Bigger front pipe to smaller diameter cat-back, bad idea? - Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community

shu5892001 · 04-12-2013, 01:48 PM

Hi,

I am currently running 2.5 inch un-resonated perrin cbe. I was wondering if I can pair it up with 2.75 inch over pipe and front pipe.

Would this work?

Are there any disadvantage?

Thanks!

shu5892001 · 04-12-2013, 02:56 PM

anyone?

BuBlake · 04-12-2013, 03:20 PM

The only disadvantage is that you'll be slightly more constricted on the cb. I believe that with NA, the main restriction is the mufflers themselves, not necessarily the pipe diameter; I don't think you'll see any difference at all unless you're going straight pipes with a turbo, but I'm no pro.

Calum · 04-12-2013, 04:37 PM

As the exhaust cools on its way through the exhaust it's volume decreases. I've often wondered why exhausts aren't designed to maintain a velocity, meaning a gradual decrease in tubing size, as opposed to maintaining just the tubing size. You MIGHT even be doing something good.

shu5892001 · 04-12-2013, 05:03 PM

Quote:

Originally Posted by Calum

As the exhaust cools on its way through the exhaust it's volume decreases. I've often wondered why exhausts aren't designed to maintain a velocity, meaning a gradual decrease in tubing size, as opposed to maintaining just the tubing size. You MIGHT even be doing something good.

boredom.is.me · 04-12-2013, 05:08 PM

Quote:

Originally Posted by Calum

As the exhaust cools on its way through the exhaust it's volume decreases. I've often wondered why exhausts aren't designed to maintain a velocity, meaning a gradual decrease in tubing size, as opposed to maintaining just the tubing size. You MIGHT even be doing something good.

Because cost - they would probably have to use seem welded tubes, which would take much longer and much more effort to produce.

mrk1 · 04-12-2013, 05:11 PM

^^^ Agreed, you could step it or reduce it down but thats not all that gradual. A pipe that slightly tapers in diameter while following the windy path under the car sounds like a night mare to produce.

Good thinking though

Calum · 04-12-2013, 05:14 PM

Quote:

Originally Posted by mrk1

^^^ Agreed, you could step it or reduce it down but thats not all that gradual. A pipe that slightly tapers in diameter while following the windy path under the car sounds like a night mare to produce.

Good thinking though

Well back to my thinking room.

mrk1 · 04-12-2013, 05:18 PM

Its not like your going to hurt anything. Ive been running a 2.5" over pipe and catless front section with the stock cat back for a few weeks. Nice and quiet

boredom.is.me · 04-12-2013, 05:20 PM

There is one option that may actually be cheaper to produce, but it would be a bit heavier. Stamped halves (top and bottom) would do the job. The initial costs would be much higher though as you would need molds to stamp with.

boredom.is.me · 04-12-2013, 05:21 PM

Examples:

The exhaust headers as will as the front pipe have stamped covers I believe.

86pro · 04-12-2013, 08:11 PM

you just will get more pick up but your back pressure and top end will be reduced. No big deal. If you were turbo then that would be close to fatal. The size difference isnt that much so not going notice it easily.

04-12-2013, 01:48 PM	#1
shu5892001 Ontario's FT86 Join Date: Aug 2012 Drives: 2016 Lexus GS-F Location: Toronto Posts: 685 Thanks: 261 Thanked 165 Times in 111 Posts Mentioned: 2 Post(s) Tagged: 0 Thread(s)	Bigger front pipe to smaller diameter cat-back, bad idea? Hi, I am currently running 2.5 inch un-resonated perrin cbe. I was wondering if I can pair it up with 2.75 inch over pipe and front pipe. Would this work? Are there any disadvantage? Thanks!

04-12-2013, 05:11 PM	#7
mrk1 Senior Member Join Date: Mar 2013 Drives: Sterling BRZ Ltd Location: New England Posts: 1,702 Thanks: 403 Thanked 1,389 Times in 671 Posts Mentioned: 21 Post(s) Tagged: 2 Thread(s)	^^^ Agreed, you could step it or reduce it down but thats not all that gradual. A pipe that slightly tapers in diameter while following the windy path under the car sounds like a night mare to produce. Good thinking though __________________ The Build Thread GT28RS - eBoost2 - 3.91 Final Drive - Supra LSD

04-12-2013, 05:18 PM	#9
mrk1 Senior Member Join Date: Mar 2013 Drives: Sterling BRZ Ltd Location: New England Posts: 1,702 Thanks: 403 Thanked 1,389 Times in 671 Posts Mentioned: 21 Post(s) Tagged: 2 Thread(s)	Its not like your going to hurt anything. Ive been running a 2.5" over pipe and catless front section with the stock cat back for a few weeks. Nice and quiet __________________ The Build Thread GT28RS - eBoost2 - 3.91 Final Drive - Supra LSD

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04-12-2013, 02:56 PM	#2
shu5892001 Ontario's FT86 Join Date: Aug 2012 Drives: 2016 Lexus GS-F Location: Toronto Posts: 685 Thanks: 261 Thanked 165 Times in 111 Posts Mentioned: 2 Post(s) Tagged: 0 Thread(s)	anyone?

04-12-2013, 04:37 PM	#4
Calum That Guy Join Date: Dec 2011 Drives: 2013 asphalt FRS MT Location: Halifax, Nova Scotia Posts: 4,865 Thanks: 5,058 Thanked 2,868 Times in 1,499 Posts Mentioned: 82 Post(s) Tagged: 1 Thread(s)	As the exhaust cools on its way through the exhaust it's volume decreases. I've often wondered why exhausts aren't designed to maintain a velocity, meaning a gradual decrease in tubing size, as opposed to maintaining just the tubing size. You MIGHT even be doing something good.

04-12-2013, 08:11 PM	#12
86pro Senior Member Join Date: Sep 2012 Drives: frs Location: tdot Posts: 149 Thanks: 13 Thanked 22 Times in 19 Posts Mentioned: 3 Post(s) Tagged: 0 Thread(s)	you just will get more pick up but your back pressure and top end will be reduced. No big deal. If you were turbo then that would be close to fatal. The size difference isnt that much so not going notice it easily.

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