How fast have you gone with exhaust manifolds?
I lost the URL for this manifold test months ago, but had forgotten that I'd printed the article out. Ran across that article by accident, this afternoon.
Here is the URL:
http://tinyurl.com/6jdgne
This guy set up a dyno "mule" engine and ran individual tests of 340 manifolds using two left side manifolds, then, two right side manifolds and then, one of each. He also tested Magnum truck manifolds (two right side manifolds;) dunno why he didn't test one of each, but I think there's not much difference in the right and left sides with the truck (Magnum) manifolds, so maybe he didn't see the necessity of doing that test.
He also tested the engine with a set of dyno headers.
It's pretty educational, especially to see how much flow difference there is between the right and left-side 340 manifolds. Too bad you can't run two right side manifolds with the engine in the car... lol!
Check it out. It will make you want to ditch that driver's side 340 (upswept) manifold...
Bill