Quote:
Originally Posted by Mike C
That whole backpressure thing is a myth. An engine is an air pump and putting a cork in the backside is of no assistance.
Go with the 2 1/2". With a compression bend exhaust like a local muffler shop will make, it's really equivalent to a 2 1/4" mandrel bent setup.
The 2 1/2" rams horns and 2 1/2" exhaust is what I did on my Jimmy with some Dynomax Hemi Turbo mufflers (long 20" case) and I have been super happy with that setup and the "butt-o-meter" suggests no power fall off compared to the Heddman headers and 2 1/4" exhaust it replaced. We are doing exactly the same on my kid's Jimmy project except running some more teenager centric Magnaflow mufflers that came off a parts truck I had.
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I'm an engineer by training so I read up on the science of it. The backpressure stuff is a myth except in a narrow band in two stroke engines. What is important in a manifold is the design so that airflow creates pressure drops where the ports meet to scavenge the gas outwards into the exhaust. But you need to reduce pressure in the exhaust as you push air through it and wider diameter system does that really well.
Putting backpressure behind a big carb and a scavenging manifold is like inhaling through a giant hose and exhaling through a tiny straw, it simply reduces performance.
There is an excellent video posted by the hot rod network where they change exhaust from 2" to 3" to open pipes whith the same engine sitting in an engine dynoamometer.
Open pipes creates better performance in all aspects except a narrow rpm band below 2000 rpm where they theorized that the manifold of the 2" system scavenges really efficiently. But add 500 rpm and overall performance increases by 30% making that narrow band of efficiency pointless. The 3" system lost what looked like 5% vs open pipes. The 2" system lost 15%. A fairly siszeable loss for just a few lbs of more metal in the exhaust.
Either way, I enjoyed reading up on the science of it. 2.5" pipes is what I'm going with. I'm using borla mufflers at 2.5" in and out. I have borlas on my Dodge viper and I have been really happy with them.