Question about the factory crush

[mizzutgr]

I searched on here for a while today but I couldn't really find anything related to mpg and removing the crush. I removed the crush , put a new, less constrictive cat on it ran 2.5 to a new magnaflow muffler and 2.5 out the back and proceeded to loose 25-40 miles off my tank. It is a 99, 4.0 5 speed with 3:07's I have a big k&n cone filter and a small exhaust manifold crack at the collector. That is a huge loss in mpg in my book. Can any body point me to some threads on this? I'm thinking about putting the crush back in it to see if that helps. Thanks

 

[1 tuff mj]

um whats a crush

 

[moneypit_k5]

subscribed, because i want to know what a crush is.

 

[Skreed]

You mean the gasket before the cat? Why would you remove that? you created an air leak now the o2 sensor is adding fuel.

 

[Jeremiah]

I think you're speaking of the "dent" in the exhaust pipe after the manifold flange. It's there on purpose to prevent your front driveshaft from stuffing into it. So, if you do any wheeling or mount a curb, you might end up with your dent again anyhow.

Is it possible your loss in mpgs isn't related to driving more aggressively with all your new upgrades?

 

[moneypit_k5]

i think hes talking about the crush sleeve that is found on the front pinion in the differential. most people just remove it because you get such a large horsepower gain. but when removed, your mpg will drop considerably, which is what the OP is most likely experiencing now.

A popular way of regaining the mpg's lost by removing the "crush" is to have a professional muffler shop reroute your exhaust directly from the header into the intake. This produces turbo, without having to buy expensive parts.

 

[Slip Kid]

Sounds like you increased the diameter of the entire exhaust and changed cats. The added volume and the freer cat would decrease the velocity of the exhaust, moving the torque/efficiency peak up to higher rpms. Removing the dent would add additional volume. Depending on type of driving, that could in theory reduce economy.

As stated above, you might be driving more aggressively with all the new mods, which might be a bigger difference depending on how stock your exhaust was before.

 

[johnnyc]

i think hes talking about the crush sleeve that is found on the front pinion in the differential. most people just remove it because you get such a large horsepower gain. but when removed, your mpg will drop considerably, which is what the OP is most likely experiencing now.

A popular way of regaining the mpg's lost by removing the "crush" is to have a professional muffler shop reroute your exhaust directly from the header into the intake. This produces turbo, without having to buy expensive parts.

What did you just say?? Both of those statements are beyond comprehension. :confused1

The OP is talking about the dent in the pipe, just as Jeremiah suggested. When I put on my Borla header, I had a muffler shop make a new downpipe from the header to the cat.

I didn't notice any increase in gas consumption. Maybe a little more prudent driving will recover your lost mileage. And get that manifold crack fixed.

 

[cruiser54]

I also believe it's driving habits as I have removed the "crush" from both of my Jeeps with no loss in fuel economy and a noticeable boost in power and throttle response.

 

[mizzutgr]

I am not driving any different than usual. I'm thinking that maybe when they removed the O2 senser and put it in the new pipe they jacked it up some how. I did get better performance in the low to mid range and it sounds real nice but loosing the mpg sucks.

 

[joe_peters]

Crack upstream of 02 sensor, o2 sensor reports "lean" to PCM, PCM attempts to correct lean condition by increasing the duration of the injector firing pulse, mileage drops.

 

[cruiser54]

Joe might be on to something here. a lazy O2 sensor won't necessarily throw a code and could cause your problem. They aren't made to last forever either.

 

[mizzutgr]

Would the larger diameter pipe and free cat expose this problem more? The mpg loss was immiediate.

 

[cruiser54]

Would the larger diameter pipe and free cat expose this problem more? The mpg loss was immiediate.

I seriously doubt the pipe size is your problem. I would investigate the O2 sensor since it was removed and reinstalled. Is it plugged in? Is the connection compromised in any way? How old is the sensor?

 

[Cottontail]

Hmmm....I still have no idea what the hell a crush is! :eyes:

 

[gradon]

That's cause your Annie with its cast manifolds probably doesn't have a crush(but probably has precats). X2 on the crack in the header adds air upstream of the o2 sensor so when it sees a leaner than 14.7:1 mixture, it'll add fuel and your mileage will go down(although I doubt the crack just happened).

 

[dukie564]

The "Dent" in the pipe just past the collector donut seal provides 1-2 psi backpressure. Jeep designed it this way to give the HO 4.0 a fatter torque curve, down low in the RPM range. it also helps gas mileage, and gives the motor a nice feel.

It is NOT a defect.
It is NOT for clearance.

It DOES looks like crap, and sloppy engineering, I agree.

Removing it will cause two things:
A slight bog in the lower RPM range.
A slight decrease in gas mileage.

The backpressure keeps a little more exhaust in the upper section of the header just past the exhaust valve which forces the motor to extract a little more heat (power) from the freshly burned fuel/air charge coming out of the cylinder.


However i doubt removing this would cause a loss of MPG. Did you reset the PCM?

 

[XJ-ARNIE]

sounds like dukie hit it. test your o2 sensor for grins as well. as you may have damaged it

moneypit...I hope your joking:twak: .....congrats by the way.

 

[Milford Cubicle II]

The "Dent" in the pipe just past the collector donut seal provides 1-2 psi backpressure. Jeep designed it this way to give the HO 4.0 a fatter torque curve, down low in the RPM range. it also helps gas mileage, and gives the motor a nice feel.

It is NOT a defect.
It is NOT for clearance.

It DOES looks like crap, and sloppy engineering, I agree.

Removing it will cause two things:
A slight bog in the lower RPM range.
A slight decrease in gas mileage.

Ummmm... no.

It IS there for clearance and it WON'T have an appreciable affect on the torque curve, gas mileage, or "motor feel"

As for the OP, how old is the O2 sensor?

Also, it's pretty normal for mpg's to fluctuate 1 or 2 mpg just from environmental factors. And all the snow on the ground could easily make that difference or you may be driving slightly different without even realizing it because of the weather, that would be my guess.

Give it another week and see if it still sucks.

 

[Porch951]

Give it another week and see if it still sucks.

That's what i would suggest, and agreed about the colder weather.

A 2.5" exhaust is pretty reasonable for a 4.0L engine. Fix the leaking header either way.