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Unread 07-03-2010, 01:33 AM   #1
SugarBear
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Flowmaster on TJ

Ok, my dad gave me his Flowmaster, with a 4" input and output on it. Now, my thing is, I'm not sure what size pipe is stock on the muffler of a TJ. I think it is 2.5"? Is that correct? If so, where can I get an adapter to fit a 4" muffler to my pipe?



Thanks!

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Unread 07-03-2010, 02:07 AM   #2
TheBoogieman
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2.25" and Advance auto or Autozone have adapters.It's better to get the right size muffler.
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Unread 07-03-2010, 05:38 AM   #3
Hemi08
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I thought if you increase the exhaust pipe size too much, it decreases backpressure which in turn decreases horsepower..like the "fart can" on imports.
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Unread 07-03-2010, 05:56 AM   #4
Demp
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ideally you want the right size muffler, otherwise there are resizing adapters that can be welded in. The one for the 4inch might sound a little hollow on a jeep since its made bigger for the increased exhaust flow.
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Unread 07-03-2010, 05:57 AM   #5
little_Jeep
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Quote:
Originally Posted by TheBoogieman View Post
2.25" and Advance auto or Autozone have adapters.It's better to get the right size muffler.
X 2....... for several reasons including size, noise, and backpressure.

If you wheel this Jeep, you don't want a real loud muffler as after a couple hours on the trail, you and everyone else will hate it.
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Unread 07-03-2010, 07:10 AM   #6
Dryseals
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Quote:
Originally Posted by Hemi08 View Post
I thought if you increase the exhaust pipe size too much, it decreases backpressure which in turn decreases horsepower..like the "fart can" on imports.
Not back pressure, scavenging. You never want back pressure, back pressure is like adding a plug. But you do want scavenging.

You want the exhaust pulse to leave the piping in a functional manner, to help do it's work. To do it's work it needs to maintain it's velocity. Think of the single pulse running down the piping, like a truck going down the highway. It creates a vacuum behind it just like a truck would. That vacuum helps the next pulse move down the piping. Since the pulse has mass, the amount of vacuum it creates is dependent on it's size. The pulse wants to expand, if it expands, it slows down, if it slows down you loose the vacuum effect.

HP is upper RPM, torque is lower RPM. A larger exhaust will normally give you more upper end HP but sacrafice lower RPM torque. So it's a balance between the two. What do you want, torque or HP and design from there. Either way, you never want back pressure, think velocity.
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Unread 07-03-2010, 07:31 AM   #7
highlander4x4
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Any exhaust shop should be able to adjust the piping and weld it in, but I would recommend getting the correct size.

Quote:
Originally Posted by little_Jeep View Post
If you wheel this Jeep, you don't want a real loud muffler as after a couple hours on the trail, you and everyone else will hate it.
My Flowmaster isn't all that loud.
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Unread 07-03-2010, 07:42 AM   #8
little_Jeep
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Originally Posted by highlander4x4 View Post
Any exhaust shop should be able to adjust the piping and weld it in, but I would recommend getting the correct size.



My Flowmaster isn't all that loud.

Neither is mine, but I don't think you can say that about all of them.
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Unread 07-03-2010, 07:43 AM   #9
little_Jeep
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Quote:
Originally Posted by Dryseals View Post
Not back pressure, scavenging. You never want back pressure, back pressure is like adding a plug. But you do want scavenging.

You want the exhaust pulse to leave the piping in a functional manner, to help do it's work. To do it's work it needs to maintain it's velocity. Think of the single pulse running down the piping, like a truck going down the highway. It creates a vacuum behind it just like a truck would. That vacuum helps the next pulse move down the piping. Since the pulse has mass, the amount of vacuum it creates is dependent on it's size. The pulse wants to expand, if it expands, it slows down, if it slows down you loose the vacuum effect.

HP is upper RPM, torque is lower RPM. A larger exhaust will normally give you more upper end HP but sacrafice lower RPM torque. So it's a balance between the two. What do you want, torque or HP and design from there. Either way, you never want back pressure, think velocity.

Post a link to all this... I'm old. It's been called backpressure for as long as I can remember
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Unread 07-03-2010, 08:37 AM   #10
Dryseals
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Quote:
Originally Posted by little_Jeep View Post
Post a link to all this... I'm old. It's been called backpressure for as long as I can remember
Wish I could, I'm old also and the term back pressure was always misused. I found an automotive engineering book (1960s) at the public library many years ago. It discussed exhaust systems very in depth. Calculations on pulse mass, heat volume etc etc. Everything down to the proper size and length of the header to obtain maximum scavenging for cam cycle valve over lap etc etc.
It was good enough that I bought a version for my self, which flooded in hurricane Ike. I've been looking for another copy since.

I've read a ton of articles on line since then, most just skim over the real details.

Do a search for exhaust scavenging. Most of it makes sense once you understand what's happening in the exhaust cycle. Then the same gas laws are applied to the intake cycle. Intake runner length does the same thing. Close the intake valve on the intake charge and what happens, the intake pulse bounces back up the intake runner, as it gets further away it creates a vacuum behind it and then retreats back down the runner. It'll bounce back and forth a few times before the energy is displaced. But if you open the intake valve at the right time, you get a ram charge from the extra energy.

I know your old enough to remember Oldsmobiles quad 4, they used this intake principal to gain extra power. All this energy can be put to use in improving performance, just remember "heat" is the real energy in the exhaust.
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Unread 12-16-2012, 09:35 AM   #11
SKINUM
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I've got a 2.5" muffler and my plan is to cut the stock pipe. Do I need 2.25 OD or ID for the reducer?

Edit***** Looks like it's 2.50 O.D. and 2.25 I.D. for the reducer.
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