3" or 2.5" Pipes?

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ADHD426

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I have a 1971 Duster the specs are below. I already bought a pair of Dynomax ultra-flow welded mufflers, which are 2.5" opening. I am planing on buying Doug Headers which are a 3" collector. Should I go with 2.5" pipes or 3"? I am also planning on doing an H pipe.

My Specs:
340 Bored over 30
904 Automatic
3.91 Gears sure grip
Martin Saine 3500 Stall 9.5" Torque Converter
Air Gap Intake
X Heads
Doug Headers
Holley 750 Double Pump with Porform Main Body and Metering Blocks.
Cam:
Lobe Center: 112
Advertised Duration Intake/Ex: 269 / 282
Duration @ .050 Intake/Ex: 223/234
Lobe Lift Intake/Ex .320/.329
Gross Valve Lift Intake/Ex .480/.494

Thanks!

Andrew
 
2.5",all the way. Size should be good to 450 h.p. 3" works well at 500 ish,will be noticeably louder.Great choice,on the Ultra Flo's.
 
2.5" will work with that build. Do you ever plan on going bigger? I'd go 3" now with an x pipe and be done with it. 3" will not be louder.
 
3 inch is overkill. 2-1/2 inch will be excellent!
 
..the H pipe will make your system quieter and should be installed 12" - 14" behind the collectors according to the Mopar Performance Guide.
 
exhaust-size-chart.jpg
 
Great thread as I am about to take my Scamp to the exhaust shop. I am still running the old slant 6 and am going with 2 1/2 " with a glass pack.

I see a lot of guys running this set up.
 
If you simply wanna waste a lotta money and have a loud car, put 3" on it.
 
My old engine was a dead ringer for yours with the exception of cam lift (mine was a .557") and TTi Headers. I ordered the 3" pipes all the way back thinking I'd likely want more horsepower in the future and, although I'm running a 360/380 crate magnum currently, I still expect to go bigger in the near future.
Go with the 3".
 
3" pipe isn't necessarily louder then 2 1/2" pipe. its all in the muffler really.

have a friends car that is running 2 1/2" super turbos and its a lot louder then my 3" super turbo system ever was.
 
Here is another area where real world scenarios override what everyone is posting here. My good friend has ran 9.20's in the quarter running 2.5" pipes all the way to the bumper! Is there a place for different size exhaust? Well sure, but again there is Huge mis information out there on this subject. Lol to the max on the chart/graph that was posted.
 
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