ECeptor

Ray,

I'm getting a set of 496HO take-off manifolds to use on my 330hp 454. The short block and cam are stock. The heads are 781's with 2.06/1.88" int/exh valves. The intake is a Performer 2.0 with possibly a 1" spacer. The exhaust is thru-hull only.

Question to you is, what mods to the stock 496HO headers should I do aside from removing the turbulator? Also, what performance gains can I expect vs the stock manifolds?

Thanks!

E

FOUNTAINATLAST

The 496 manifolds won't fit, flange to the block is different. Only fit 496 motors thats why 496 headers are so pricey, one motor application.

Raylar

No, not quite correct. The exhaust port outlet locations and bolt flange configuation on the new 496 motors are an exact match with any Gen version big block chevy heads. Mercury had Edlebrock make the flange bolt configuration on some of Mercury heads different on 4 locations out of eight so the header bolts on the CMI headers would clear the tubes.These were the earlier header designs, where the outer jacket tube continued all the way to the flange. The newer CMI's and some others now stop the outer tube about an inch short of the flange and these modified bolt locations are no longer needed unless you are using the big 2 1/4"-2 1/2" big tube sets. You can install 496 manifolds on just about any 396, 427, 454, 502 GM heads and most aftermaket big block heads except for the specially drilled Mercury/Edlebrock heads and vice versa as long as the headers or maniflds are tapped for the water temp sensors and they clear obvious physical obstructions and the headers were not drilled to fit the Mercury/Edlebrock heads. Just a little clarification here.

Ray @ Raylar

ECeptor

Thanks for the fit-up clarification.

Any historic data on performance vs stock Merc 454 manifolds?

Raylar

Marine engines respond to exhaust modifications pretty much based on how much better the other breathing systems are working, ie: intake, cam, heads etc. Some of the really old type Merc log manifolds flow like crap and suffer from the danger of water leakage at the riser flanges. almost any aftermarket manifold will flow better than these and usually make some noticble improvement in power, but the extent and size of the improvement will be dictated by how much better the motor needs to breathe on the exhaust side. In other words, if a marine exhaust is keeping up with the engines exhausting needs well the increase in power will be nominal. If the naturally aspirated motor needs more exhaust flow or help due to bigger cam, better flowing heads and intake system then most likely changing the exhaust system to a better flowing type will net bigger gains in power. This is not quite the case on supercharged or turbo motors, they need any help generally on the exhaust side that they can be given. Again its going to work differently on a motor by motor basis, no happy quck all the time "deal" here!

Ray @ Raylar

ECeptor

Thanks for the reply. At worse case, I know I've banked a good 50lb weight savings. Best case, I've picked up (hopefully) 10-15hp.

For $350, I can't go too wrong. The manifolds will work nicely when I get to re-do my bottom end, upgrade the cam and finish upgrading the heads.

Again, thanks for the technical info!