I had a custom dual out flowmaster exhaust.

By and by... (a year later)... we all got tired of the noise.
It sounded great outside, but the droan of a chambered muffler was just too much inside the cab.

So, as luck would have it, a 19yr old (with factory exhaust) joined our club.
Long story short we traded.

Since then, I have noticed a big drop in mpg. Could it be true that a free flowing
exhaust frees up some mpg's?
At the same time I added a 1.75" leveling kit to the front. (to fix the bumper/winch sag)

So... is it the new stance (I've always had a lil rake) or the exhaust causing the mpg loss?

 

What is the exact drop in mpg? I guess I would consider 1mpg a big drop with my setup (since I av 13-14). You are adding a substantial amount of weight back to the JK with that giant suitcase of a muffler. That coupled with a confused computer may be giving you a drop. I would disconnect the batt for 10min and then reconnect. Maybe after the computer has a run with the old muffler things will be a little closer to normal.

Also, were you babying the accelerator because of the louder exhaust? A smaller foot would give you better mpg

 

How much is a "big" drop? I'll bet it's a combination of the two. I doubt if the exhaust alone would account for too much without modifications to the intake side also.

Great looking Jeep BTW!

 

I don't know the answer on this, but I do like the way your jeep looks .

 

I would say the exhaust. You had made it easier for the engine to breath for a while and then stiffled it again. How much did you lose? JOE>

 

Might need to let the ECU re-learn the fuel curves, since it may be running a little rich to compensate for the increased back pressure?

 

Thanks for the compliments.

I have not figured the mpg after the change, but I was getting 15-17.
This past week I could almost watch the gas needle go down.

The reset idea is a good one I never thought of that.

 

Since flowmaster mufflers don't flow well (measure your stock muffler versus a flowmaster, the floormasters don't even squeek out 500cfm) and the little 3.8L even at 6000rpm doesn't even flow 500cfm, your fuel losses are probably coming from elsewhere.

 

I am with you on that. I felt zero gains from the exhaust.
I will reset the computer and go from there.

I am thinking of pulling out the factory isolator and drop it a 1/2"
I actualy like the tad bit of rake anyway.

Before...


After... Its almost perfectly level now.


FYI - Its a RC 2.5" BB w/ Trailmaster 1.75" pucks, and Daystar 3/4" pucks in the rear.

 

I was.