gas mileage down 10% after switch to Flowmasters?

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May 31, 2005
Pocatello, ID
I had a pair of Flowmaster mufflers installed on my '98 Mustang. Ever since then, my mileage has been down about 10%. This has been nearly three months, so it's not based on one tankful. I have reset the computer by disconnecting the battery... since that time I've noticed a change in temperment, but the mileage is still low.

My theory... flowmaster makes the claim that their mufflers send so much heat out the tailpipe that engine components such as belts and hoses will last longer. Well, I don't know about that, but I'm wondering if there has been a change in temperature at the O2 sensors to the extent that this could cause a drop in mileage? Maybe, (hopefully) this would be happening only because the sensors were marginal to begin with.

No other exhaust system mods, the sensors are original (I believe) with 80K miles on them. Does my theory make sense?

I also thought it could be "winter gas", but last winter my mileage wasn't this bad.
Could it be your hitting the throttle harder (subconsciously) so you can hear the new mufflers?

The other possibility is that the mufflers flow well enough to allow a performance increase, and with better power may come more fuel used. It's just a stab in the dark.
No, it's not my driving style. In fact I've been driving like a grandma in an effort to get better mileage.
less egr backpressure could mean less egr, which if you have knock sensors is giving you less timing and less overall efficiency.
Hmmm... that is an interesting thought.

Here is a little more background info: After the mufflers were first installed, the car turned into a wild beast! I mean it really felt awesome. It had more power than before, for sure. But I noticed the mileage drop, so I then reset the computer.

After resetting the computer, the car feels more normal, or more like before. I think mileage got a small amount better, but not much. To give an idea, the trip odometer used to give me about 280 miles per tank before the mod. Directly after the mod it went to about 240! Then after reseting the computer it's now around 250...
I think modern engines with computer controlled fuel injection are set up to operate within certain flow parameters, and that means flow in and flow out. If the parameters get outside of what the computer can adjust to, gas mileage and/or power is going to take a hit.

It's not like the old days where putting a pair of glass packs on your V8 would not only guarantee a better sound, but better performance. Now the exhaust system has to be specifically tuned to the engine/FI system.

Mufflers are more of an integral part of the powertrain than they used to be. There are several companies that make aftermarket exhaust systems for my 300M, but only Borla guarantees that their system matches the OEM flow requirements.
Yes, thats the thing, only the mufflers were replaced. According to a lot of Mustang aficionados, this is hardly a mod. It's like a "sound only" mod, without a lot of performance expected. In my case, the originals were showing a little rust around the edges, and I just wanted to replace them with something a little better............
I had flowmasters put on my 95 Mustang GT and my 03 Cobra. The mileage did not change. I don't think the small difference in flow should have any affect. From what I see on the Mustang site, it appears that installing an axle back system on the 05 Mustang, even with the new computer, has very little effect on anything but sound.
Changing exhaust "back pressure" can certainly affect motorcycle engines, sometimes requiring a richer fuel mixture to keep from losing low-midrange power. Maybe it holds some truth in cars, too, and your computer is richening the mixture some to compensate? Seems like a bit of a reach, though....
How does the weather of the last three months compare to the weather of the same time last year?

Do/did you use a single brand of gas or any old brand and has anything changed with regards to that?

Have you noticed the smell of gasoline on your neighbors' breath?
Eh, well, this winter has been cold. But I'm seeing a solid 10% here. It could be that the mufflers are a coincidental event here.... but no check-engine lights...
In the old days when you installed "Thrush" mufflers on your car (as in "I can see from the inlet to the outlet") you needed to open up the jets a a bit. I assume your ecu does that automatically
Not sure why there's such a significant drop...but there is a reason they are called Blowmasters on some message boards.

From what I understand, Flows decrease air flow.

From what I understand, Flows decrease air flow.

I've heard this about their original Flowmaster mufflers. The Deltaflow series should cure this.

At the same time my buddy had a Flowmaster catback on his 95 Camaro a few years ago and that thing was a beast. Besides the guy did say he felt a difference.
Thanks for all the responses. These are the Delta-50's, so they're not the all-out race mufflers. They were professionally installed by a local guy that's been around forever and has a great reputation.
There is another thread on this board that talks about wind resistance with cold temperatures. Very interesting... maybe I just need to wait until I get a full tank with temperatures in the 70's....
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