5 tailpipe, will this be a problem?

Anyone with experiences? I went with carsound cat and flowmaster 70series. 2.5"throughout.

I'm sure much of it depends on muffler/cat combo, as someone said when it comes to exhaust any change in the system affects the whole system.

 

[thedarkprince]

Wouldnt the smallest pipe size be the control? I understand that the air does cool down past the cat and reduces volume accordingly, but how much difference would a 1/4" in diameter make way back after all that? I mean once you have you're smallest pipe being whatever it is, whatever is bigger behind that, I mean, would going from a 2.5" to a 5" TAILPIPE really matter? (no I am NOT going to put a ricer 5" tailpipe on, its just an exaggeration)

 

[FUNKYTEE5]

Wouldnt the smallest pipe size be the control? I understand that the air does cool down past the cat and reduces volume accordingly, but how much difference would a 1/4" in diameter make way back after all that? I mean once you have you're smallest pipe being whatever it is, whatever is bigger behind that, I mean, would going from a 2.5" to a 5" TAILPIPE really matter? (no I am NOT going to put a ricer 5" tailpipe on, its just an exaggeration)

The most important pipe diameter is what hooks up to the header and runs back to the muffler/cat. From there back it can neck down (2.5-2.25) slightly due to exhaust gasses cooling/contracting and there will not be any adverse affects unless you have a race motor or sumptin. It is good to have free flowing exhaust from the muffler back, but the head pipe diameter will mainly determine how well and at what rpm the system will scavenge exhaust from the cylinders. Backpressure is often mixed up with scavenging and they are not the same! Scavenging is when the velocity of the exhaust gasses through the system will help pull exhaust out of the cylinders at a certain rpm

FUNKYTEE5

 

[xj_chris]

Well when you think about it, the '01 exhaust has alot of flow changes:
1) 3 individual pipes into 1
2) pre-cat
3) 2 outlet pipes converge to a single pipe
4) Catylic convertor
5) Muffler
6) tailpipe turns
If I remember my fluids right, #1,3 should increase velocity and decrease pressure (hence contributing to better scavenging, unless it is a high RPM engine), while #2,4,5 are all obstructions and should decrease velocity and increase back pressure.

But either way, the cross sectional area of the 2.25 pipe is 3.975 sqin while the 2.5 pipe is 4.907sqin... almost a 24% increase in area for the gasses to escape with only 1/4 inch increase in diameter!

 

[Bloose]

The most important pipe diameter is what hooks up to the header and runs back to the muffler/cat. From there back it can neck down (2.5-2.25) slightly due to exhaust gasses cooling/contracting and there will not be any adverse affects unless you have a race motor or sumptin. It is good to have free flowing exhaust from the muffler back, but the head pipe diameter will mainly determine how well and at what rpm the system will scavenge exhaust from the cylinders. Backpressure is often mixed up with scavenging and they are not the same! Scavenging is when the velocity of the exhaust gasses through the system will help pull exhaust out of the cylinders at a certain rpm

FUNKYTEE5

Exactly!

B