[RookStang07]

Hey guys, I have a completely stock Mustang GT I just bought, anybody know how much horsepower I would gain if I bought Flowmaster 40's with 3.5 inch tips? Please get back to me. Thanks!

[vashthe3rd]

Axle-back exhaust mods will do very little on a bone stock GT.
HP increases go up as mods go up.

[bigwilly43729]

The tips are the key. Bigger tips = more HP.

Doubt you'll feel any significant difference on the butt dyno.

[racin366]

Not much power, better sound though.

[fobra559]

A 2rwhp gain with a slight loss of tq. Just mainly for sound

[stlwagon]

Gains will be minimal but it'll sound faster!

[Ricesmoker]

I bet you get 50 RWHP out of those mufflers!
Chrome are tips good for 5 RWHP

[jwal]

Originally Posted by Ricesmoker

I bet you get 50 RWHP out of those mufflers!
Chrome are tips good for 5 RWHP

Vinyl Flames/neons - good for another 10 hp
giant rear spoiler - good for at least 20, or so the highschool kids with civics say

[hotshot]

How much would a complete exhaust yield someone? Like LT+O/R X+axlebacks. Maybe 25-30?

[b1stang]

if your looking for a better sound do it. If it's horsepower your looking for start on the front end, intake's, supercharger, etc.

[stlwagon]

Originally Posted by hotshot

How much would a complete exhaust yield someone? Like LT+O/R X+axlebacks. Maybe 25-30?

Not quite that much. More like 15-20..

[Thereaper]

If you want more HP get a CAI and 93 octane tune.

[NCStangin]

Originally Posted by Thereaper

If you want more HP get a CAI and 93 octane tune.

No question the quickest and best way to pick up at least 20+ (conservative) Rear Wheel Horsepower. The best initial bang for the buck modification you can make. But exhaust and Mustangs are just like Harleys, you've got to change the mufflers. The 4.6 does sound schweeeet running through RPMs and some better flowing mufflers.

[six5mustangfan]

My advice is to buy axle backs strictly based on the sound. A lot of exhaust companies lie about or decieve you into thinking that you will gain a lot of horse power when in fact you will gain very little if any. Pick the one that sounds best to you!

[NCStangin]

Originally Posted by six5mustangfan

My advice is to buy axle backs strictly based on the sound. A lot of exhaust companies lie about or decieve you into thinking that you will gain a lot of horse power when in fact you will gain very little if any. Pick the one that sounds best to you!

Excellent point.
I would also recommend getting mufflers that use some type of packing as well (glass, stainless wool, whatever) like a turbo-style or derivatives there of. They make excellent sound, flow just as well, and don't tend to have the harmonic droning like the chambered (Flowmaster) style.
I have the Ford Racing 49-state axle back and love it. The only remotely droning noise is when throttling down in gear (stick) it makes a very sweet sound at 2200-2400 RPMs. Nothing like the 2 and 3 chambered Flowmasters I have run on 5.0's which would drown out everything else resonating through the unibody at about the same RPMs.