Scrap the SM's.

For that I would go with Thunder n Lightning Headers/Manifolds; http://www.diamondperformanceparts.c...cat=282&page=1


A 598" I built with T&L's. https://youtu.be/u-5P0vw1UqQ

Blown the Gen II's are ok with the right pipes but really need Gen III's. NA, the Thunders are the way to go!

On that combo i would guess about 40-50hp difference,the big n/a engines really like headers they really like the big tubes 2 1/8 now if it were a blown 509 making 700 the headers would be good for 20 or so at best

Man those things are Uugly, sounds like a swap cam

Good info.

How about a blown 540, making 900-950?

And that's when bling can fuk you.

At 600hp+ level NA it's either this or correctly sized tubular headers.

BT: I've seen these in Merc Racing blue and they look pretty good. Obviously my opinion.

im thinking I don't have enough room between the motors for the 2 1/8 headers....side by side boat

My buddys pro charged 509's have stock HP500 Gils, with boat running 107, they gotta be making 825-850 HP, seems that exhaust is fine for him, he researched it years ago, everyone said with the boost it would not matter much at all... I think boost is a totally different animal..

All about personal preference I suppose.

Headers look like "money" because they cost you "money" and lots of "money headaches" down the road.

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I personally dont think those thunder n lightning manifolds look bad. I mean sure,stainless tube headers look cooler, but its not like the GIL or SM regular manifolds are pure sex. At least imo.

According to pipemax, a build like that maxing at 6000 rpm, making 750ish hp

For low to mid range power, a 2.06 to 2.131 diameter primary, with 18.8 inch x 3.829 to 4.079 diameter collector.

For mid to upper rpm power, a 2.206 to 2.331 diameter primary, with 18.8 inch x 4.079 to 4.579 diameter collector.

Thats a mile, wait, 10 miles, away from a typical stainless marine standard manifold. In the car world, if you stuffed a 750hp 572ci big block in your car, and ran a 1 7/8" shorty header, with a 3" inch collector, someone would smack you across the cheek.

I believe MER Performance recently dynoed a 650ish hp 509 build. Made close to 640 with the 2.25 dyno headers. Bolting the stainless marine manfolds on, it made right about 600hp, and lost a bunch of torque as well. About 30ft lbs I think at peak tq .