# Exhaust Modifications



## 219 (Mar 19, 2004)

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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## NDJollyMon (Aug 22, 2003)

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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## hatcityhosehauler (Feb 13, 2004)

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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## fixjet (Jan 19, 2004)

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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## NDJollyMon (Aug 22, 2003)

Going to add the TB Spacer too.

More air! More air!


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## 219 (Mar 19, 2004)

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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## NDJollyMon (Aug 22, 2003)

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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## JimWilson (Feb 24, 2004)

NDJollyMon said:


> Going to add the TB Spacer too.


Don't waste your money -- the only thing you'll gain is an annoying whistling sound. And don't ask me how I know that...


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## JimWilson (Feb 24, 2004)

[email protected] said:


> I was told that I should expect about 10-15% increase in horsepower.


That's highly unlikely. AAMOF, in order to get those kinds of numbers you'd need to swap the entire exhaust system, including the headers. A 10%-15% increase in the HP of any motor is not as easy as bolting on a muffler. I wish it were that simple...


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## NDJollyMon (Aug 22, 2003)

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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## JimWilson (Feb 24, 2004)

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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## DernMooseAK (Apr 16, 2004)

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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## JimWilson (Feb 24, 2004)

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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## NDJollyMon (Aug 22, 2003)

Jim...
How did you figure out I was PSYCHO!


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## fixjet (Jan 19, 2004)

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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## kjp1969 (Apr 25, 2004)

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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## DernMooseAK (Apr 16, 2004)

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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## mjwencl (Feb 25, 2004)

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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## fixjet (Jan 19, 2004)

DernMooseAK, I didn't say they didn't help preformance ( I wouldn't have bought 3 systems for my trucks if they didn't ) I just don't believe the MPG claims. In my 3 GM trucks 1 mpg maybe on average.


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## jrayburg (Jan 27, 2004)

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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## mjwencl (Feb 25, 2004)

My vehicle with stock everything get's 19.7 MPG at 55 MPH on flat terrain with there is no prevailing wind (in other words, calm). Tires are OEM stock and inflated to 50 psi fronts / 80 psi rears.

Towing MPG is still undetermined as I'm still putting on my 500 miles of trailer towing break-in use limited to 50 MPH. I'm currently at 10.0 MPG at this speed as it doesn't shift up a gear until you hit 55 MPH or higher. Yes ... I tried it once already.







The Instant Economy MPG rose to around 13.7 MPG then.









Update: Pulling Trailer Average Economy has been around 8.8 MPG. That's pulling everywhere under all conditions. Stop and go traffic to 70 MPH interstate traveling. That figure is for over 2500 miles of travel also.


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## Pakeboy2 (Aug 13, 2005)

On our '99 Suburban, I put in a K&N filter, MagnaFlow muffler and a throttle body spacer. MPG went up dramatically over stock. Wife is happier!!!


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## aplvlykat (Jan 25, 2004)

I also modded the exhaust with a flowmaster big block 70 series muffler, the sound over stock is minimal. I also modded the intake. If you look at how the intake air on a 1999 suburban comes into the filter box it enters through a 3 inch tube from the fender well and it draws air from behind the headlight asembly through a 3 inch opening. What I did was to add a second 3 inch tube into the filter box from the wheel well and added a second 3 inch opening behind the head lights. then I installed a scope inside the grill next to the air conditioning condensor to force feed air to the larger intake air openings. I did a simular thing to my 2003 tahoe, on that one if you remove the air filter box and frame there is a direct pathway to the underside of the truck. I installed a 3"x9" duct with scope, reinstalled the stock air box which I had to cut out the bottom to fit the new air duct, then I used a K&N high flow filter. In both cases there seemed to be better performance and increased gas mileage. I just clocked the MPG on a 200 mile trip and I averaged with my 7.4 liter motor 16.4 mpg not towing, I wish I had checked before but I didn't. Is this good for a 7.4? what is everyone else getting on the freeway with simular motor? Kirk


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## MJRey (Jan 21, 2005)

Randy,

I've got the exact same truck as you and I've been looking into mods such as intake, exhaust and performance programmers. Based on much reading on F150Online.com I decided two weeks ago to order a Superchips Custom Tuner, SCT-2, from a place back east called Troyer Performance. I also looked hard at the Magnaflow Cat-Back exhaust for our trucks, part number - 15755, and it looks real nice and they show on their web site test data and sound clips of one installed on an 03 Expedition. The only thing I didn't like was that the chart they show of horsepower and torque is only from 3400-4400 rpm. What I'd really like to see is a chart going down to about 1500 rpm so I could see if it helps any at lower rpms where I'm usually towing at. Both the SCT-2 and the Magnaflow were about the same price so I decided to try the programmer first. I should have it in another week and I'll let you know how it works out. One nice thing about the SCT-2 is that I'll have three programs to choose from; 87 Octane-Towing, 91 Octane-Towing, and 91 Octane-Performance (no towing). I'll probably try the 91 towing program first to see if the mileage improves. I won't be towing the trailer anywhere until the second week in September so I'll have to wait until then to see how it helps pulling the Outback. The trip I've got planned in Sept. will take me on almost the exact same route I last towed the trailer on so it will be a good comparison between the stock programming and the custom programming.


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## Reverie (Aug 9, 2004)

Is it just me or aren't we supposed to tow with the Overdrive OFF? The gain in MPG even at $2.50 a gallon would have to significant to justify paying for a new or overhauled transmission.

Reverie


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## BoaterDan (Jul 1, 2005)

I'd just add that exhaust system results are better the closer you are to the engine... i.e. you'll get better results with a set of "shortie" headers than you would from the cat-back or just a muffler.

Those 10-15% figures come from tests performed on a 800 HP built motor, which of course is grunting and groaning through a stock exhaust like my 2-year-old trying to load up his diaper.

The TB spacer, and all those tornado whirlwind crap products are pretty well known in modder circles (I had a TransAm in my BK days) as being useless, or at least insignificant.


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## tdvffjohn (Mar 10, 2005)

My truck

KN air system.........improved MPG slightly

Custom 2 1/2 exhaust, duels thru one Flowmaster muffler.............love the sound, did not notice any other improvements.

TB spacer........bought as a gift from my son..........whistles and was useless as an improvement. (but on my sons s 10 blazer with 4.6, it improves power and mpg. so go figure.)

John


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## huntr70 (Jul 8, 2005)

My truck.....K&N Air intake, Flowmaster dual side exit exhaust from Summit, Synthetic fluids.

The only thing that gave me any better fuel mileage was putting the topper on the bed. It keeps the air flowing over instead of swirling. Actually gained me 2 MPG over a Tarp cover. I averaged 9 MPG on our recent trip to Maine. That was the 360 ci engine and towing OUT of overdrive. Also had quite a few LOOOOOng hills on I84 from NY to Mass,. but hey, you don't buy these toys to worry about putting gas in them!!! Definitely getting a 3/4 ton next....diesel???.....If I can oversee the $6000 price tag above and beyond a gas...

Steve


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