btorok

Hey all,
I put my new Tanabe Medallion on last week, and I'm actually getting WORSE gas mileage now. I thought reduced back pressure was supposed to help with it. Anyone heard of or seen this? Or did I just buy a crap exhaust? Appreciate any input on this.

juniorsdad

maybe you're stepping on the gas more so you can hear your new exhaust more...

btorok

Lol...no I was actually lighter on it bc I wasnt used to the sound....even though I do have a lead foot.

fstrnldr

I can't say i've watched my MPG w/ each mod, but i know w/ I/H/E, and the emanage ultimate, i still average about 23 on the interstate between 80-100mph w/ AC and very aggresive driving. I would give it some more time, and see how it averages over a longer period of time. Could be gas quality, or the ECU learning if you reset it after the install.

btorok

Hmm. I'm only getting about 18. I do have a bit of a lead foot though. I didnt reset the ECU after installing the exhaust though. Wonder if I should. I dont even know how to. I think I'll try to lay off the pedal for a while and set the cruise under 80 and see how that goes.

DemonIS300

resetting the ECU won't affect the the exhaust, and yes usually after installing an exhaust, you will lose gas milage because you open more room for exhaust fumes to go through, it's not restricted anymore which in turns give you more power but waste more gas.

fstrnldr

resetting the ECU will effect the gas MPG until it relearns everything and is perfectly tuned again.

As far as the exhaust hurting milage, I'ld like to hear your theory on how this is possible. Letting the engine breath easier doesn't make it use more gas. You only need so much fuel to make a set number of HP, and a certain number of HP to maintain a certain speed. So if hes driving the same speeds as before, then he still needs the same HP to maintain that speed, and the same about of gas to maintain that same about of HP. You can make a motor more effecient or ineffecient w/ certain mods under certain conditions, but a cat back isn't enough of a change to have that drastic of an effect on things. If it was it would offer up a much larger HP increase that it does.

DemonIS300

there's a difference when it's tuned and when it's not tuned, stock car are not tuned for aftermarket product, i'm not saying you are wasting a crap load of gas for just an exhaust. I'm saying you are not gonna get the same gas milage as stock.

fstrnldr

actually it is tuned, (see my ECU reset question) The car comes w/ Oxygen sensors for just this reason, to keep the car tuned under normal driving conditions. Like i said before the cat back isn't a nearly big enough change to worry about, but for any minor change the computer has the ability to adjust the fuel trim to keep the engine where it is happy. Still won't see much if any of a change in the fuel trim to compensate for a cat back. As far as WOT, anything you do that will allow more air through the engine is really only going to help, the IS runs increadibly rich stock. My I/H/E car above is still pulling about 20% fuel on average from the Emanage tune, all to keep it at about a 13.0:1 a/f ratio in third gear.

Oh and no you shouldn't get the same milage as stock, you should be able to get better, assuming the same driving style and conditions. Only airflow type upgrade that may really start to effect MPG is a header since there is more tuning that can or can't be done w/ it. IF you get a header thats tuned for a high RPM and has very poor eficency in the lower RPM where you are cruising most of the time, i could see a milage loss, but not very likely w/ the current products on the market for bolt on's w/ the IS300.

I'll say it again, if you are getting worse milage w/ the cat back, check out the rest of the car: dirty air filter, recent ECU reset, worn spark plugs, low air pressure in your tires, etc etc. There is something else going on.

btorok

Appreciate the input guys.