Re: Mercruiser 420 HP specs.
 

Just Joining the band wagon. I also have two of these Intakes just laying around. I like to run the Holley cast iron hi rise with squarebore. Were on cars 65-68 . They run great if you bore out the throttle bores. They are an excellent manifold. Mercruiser would have used them but didn't because they were no longer a production item when Mercruiser got into the hi- performance realm. The manifolds I'm talking about are large oval port. On a naturally aspirated engine, these and a set of 781 or 049 heads with big valves and some porting will outperform a 454 mag with 990 rectangle heads and rectangle intake up to 6000 - 6500 depending on compression , cam, carb, etc... BTW I have the cast iron Holleys. BBB

Re: Mercruiser 420 HP specs.
 

I just scored a GM aluminum high-rise dual plane intake part# 6269318 with the brass in the water jackets. I'm going to see how it works compared to my Weiand Xcellerator. The engine has the Stainless Marine manifolds. Something isn't quite right. The boat is only running 54-55 MPH @ 4900 RPM. I would think that with 420HP it should be at least in the 63-65 MPH range. The problem might be in the boat, I can't seem to be able to trim the drive up much at all until it starts to porpoise.