97 XJ 4.0. I went to a muffler shop yesterday to get some 2.5 inch exhaust from the header back put on. i have a high flow cat (magnaflow) and a flowmaster super 40 muffler. the guy said that on his 91 he did the same thing i did and lost alot of torque (his was a standard) mine is an automatic. He said its probably not worth the effort of putting it on. He said he would charge me $150 bucks to install everything if i really wanted it though. Is this true that i would lose torque? Ive been reading alot and it says nothing about loss in torque that its a good upgrade. Thanks in advance

 

It is a great upgrade. I have 2.5" high flow cat, 2.5" glass pack, 2.5" mandrel bent piping through out.





with 3.07 gears i can still spin my 33" super swampers.
I didnt loose any torque at all.

You engine is a big air pump. With the 2.5" exhaust i would upgrade your intake and you will feel a good power boost.

 

Technically, yes, you could loose torque, with bigger more open exhaust, you may pick up some HP at the same time. This is the trade off with freer exhaust. You can pickup top end HP, but loose bottom end torque.

 

with that flowmaster muffler, you should be running a 2.25" tailpipe. set up that way it will help you with bottom end torque. i plan to set up my jeep witha system very similar to yours. my dad gave his old manifold/factory header that was cracked at the collector. i cut the collector off, made a new "y" out of 2.5" pipe and put a 3-bolt, 3/8" thick flange on it. i left the pipe sticking out a little so that i can put a chevy big block exhaust gasket on it. there is not a lot of room near the collector because of the front axle being there so getting 2.5" pipe in there will be tight.

the flowmaster muffler is a scavenger muffler and needs a slightly smaller tailpipe to help it work properly.

zedpapa

 

I just installed MUD FROG exhaust system from the venders section on here i used the super 44 it was all 2.5 from muffler on back. Made a HUGE DIFFERENCE in throttle respone. Jeep runs alot better.

 

so 2.5 inch from the header to the muffler, then 2.25 inch out the back? I was probably gonna dump it behind the axle.

 

2.5 is great... then upgrade your intake system to level it all back out again...

 

Your engine is an air pump as was mentioned but time for a little explanation on why some things work and some doesn't.

• Larger pipes can loose bottem end torque but increase the top end. The reason is exhaust/intake velocity. The larger the pipe wether intake or exhaust will be tuned for more rpm. Because the charge traveling the pipe does not keep the velocity and create in effect a vacuum behind the charge drawing the last bit of spent gases out past the valves. It takes more RPM to achieve this effect. Hot Rodders have known this for years on the dynos. 1 5/8" headers typically make a larger torque number than 1 3/4" and 1 1/2" was to small and lose in Hp.

• This same principle works on the intake in reverse. LONG intake tracks make for large torque numbers. They increase the filling of the cylinder and cause a swirl effect and even a ram air effect as the valve is closing and the piston travels up. Now as RPM increase that long intake runner becomes a hinderence to HP. The valves are not opening long enough for the charge to travel the length of the long runner. Yo want to a very good example of how to do both and torque curve that is FLAT, look at the FORD SHO V6 it has a two stage intake long runners to make low end torque and then butterflies that open at 3750 RPM that allow a short runner. Then when taken to the other extreme look at the Ford BOSS 302 from 68-70 Mustangs. This was an engine with 351C 4 barrel heads. It was a true dog on the street and made no power below 3500 rpm the intake and exhaust ports way to large for the application. But when tuned with the right cam and compression for the track. Man watch out they was pumping 500HPfrom those engines then.

I have not played with the 4.0 enough to know the answer to your question, but I can tell you that larger is not always better with intakes or exhaust systems. You want a tuned exhaust system that will give yout hte broadest torque curve possible for a Jeep.

Ok I'll get off my soap box now.

 

upgrade the intake to what?

 

Neon injectors... bored throttle Body... 99+ intake... CAI...

 

I've messed with my exhaust system a few times.

stock size piping to stock piping with flowmaster super 40: No change except sound

2.5" highflow cat, 2.5" flowmaster, 2.5" taipipe: much louder sound, more high end power, slight loss of low end torque. This is with a CAI and Amsoil EA filter. The low end difference was most noticeable when climbing hills on the highway. It wouldn't really chug up the hills like it used to.

My next setup:
stock cat, 2.5" midpipe, 2.5" flowmaster, 2.5" tailpipe.

The reason for this is that the stock 2.25" cat will provide SOME restriction so that I won't loose low end torque, but still gain a little from the rest of it being 2.5". MY 2000 has the California emissions package with the two pre-cats and 2.5" downpipe. Having 2.5" exhaust all the way through made it not as restrictive. Better high end, but less low end. Since I don't race the jeep, I need a balance between both.

 

Is 2.5 better for fuel economy? I dont really need much low end torque since my XJ is a DD and im on the highway 10 times as much as the dirt. Its really lacking in the high end with stock 2.25 piping, stock cat and no muffler. I run a K&N CAI and i plan on getting a 63mm TB, its a 99 btw.

 

Just remember the principles of air or fluid flow.. you can have a 8 inch pipe for most of the run with one 3 inch pipe elbow/bend.. and you will have the flow of a 3 inch pipe/bend.. this is defiantly a hit and miss without all the math involved. tuning is a science..

 

I ended up putting the APN header on, New 2.25 inch exhaust tubing to the cat, 2 1/2 from cat to muffler, 2 1/2 from muffler and dumpped it before the axle with a 3.0 inch turndown. felt a big increase in torque and high end. Is a TON louder! I haven't heard an XJ this loud yet! (without being straight-piped of course

 

Pipe size only applies to headers. After the header, it does not matter what size pipe you put on as long as it is large enough to handle a couple cylinders worth of exhaust at the same time because, in a four stroke, that is all that is happening.

The rule of thumb is you drive torque and race horsepower. Since 99.99% of your engine useage is low RPM (as opposed to redline RPM) the horsepower gain you get from big headers is not worth the loss in torque. You won't ever get to enjoy it. The difference is not meaningful to most drivers in any case. Racers race horsepower, not torque, so they like big tubes.

All that said, this only applies to HEADERS. The initial scavenging of exhaust from the port to the end of the header tube is where torque OR horsepower is made, not at the tailpipe or the cat. You need an exhaust system large enough to handle the output from a couple of cylinders and a 2.5" tube is probably more pipe than is needed but who cares, it won't hurt anything and might sound cool. We all know that if it sounds cool, it is faster. Just like when you wash it.

Anyway, that is the purpose of headers - to equalize and put into rhythm the exhaust pulses. This is also why drag racers and other race cars don't have exhaust systems after the headers - they don't mean anything to power generation.


Also, big tubes don't mean anything if you have restricted heads with poorly flowing intake and exhaust ports. You can only scavange what you can draw into the motor in the first place. The header manufacturer is going to have dyno numbers for torque and HP. Just follow their recommendations for whatever you are trying to achieve.