Lets set this straight.

[wolfpackjeeper]

19mpg hwy on 33's with 4.10's. 3 inch cat and muffler

You are running a muffler silly. He said strait pipe, no cat no muffler. My Post says with no cat or muffler your economy will go down. Believe it or not that muffler alone will add some low end torque and let the O2 sensor do its job.

 

[mattbred]

You are running a muffler silly. He said strait pipe, no cat no muffler. My Post says with no cat or muffler your economy will go down. Believe it or not that muffler alone will add some low end torque and let the O2 sensor do its job.

A properly intact cat shouldn't effect backpressure much at all. I ran one without (PO didnt have one installed) and I got the same mileage as I do now. I also ran with an open header and didn't notice any performance drop at all. This was with the RENIX (89) so it may be different with the later computers and head designs.

 

[outlander]

x50 on the dyno numbers.I can't believe that with all the tech on various different boards floating around nobody has dyno'd this controversy.
If I had access to a dyno I would step up and do it.....

 

[ehall]

More air out = more air in = more power. Simple.

an open exhaust does not produce "more air out" it produces lazy air out

backpressure==scavenging==more power at low RPMs

open exhaust==sucky low-end

 

[techno1154]

In all honesty though, I would like to see the dyno results of a series of tests.

1. Bone stock
2. 2.5" exhaust
3. Aftermarket headers and stock exhaust
4. Aftermarket headers and 2.5" exhaust
5. Aftermarket headers and 3" exhaust
6. Open Headers

A bone stock XJ do have different exhaust.

My first XJ; 1994 OEM manifold and 2.25 inch pipe all the way out. No previous XJ experiance and it did well acording to my Butt dino. Exhaust manifold leaked. Replaced it with a Borla. No further exhaust mods. It exhibited low torque/power off the line to about 2000 RPM. Then the Borla showed some inprovements but only above 2000 RPM. Well I was happy with exception of when I was trying to go 0 to infinity from a stop light.

Enter my 1996. 2.5 inch from the crossover pipe to the outlet of the muffler then down to 2.25 inch. Standing starts is excelent when it came up against any other SUV in it's class + many of the Rice Racers. Replaced the the stock muffler which was suffering the symtoms of old age one year ago with the Rusty's cat back with the FM 50 series. According to the Butt dyno, there is an all around improvement in the performance of the XJ. Bear in mind my XJ very seldom see 3,000 RPMs. But even so from a standing start at a light with 3.55 gears and 30" tires still put a smile on my face and a question on the faces of those in the Ricers if even if for just the first 100 ft.

So yes, both my XJ's had stock exhaust on them. The 1994 had 2.25 from manifold to tip while the 1996 have 2.5 from under the engine to the exit of the muffler. The butt says the 2.25 tip had more low end torque. High RPM and top end, :dunno:

its timefor me to get off the :soapbox:

 

[jeepman121]

Whoa! I do have a Muffler...It's a Generic Flowmaster called a thunderbolt. It's Identical internally to a Flowmaster turbo muffler. Ther is no cat however.

 

[jeepman121]

Ok so heres a better question I suppose...is removing the cat and putting a flowmaster turbo muffler better than stock?

 

[1990XJlimited]

as an ase certified tech, it is common knowledge that restriction = power loss. so obviously more powerstraight piped, but your low end tourque will suffer. so if you wanna street race a cherokee go for it. but for offroaders like me go with a 3 in catforward system

 

[87manche]

as an ase certified tech, it is common knowledge that restriction = power loss. so obviously more powerstraight piped, but your low end tourque will suffer. so if you wanna street race a cherokee go for it. but for offroaders like me go with a 3 in catforward system

ASE certified or not, it's a pretty bold statement that in all cases more restriction leads to a power loss. Ever hear of a tuned pipe?

Unless you have access to a flow bench and a dyno no one here can make any reasonable claim about a 4.0 and it's exhaust system.

So until someone ponies up the money to have a head ported and flow matched so we can test a variety of exhaust systems on an engine dyno then this thread should just stop.

Do what you want with your exhaust, but it's still a low reving tractor motor, and discussion after discussion on this same topic with no engine dyno results is just stupid.

 

[Hellbent]

19mpg hwy on 33's with 4.10's. 3 inch cat and muffler

It has since fallen off with 4.88's

Not allowing a motor to breathe properly doesn't help it. Changing the length and diameter of the headers can affect torque and HP output. Putting on a more restrictive exhaust (note I am not saying header) only serves to stifle the performance of the header. ( a stock exh manifold is not a header.)

Of course if you remove the oxygen sensor from the exhaust stream it will run like crap.

I swear this is like that frickin plane on a treadmill thing. More air out = more air in = more power. Simple. You people that claim the backpressure is beneficial are seeing a different power curve because you are stifling the performance of your engine at higher rpms, of course you see better low numbers than high numbers. hasta

well put, thank you!

 

[BillBraski]

What everyone but 1 post has missed is the fact that a engine MUST have decent exhaust scavenging, so when the exhaust valve is nearly closed and the intake is just opening, to help pull the new air/fuel mixture into the combustion chamber/cylinder. That will effect your power numbers, period. But it will really effect the low torque becuase, as said before, its all about velocity. The more exhaust velocity the more scavenging = more power and better economy. The problem with the 4.0 is its not a real good breathing head/intake/exhaust, so just adding a header will not do all that much. Just larger size pipes does nothing. Its all about a smooth, steady, flow. The Flowmaster Delta mufflers are good for scavenging.

But, if you were to machine the head to raise the compression, port match the intake to the head, polish the intake manifold runners and remove any casting obstruction(s), do a valve job with a little larger valves (3 or 4 angle seats), polish the combustion chamber and use a ceramic coated header or a big cast iron exhaust manifold, not to mention larger lb./hr injectors, bigger throttle body and a low restrictive cold air intake, it would be a very high RPM motor. All that I just laid out would cost more than most XJ's are worth and is a waste of time becuase there are many other platforms that would be better for high RPM power. So, a little better flowing exhaust wont kill you. Just not too much.

If the OEM could put the muffler right off the collector, they would. It would cost less, but it would be louder and you would have nearly no scavenging effect.

 

[falcon556]

If unrestricted flow is good, how come the factory pipe has built in restrictions.
If you remove the manifold and look down the exhaust pipe you will see one or more (depending on year) "dents" that restrict gas flow.
I was told that they are supposed to tune and improve flow.

 

[BillBraski]

If unrestricted flow is good, how come the factory pipe has built in restrictions.
If you remove the manifold and look down the exhaust pipe you will see one or more (depending on year) "dents" that restrict gas flow.
I was told that they are supposed to tune and improve flow.

Who said unrestricted flow is good? I didn't. I just said velocity was very important to exhaust scavenging.

 

[mattbred]

If unrestricted flow is good, how come the factory pipe has built in restrictions.
If you remove the manifold and look down the exhaust pipe you will see one or more (depending on year) "dents" that restrict gas flow.
I was told that they are supposed to tune and improve flow.

This dent, or "crush" in the downpipe is there not for exhaust reasons, but for front driveshaft clearence. If anything it would hinder exhaust flow. Also, as far as I know, if you have more than one of these, then you must've cranked your exhaust on a rock or something.

As I and BillBraski have stated, it's the scavenging effect that you want, a higher exhaust velocity that "sucks out" the exhaust from other cylinders and "sucks in" the air/fuel mixture. This really helps the low end because that is when the engine needs the most help.

If you were to cut the exhaust pipe at the end down to 1", this would increase back pressure a shitload, but it would really hamper the engine's performance. This is why there's a balance, but only for the velocity/pressure effect. Back pressure itself only slows down the pistons.

 

[WB9YZU]

This is basicly to the OP, but it applies to all that just look at the exhaust pipe as a restriction.

The main point of all this hoopla is that you just can't slap ideas like "less restriction is better".

Engines are more than just intake...
...and they are more than exhaust.

To truly obtain efficiency, you must tune the system; all of it.

As Goodburbon indicated, how you tune it, just like a Pipe Organs pipes being a different lenght and diameter change the pitch, changing the length and diameter of header pipes changes the torque and HP curves.

A system which is set up to run with a bit of restriction in the exhaust may run lean when unrestricted. Which, for all of us old farts, was the reasoning why converting from single exhaust to duals worked to increase gas milage.

An open header by itself runs like crap, because as 2 posters have indicated, it doesn't scavenge well (one outgoing exhaust pulse, helps clear another cylinder more effectively which allows for more charge to be taken in). You actually have to add pipe to a header to tune it. In most cases, well tuned exhaust system with a muffler will out perform an open header.

The factory makes exhaust systems to fit the application, and to fit the chassis. Are they optimal, well no. Are they better than straight pipes? Most likely, for the application.

For low end torque, you want to slow things up a bit. You want longer intake runners and smaller valves and longer headers to hake torque. At the point where you are making power, exhaust velocity isn't a big concern.

For high end RPMs and HP, you want to speed things up. Shorter intake runners, larger valves, shorter headers, and less, but well tuned exhaust.

The factory is all about overall performance and dealing with the chassis, so they compromise on overall performance and made a vehicle which comes off the line OK for a 4WD, tows OK, and can pass another car at 65mph with a fair bit of margin.

The answer to the Original Posters question is moot.
I believe it was in the 3rd post. "Compared to What?"

IMO, if it feels good to you, fine.
But if your going to throw out ideas like "Open exhaust is better" and "Restriction is bad" be prepared to back it up with 1/4 mile time slips with both top end MPH (HP) and 60' times (Torque). When you spend your time at the track figuring this all out, and compensate for weather and track conditions, I think you will be suprised at how wrong your presumptions and blanket statements are.

 

[mattbred]

An open header by itself runs like crap, because as 2 posters have indicated, it doesn't scavenge well (one outgoing exhaust pulse, helps clear another cylinder more effectively which allows for more charge to be taken in). You actually have to add pipe to a header to tune it. In most cases, well tuned exhaust system with a muffler will out perform an open header.

Not so with my case. I cut my exhaust off where it connects to the cat (right at the tranny mount) so it was the manifold feeding into the downpipe and that was pretty much it - into open air. It ran beautiful. This was on an '89 (RENIX) with the 4.0L. It felt like it had more power than with the rest of the exhaust on, to tell the truth. Idling up the driveway felt better, but it might've just been the awesome sound. In any case, I did NOT notice any sort of "running like crap" or running worse than with it all connected.

For low end torque, you want to slow things up a bit. You want longer intake runners and smaller valves and longer headers to hake torque. At the point where you are making power, exhaust velocity isn't a big concern.

I'm by no means an exhaust guru, so can you explain why we need a slower exhaust in lower RPMs?

But if your going to throw out ideas like "Open exhaust is better" and "Restriction is bad" be prepared to back it up with 1/4 mile time slips with both top end MPH (HP) and 60' times (Torque). When you spend your time at the track figuring this all out, and compensate for weather and track conditions, I think you will be suprised at how wrong your presumptions and blanket statements are.

Well, the same could be said to "backpressure is good" and "restriction is good". Proof is needed for all statements, really.

 

[BillBraski]

Not so with my case. I cut my exhaust off where it connects to the cat (right at the tranny mount) so it was the manifold feeding into the downpipe and that was pretty much it - into open air. It ran beautiful. This was on an '89 (RENIX) with the 4.0L. It felt like it had more power than with the rest of the exhaust on, to tell the truth. Idling up the driveway felt better, but it might've just been the awesome sound. In any case, I did NOT notice any sort of "running like crap" or running worse than with it all connected.


I'm by no means an exhaust guru, so can you explain why we need a slower exhaust in lower RPMs?



Well, the same could be said to "backpressure is good" and "restriction is good". Proof is needed for all statements, really.

More back pressure, more low power. For the last time, exhaust scavenging is key for higher RPM power. I'll give you some examples of exhaust scavenging: A muffler that pulls the exhaust gases through. A X or H pipe creates scavenging by similar prinipal as a muffler like a FlowMaster Delta Flow. Anti-reversion cones are what I use for the 1/4 mile, they have ultra low restricion and help smooth out the pulses.
But, what is exhaust scavenging at the cylinder head? Its when the camshaft has overlap between the intake and exhaust valves, the exhaust "vacume" helps pull in the new air/fuel charge. I think most factory cams have 5 to 20 degrees of overlap (Its just a guess becuase of emisions standards), while my Chevy's 302 has much higher overlap and as a result the power band is from 2200 to 6800 RPM. The smaller the overlap, the more low RPM torque the engine will produce and the more efficient it will be. Most stock exhaust manifolds don't do a whole lot for scavenging, thats why long tube headers help create more low power.

A stock 4.0 H.O has plenty of power for me, If I want more power in a XJ I'll stroke a 4.0 to 4.7 or keep it cheap and drop in a small block chevy or chrysler. Now that I think of it, a 5.7 hemi would kick ass in a XJ! If it would fit!

 

[mattbred]

Ok, you seem to know a lot about this. Can you explain to me why back pressure actually helps low end torque? Wouldn't scavenging help it moreso at lower RPMs, when the engine has less power overall to deal with pushing out the exhaust? To me, that seems counter-productive. Creating more backpressure for the piston to push against when it has less power to push it with.

Wouldn't scavenging help both low end AND top end? I can't possibly see why back pressure itself is a good thing. Is it maybe because it's sucking the fuel into the exhaust with the overlap? And thus less actually compressing and exploding on the power stroke?

 

[WB9YZU]

Not so with my case. I cut my exhaust off where it connects to the cat (right at the tranny mount) so it was the manifold feeding into the downpipe and that was pretty much it - into open air. It ran beautiful. This was on an '89 (RENIX) with the 4.0L. It felt like it had more power than with the rest of the exhaust on, to tell the truth. Idling up the driveway felt better, but it might've just been the awesome sound. In any case, I did NOT notice any sort of "running like crap" or running worse than with it all connected.


I'm by no means an exhaust guru, so can you explain why we need a slower exhaust in lower RPMs?



Well, the same could be said to "backpressure is good" and "restriction is good". Proof is needed for all statements, really.

The header is the group of pipes that connect the cylinder head to the Collector (where the pipes join). The Collector makes the transission between the header and the rest of the exhaust system. XJ headers do not have a collector per se, they Y the header pipes together and the resultant connection to the exhaust system is not much larger than the header pipes. They are very poor headers, but better than Log manifolds.

You did not make an open header system, you made a header with a piece of exhaust system on it. You made an untuned system. Did it feel better on your seat than the full exhaust? Perhaps, but show me your 60ft times to show me you made more torque, or 1/4MPH figures to show me you made more HP.

I did not say you need a slower exhaust, I said "that for low end torque, you want to slow things up a bit". The fact is that statement is a dicodomy. Longer runners and smaller valves at low RPMs actually increase intake velocity and help fill the cylinders better. The reason I used that phrase was that most folks think "Performance = Short, large diameter intake + Large valves" which is not always true. Still, the result is that though you will not have the volume of exhaust to deal with on the exhaust side, a poorly designed exhaust system will harm you just the same.

It is not about restriction, or backpressure, though a poorly designed system can product that. It is about tuning for the application. A straight pipe can have as more restriction as a well designed exhaust system.

 

[WB9YZU]

Ok, you seem to know a lot about this. Can you explain to me why back pressure actually helps low end torque? Wouldn't scavenging help it moreso at lower RPMs, when the engine has less power overall to deal with pushing out the exhaust? To me, that seems counter-productive. Creating more backpressure for the piston to push against when it has less power to push it with.

Wouldn't scavenging help both low end AND top end? I can't possibly see why back pressure itself is a good thing. Is it maybe because it's sucking the fuel into the exhaust with the overlap? And thus less actually compressing and exploding on the power stroke?

Skip this hangup you have with backpressue, it means nothing.

You don't seem to understand. Let's see if I can put this differently. CB antennas...
You can tune for Channel 40, but your tune will be off for channel 1. You can tune for channel 1, but then your tune for channel 40 will be off. So you tune for the channel you use most, say channel 19. Now your tune is OK at channel 19, but acceptable at channel 1 and acceptable at channel 40.
Exhaust works the same way.

I have no idea what your second point is, it makes no sense.
Scavenging empties the cylinder more completely by using the exhaust pulse from another cylinder. It does not push exhaust up back in to the cylinder.

Exhaust from an engine is not a straight flow of gases, it is pulses and wavefronts.