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I was wonder if anyone here has ever put a Flowmaster muffler on their TV. I just put one on and I noticed an increase in the horsepower when not towing and no or little change when towing. I have a 2003 Expedition with a 5.4L 4X4 with a complete towpackage. I noticed that before I changed the muffler the car would change into Overdrive on very flat highways (I always travel the speed limit - 55 mph in CA when towing a trailer). After I changed over the car very seldom changes into OD. Thus my mpg is lower by 1 mpg. Can anyone offer any explanation for this?
 

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I put a FLOWMASTER muffler onto my TAHOE. I didn't tow my trailer before installation, so I don't know about an increase in HP or MPG there. I did have a kick in HP while not in tow, and she seems to hum along nicely while towing too. I have the FLOWMASTER model made for SUV's. (can't remember exact #)

If I gained any MPG, it surely wasn't much. (I stomp on it more to hear her roar!)
 

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This is purely speculation, and hopefully one of the mechanic types here can elaborate, but did you also increase the intake capacity? I thought I had read in an earlier post that you should modify both the intake and exhaust to maximize any performance improvements.

air in = air out


Tim
 

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That is correct. If you just improve the exhaust you reduce backpressure to much. Add a high flow filter like Amsoil or K&N to increase the intake to match the outflow. Mine works very well and has improved both this and my previous Sub. I don't know about fuel savings ( doubt it ) but the truck has a little more torque. Save your money on those high buck intake systems and just add a high flow filter to the stock airbox. They look neat but they do pick up most of their intake air from the hot air in the engine bay.
 

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Discussion Starter · #6 ·
I was told that I should expect about 10-15% increase in horsepower. Does anyone know if that is true? Since my '03 Expedition 5.4L had 275 horses I should now have over 300 right?
 

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From FLOWMASTER FAQ:

What kind of performance gains can I expect when installing Flowmasters on my vehicle.

Performance gains will vary dependent upon your application. The more power your vehicle makes, the bigger the gains will be. A typical power gain will be around 5%-10%, with increased fuel economy from 1-2 mpg.
 

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I don't know what (if any) kind of performance increase I obtained by adding a FLOWMASTER MUFFLER. I didn't notice much increase in MPG, but I really noticed the engine seemed a little tougher.

Maybe it's all in my head, but the only true test would be to hook it up to a computer.

Do I think it made any difference? ...heck yes. (and I love the sound)
 

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NDJollyMon said:
I don't know what (if any) kind of performance increase I obtained by adding a FLOWMASTER MUFFLER. I didn't notice much increase in MPG, but I really noticed the engine seemed a little tougher.

Maybe it's all in my head, but the only true test would be to hook it up to a computer.

Do I think it made any difference? ...heck yes. (and I love the sound)
Except for the sound the "improvements" are mostly psychosomatic (all in your head
). Unless your old muffler was all clogged up, then you might have gotten some decent improvements from the swap (which you would have gotten almost the same amount -- minus the nice sound -- by replacing the muffler with just about any other one).
 

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A high flow muffler like Flow Master, Gibbons will increase the Horse power as well as MPG. Stock mufflers are very restrictive. not to make you mad but for noise reasons. People dont buy 30k and up vehicles that are loud. The more noise the better the performance.
I replaced my stock muffler with a Flow Master, before the tank was done avg. MPG went up by 4 MPG while playing with new get up and go. Couldn't take the noise so went to modified stock muffler, drilled out baffles.
Will probly go with a Gibson when I get back to lower 48.
As far as increaseing intake due to freeing up exhaust, no. It will help, but not needed. Only on High horse power engines, your cats will give all the back preasure an engine needs.
 

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DernMooseAK said:
I replaced my stock muffler with a Flow Master, before the tank was done avg. MPG went up by 4 MPG while playing with new get up and go.
You're saying that your mileage went up 4 MPG's merely from an exhaust change??? Sorry to sound disagreeable, but that seems highly unlikely. If you could get that type of massive improvement that easily Ford, GM and Dodge would all be doing that from the factory (especially with the CAFE ratings breathing down their necks).

Are you sure there wasn't supposed to be a decimal point before the 4, as in .4 MPG increase?
 

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4 mpg increase I find hard to believe. I've had Cat Backs on 4 different trucks and never saw more than 1 mpg increase. 2 Gibson's, 1 Flowmaster, and 1 Hooker and all of them increased power alittle but don't expect it to pay for itself in fuel savings. Best I can say is they just seem to make the truck not work as hard at interstate speeds and when towing. But that doesn't translate into big fuel savings. If you have a newer GM, say a 03 or 04 HD don't bother, as they have excellent exhaust systems that you won't improve on much.
 

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Intakes, exhausts and headers all aim to remove restrictions in the airflow that are at factor, by and large, only at high RPM. You engine might only flow 200cfm at 2500 rpm, but is capable of flowing 400 cfm at 5000 rpm. So it makes sense that a free flowing exhaust will help at high rpm, but not low. I'm no engineer, but I've tinkered a bit and that's the consensus in racing circles. If you're towing a lot at 5000 rpm, you may have other issues!


Now an ECU reprogram changes the way the spark and fuel is delivered. The really aggressive maps call for premium fuel in order to advance spark farther than low octane will support- that produces torque (at least it does in my Miata!) For low rpm grunt, that may give you what you want, at a price.

Best way to get more towing power is obviously to get more cubic inches- like a GM vortec 8.1.
 

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fixjet / Jimwilson,
I will concede that the mpg would probably have dropped some, around to 2mpg better. But if you dont see a change in mileage with a better flowing muffler than it is your driving habbits that have changed.
The big three dont put high flow mufflers on there vehicles because they are to loud. Would you buy a Yukon XL that you had to yell to the person standing next to it to be heard? Or is resinating so bad you get headaches (with a flowmaster 40 thats what you do/and get) A mufflers definition is to muffle/reduce noise.
By your own admision you said the engine doesn't work as hard, if an engine isn't working as hard it isn't using as much fuel, an enginge is nothing more than an air pump. The easyer it is to move the air in/out improves it performance.
My last point, if they didn't give you at least performance if not mpg why would the after market buisness be so big? Wouldn't take long for the word to get out it was a waist of money to upgrade your exhaust.
This is my 3 vehicle that I did exhaust work on, and they all saw improvements.
85 Bronco/90 Suburban/03 Yukon XL
My 2 cents worth
 

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fixjet said:
If you have a newer GM, say a 03 or 04 HD don't bother, as they have excellent exhaust systems that you won't improve on much.
I have the Chevy 2500 Crew with the 6.0L V8 and a 3.73 geared rear. The stock exhaust is massive and it sounds GRrrrrrrreat! When my foot really gets into it, it reminds me of NASCAR!
Sweet!


Cheers!
 

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I stuck the 70 series Flowmaster muffler on my '03 F250 CC 5.4L. Maybe in slight increases in power and mileage are in my head. But...I'm easy to please.

Works for me.
 
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