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Unread 04-21-2013, 08:54 PM   #1
Alex77cj7
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1977 CJ7 
 
Join Date: Jun 2012
Location: Lafayette, Louisiana
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cj7 304 exhaust pipe size?

okay so i need some help with exhaust.. I ordered a pair of fender well headers. Im going to run thrush welded (flowmaster copies) mufflers with the pipes exiting in front of the tires on each side. the headers have 3" collectors. I Dont Know how to calculate or figure out what size pipe i need to have the proper amount of back pressure and performance. I have a 304 with the following upgrades:
-bored
-performance pistons and rods
-crankshaft
-summit racing sum-8600 cam
-edelbrock intake manifold
-holley truck avenger 600cfm carb
-msd 6al ignition
-msd distributor with team rush upgrade
-electric fans

So i guess my questions are:
What are size pipe are yall running with fenderwell headers?
What size pipe should I run?
What length of pipe should i run in between the collector and muffler?
How much back pressure should i shoot for?

Ive never dealt with the sciences of performance exhaust so i dont really know anything about that. I'd rather not bring it to a shop for them to just run a small amount of mostly strait pipe..

Thank you so much,
Any help is greatly appreciated!
Alex

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Unread 04-21-2013, 09:12 PM   #2
Alex77cj7
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im estimating and others have estimated that im making approximately 300 hp. I dont know how accurate that truly is but it sounds reasonable with the upgrades made. I was considering staying with the 3" pipe size since thats the collector size but idk how well that will perform. I had a bronco that i cut the mufflers off of and ran with strait pipe that dumped out before the axle and it seemed like it barely had any power after i did that but after putting some mufflers on it seemed to get most of the power back. but then again that was a completely stock 302 engine with stock manifolds and true dual exhaust. the pipe on it was only 2.5" though. I just dont want to make the mistake of running the wrong pipe again and risk having poor performance. Headers come in tomorrow and id like to get it all done asap.

Thanks again,
Alex
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Unread 04-21-2013, 09:51 PM   #3
Alex77cj7
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also the muffler inlet and outlet is 2.5" but i test fit a piece of 3" and it fit perfectly over the inlet pipe.
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Unread 04-21-2013, 10:04 PM   #4
ASteve
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Alex,

I have a 304 as well and I just installed Dougs fender well headers, 1 5/8" primary tubes, 3" collectors reduced down to 2.5". Then I installed Flowmaster Delta 50's slim versions with 2.5" in and 2.5" out. From my research this was the best performance sizing for my application. RV Cam, Edelbrock Performer Intake, Holley 600cfm carb, headers, Team Rush, MSD CDI, MSD E-Coil...

http://www.jeepforum.com/forum/f8/da...l#post15241248

SC
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Unread 04-21-2013, 10:10 PM   #5
Alex77cj7
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Quote:
Originally Posted by ASteve
Alex,

I have a 304 as well and I just installed Dougs fender well headers, 1 5/8" primary tubes, 3" collectors reduced down to 2.5". Then I installed Flowmaster Delta 50's slim versions with 2.5" in and 2.5" out. From my research this was the best performance sizing for my application. RV Cam, Edelbrock Performer Intake, Holley 600cfm carb, headers...

http://www.jeepforum.com/forum/f8/da...l#post15241248

SC
So you run 2.5" pipe through out? I think that would be best too. I found a hp chart and it said for 300 horsepower with dual exhaust that 2.5" would be best but I figured that was with going all the way to the rear of the vehicle not just short piece if pipe and exiting before rear tires.
Thank you so much!
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Unread 04-21-2013, 10:15 PM   #6
ASteve
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Look at my link I posted. It shows the entire set up...then scroll down and there is a sound clip.

If I were going out to the rear I would not install fender well headers, heck if I were to start over I would go with in frame headers...and out the rear...

SC
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Unread 04-21-2013, 10:16 PM   #7
Alex77cj7
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Quote:
Originally Posted by ASteve
Alex,

I have a 304 as well and I just installed Dougs fender well headers, 1 5/8" primary tubes, 3" collectors reduced down to 2.5". Then I installed Flowmaster Delta 50's slim versions with 2.5" in and 2.5" out. From my research this was the best performance sizing for my application. RV Cam, Edelbrock Performer Intake, Holley 600cfm carb, headers, Team Rush, MSD CDI, MSD E-Coil...

http://www.jeepforum.com/forum/f8/da...l#post15241248

SC
Also what length of pipe did you use in between the header and the muffler? Isn't there a certain minimum length that is needed?
Sorry for all the questions. I just really don't want to mess this up..
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Unread 04-21-2013, 10:19 PM   #8
Alex77cj7
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Quote:
Originally Posted by ASteve
Look at my link I posted. It shows the entire set up...then scroll down and there is a sound clip.

If I were going out to the rear I would not install fender well headers, heck if I were to start over I would go with in frame headers...and out the rear...

SC
Thanks. I totally missed seeing the link the first time! Sounds great and looks great too! I like the look of the fenderwell headers better with the side pipes. I was going to run the in frame ones but I didn't want to have to bring my jeep to an exhaust shop or have to fight with getting the right fitment and everything. This is pretty much straight forward. Thanks for all the help.
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Unread 04-22-2013, 08:58 AM   #9
IFADAT
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Purple Hornies are what I used. I wish they were longer but they sound good.

Click to View Search Results for Google (Search Results) Google (Search Results)

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Unread 04-22-2013, 09:41 AM   #10
Alex77cj7
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Quote:
Originally Posted by IFADAT
Purple Hornies are what I used. I wish they were longer but they sound good.

Video Link: http://www.google.com/search?client=safari&rls=en&q=purple+hornies&ie=UT F-8&oe=UTF-8
Thanks! Sounds good!
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