Mild Thunder and I figured around 35-40 HP difference between a dyno header and my Stainless marines, I will say theres going to be around that.

+1, and Smitty does such a great job at them

Unfortunately not, alot of differences due to fact water is going to be shooting out of exhaust and cant run on shop floor vs my dyno muffler configuration. Recently switched back to my 5" inlet/outlet hushpower mufflers.Read they are a 2200 scfm rated each. These emis have 4' od, 3.5" ID 36" long dry to tip risers. After port matching the exhaust manifolds yesterday and getting everything bolted on, this is what it all looked like:

So here's a question for you Scott (since your a flow expert/pipe max calc kinda guy), I need to extend these risers further to collect the water at least oustide (dump into/over trash cans then pump back out of them) so my tentative plan is to go buy some 4 x 6 clamp-on fernco rubber couplers, buy a 10 ft piece of 6" pvc, chop it in half, clamp on two 5" pieces on each tail.going from a 3.5 to 6" id pipe SHOULD give it enough expansion so that it (exhaust flow) doesn't this of it as a "extended" tailpipe, thoughtful I will be adding water at that point also. I have Allison parts issues to resolve for customers truck and other stuff before I can get back to this but hopefully today I can get some data. I do have 1 o2 bung wlded in a riser to get some o2 data two. Other thing is its MONSOONING here this AM so until air drys up no reason to dyno as CF will be over 10%, all the other pulls were at around 5% give or take 2, once it heads too far from that I dont trust accuracy of the data as much unless its back to back during THOSE conditions, Smitty

Im thinking at THIS hp level peak is going to drop 400 rpms, hp is going to be down to about 650 and torque HOPEFULLY goes up 10 ft lbs, not going to be thrilled if this whacks 60 or 70 hp off the top!

Yeh, the 6" should be looked at as atmosphere I would think. Well, you know what thinking/assuming does .....so let me look into it. Will be back in a bit.

BTW: i do know that running the 3.5"-4" exhaust 'uncoupled' into the 6" will be fine...I just need to look up if the two coupled will be okay.
Uncoupled: one slid into the other. Not connected w/ a sealed connection.

Okay, I'm back...got real busy...lol...anyway, gonna stand by y answers.

6" sealed to the 4" should be close enough,but down into 6" unsealed will definitely act like being in atmosphere.