Lets set this straight.

OEM Tech??

for craps sake, can we stop asking about exhausts!

it's a motor that redlines at 5 grand, does anyone think that a full flow exhaust is going to make that much difference?
it's a damned tractor!

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

goodburbon said:

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

Click to expand...

no, no dyno for you!



yes, I'm a bit froggy today.

See, i really think it's better all around. I have to give it less throttle to accelerate, have serious passing power and I'll let you guys know the mileage results after a few tanks. I also have a 92 5 speed with 3.07 gears...wicked fast 2nd goes 60mph and it ran alot faster when I straight piped it and blew all the glass out of the cherry bomb.

87manche said:

for craps sake, can we stop asking about exhausts!

it's a motor that redlines at 5 grand, does anyone think that a full flow exhaust is going to make that much difference?
it's a damned tractor!

Click to expand...

uhhh...yeah. flow is power. that little 2 inch stock pipe with the ridged bends is so restrictive.

You have to define better.
Does your XJ, pull a trailer better with a slightly restricted exhaust at 55 MPH, likely.
Does it accelerate through peak torque quicker on a flat street into the upper RPM band and maybe loose some torque in the process with a more open exhaust, likely.

horsepower = torque * RPM divided by 5252

You can lower one and raise the other, move them around in the RPM band some and mess with peak RPM. But in the end you have three variables and one constant. Messing with the exhaust changes the curve some.

Most XJ's are heavy, have a motor designed more for torque than peak horsepower, often turn big tires and have mechanical limitations on the highest RPM's that the motor will survive.

I do a little street racing from time to time. I shift early and try to keep the motor in the peak torque band. I regularly beat lighter cars with a better horsepower to weight ratio. How? Because my peak torque band is flatter than their peak horsepower band. My steady acceleration can beat there peaky motor and quick shifts. If I drive it much out of the the torque curve, which falls off fast after mid RPM, they win. I also try to pick my road, and try to stage the race from 15-20 MPH through around 75 MPH (lower if possible). HO motors have moved peak torque a little higher into the RPM band and seem to have narrowed the torque curve a little. Butt dyno.
My HO is marginally a little faster than my Renix, but really not much. They accelerate a little differently, but the end results are largely the same.

I imagine there is some room for tuning the exhaust. But it would likely be easier IMO, to tune it out of the envelope, than to get it much better.

no, there is a certain point where it will get better, and for a 4.7stroker, my best results have come with a simple 2.25in setup, with a muffler. With no backpressure, the exhaust gasses just wave at the o2 sensor on the way by and it does not get accurate readings.

and flow is not power. My camaro ran open headers all the way to the exhaust shop after I installed a set of long tubes. It ran horribly, no power before around 3k rpm, and no torque ever. After fitting it with a mild exhaust, it ran leaps and bouds better. And I put some restrictive, s-curve mufflers on it instead of a strait through design on purpose

But its louder.. It must be faster!!!!

87manche said:

it's a motor that redlines at 5 grand, does anyone think that a full flow exhaust is going to make that much difference?
it's a damned tractor!

Click to expand...

:lecture:
X2

What I'm asking is the 4.0 better or worse with an open flow exaust.

jeepman121 said:

What I'm asking is the 4.0 better or worse with an open flow exaust.

Click to expand...

What I'm asking is better or worse than what? Not trying to be an a hole, but are you pulling stumps or racing ricers?

Well I'm bone stock...wanted better fuel efficiency, better sound and acceleration. I have noticed serious passing/highway power. thats no doubt. and it seems to be the same or maybe even a little quicker from the jump too.

your fuel economy will probably go to crap with no cat or muffler. Fuel Injection needs the pressure so the O2 sensor can get an accurate read on the exhaust gases

wolfpackjeeper said:

your fuel economy will probably go to crap with no cat or muffler. Fuel Injection needs the pressure so the O2 sensor can get an accurate read on the exhaust gases

Click to expand...

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

Back pressure isn't good, but a 4" exhaust isn't good either. You want the exhaust to have good velocity so that it will "suck" the exhaust out of the cylinders at low RPMs. This is what helps low end torque, helping the piston out so it isn't doing all the work.

More backpressure and less velocity would make the piston do more work and slow down, since it is pushing against all that extra pressure. It slows down the engine as a whole, which would make you lose torque.

The reason super huge exhaust diameters aren't good for little motors is that they lose the velocity, and create turbulence. This disrupts exhaust flow and can create even more back pressure than before.

Hope that was understandable.