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Old 05-09-2015, 08:52 AM   #1
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Exhaust diameter conumdrum

Hey, so I recently bought a 2012 V6 Mustang with the perf. pack, and the only mod it has is a JLT CAI. I'm looking into exhaust upgrades, but having trouble putting together something where the exhaust tube sizes match up. This is what I'm looking at:

BBK long tube headers [1.75" diamater tubes, collector size unknown]
BBK shorty catted X pipe [2.75"]
Stock X pipe-to-muffler section [2.25"]
Roush axlebacks [No idea, I'm assuming 2.25"]

So if I went this route, the x pipe tubing would be a half inch bigger than the rear section of the exhaust, unless I went with an actual catback which are all 2.5", but all the actual catbacks they make I either don't like or are too expensive.

The other way would be to get a muffler shop to fabricate a new X pipe-to-muffler section with 2.5" tubing, but then I'm not sure if the Roush axlebacks would fit on 2.5" tubing.

Of course the easiest and cheapest way to go would be to get a set of shorty headers and the Roush axlebacks and leave the rest alone, but I really want the long tube headers.

So, does anyone have any experience installing larger diameter exhaust components? What did you do for the rest of the exhaust piping?
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Old 05-09-2015, 10:10 AM   #2
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Exhaust diameter conumdrum

I would see if a shop can fab up 2.75 from the longtubes back and then try to find some axlebacls that are 2.75. It might be cheaper and then you know you will have a solid matching diameter back.

Unless you need the cats, then do the LTs, 2.75 x pipe, then have it fabbed 2.75 from the x pipe out the back.
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Old 05-09-2015, 11:07 AM   #3
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Most mid-pipes and piping all the way back to mufflers, are 2.5".
-2.25" is exceptable, but never any larger than 2.5" without a blower.
-I would consider purchasing a set of cats and have them welded on as close to midpipe intake as possible. MAC ProChamber, for example.
-Don't go catless, the cats keep the low end torque and exhaust tone nicely.
-A true dyno tune following the installation will give you maximum power results, especially low-end torque.
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Old 05-09-2015, 11:10 AM   #4
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Not to high jack the thread, but still relevant, is there a general rule of engine displacement vs exhaust diameter on NA cars?
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Old 05-09-2015, 01:16 PM   #5
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Originally Posted by Wysh View Post
Not to high jack the thread, but still relevant, is there a general rule of engine displacement vs exhaust diameter on NA cars?
Frankly, I dunno. I don't think many people actually DO know, because on all the car forums I've ever been on, everyone says something different.

My opinion is that it doesn't matter to performance. Some people say that if you go too big, then you don't have enough "back pressure." Which would make sense, but if you've ever seen the super hot-rodded drag cars, most of them run open headers. Not a lot of back pressure there, and I'm pretty sure they care about performance.

It may make more of a difference if the exhaust piping is longer though, if it's too big then the exhaust may lose velocity on it's way out...or something. I dunno, honestly I'm just talking out my ***. Regardless, the largest size exhaust piping any aftermarket manufacturer uses for V6 Mustangs is 2.75" and most of it is 2.5"


I don't care much about that debate though. My issue is that if the headers and X pipe use larger tubing than the rest of the exhaust, then the whole rear portion of the exhaust becomes something of a bottleneck. I'd rather have it the same diameter all the way out. Or at least closer than a half inch difference.
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Old 05-09-2015, 03:37 PM   #6
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I just recently got straight pipes with 2.5" pipes from the cats back and it's cheaper and can definitely be done. Just find a reputable muffler shop


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Old 05-09-2015, 04:18 PM   #7
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Quote:
Originally Posted by Wysh View Post
Not to high jack the thread, but still relevant, is there a general rule of engine displacement vs exhaust diameter on NA cars?
YES- Too large of diameter, you loose torque. It.It is critical and always advisable to get a dyno tune to maximize your new exhaust efforts.

EXHAUST ARTICLE:
What Are The Best Exhaust Systems? How Exhaust Systems Work - Performance Benefits & Install Tips
LINK #2:
How To Calculate Muffler Size and Exhaust Pipe Diameter Page1 - High Performance Pontiac Forums at Hot Rod Magazine Magazine
LINK #3:
Performance and road car exhaust
**From what I have learned reading flow charts and test results, great results come from longtube headers, MAC Prochamber mid-pipe, Flowmaster 10 Series chambered muffler.
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Old 05-09-2015, 07:03 PM   #8
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Your going to lose torque but still gain horsepower, it just depends on how large you go.


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Old 05-09-2015, 10:04 PM   #9
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The amount of torque you lose going to a 2.75 exhaust is not offset by the amount of hp you'll gain by having an exhaust larger than 2.5" .


It is not backpressure that is good for an exhaust, backpressure is never good. Velocity is inversely proportional to torque is the real issue. Smaller diameter = higher velocity = more torque. Larger diameter = lower velocity = lower torque, BUT can maintain a higher velocity in the upper rpms where the smaller pipe can choke up, and that's where you see slightly higher hp gains. However, in this case, they do not offset.

2.75 is typically the GT exhaust size, are you sure you're looking at the right x-pipe.
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Old 05-10-2015, 12:08 AM   #10
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Yeah from my research 2.5" is your best bet.


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Old 05-10-2015, 08:03 AM   #11
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Quote:
Originally Posted by robb15033 View Post
YES- Too large of diameter, you loose torque. It.It is critical and always advisable to get a dyno tune to maximize your new exhaust efforts.

EXHAUST ARTICLE:
What Are The Best Exhaust Systems? How Exhaust Systems Work - Performance Benefits & Install Tips
LINK #2:
How To Calculate Muffler Size and Exhaust Pipe Diameter Page1 - High Performance Pontiac Forums at Hot Rod Magazine Magazine
LINK #3:
Performance and road car exhaust
**From what I have learned reading flow charts and test results, great results come from longtube headers, MAC Prochamber mid-pipe, Flowmaster 10 Series chambered muffler.

Link #2... **** ya math and science. That makes perfect sense.
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