Assuming a 10 percent increase in volumetric eficiency, you will still need to spin the engine to 6000 rpm to see 530 hp. The most Merc ever got out of a 454 was 450 hp and that engine had a hyd roller, 800 Holley and a single plane intake. 550 is pretty easy with a small blower and it won't be nearly as bad tempered.

So you think a small blower is the easyest way,what kind of a blower did you had in mind and is it just bolt on or do i have to do some internally job.

I was thinking along the lines of a Wieand 256 blower with a single carb running 5-6 lbs of boost. It would be safe on a stock engine without an inter cooler on 93 octane and make 550 hp easy. You could find a used 525sc blower for less money but they really should use an intercooler if you're going to try for 550 hp. Sure there are things you should do to ensure long life. Larger oil lines and fittings and a larger oil cooler are at the top of the list. The engine should be in good shape to begin with, good compression and not too many hrs. on the valvetrain. The stock cam in a 420 is the same cam as the 525sc. It's a pretty good sc cam although it's mild and a flat tappet hyd. If the engine is in good shape, it's mostly a bolt on.

Just an FYI, Dave Wessledye just rebuilt an old pair of real simple 454cid engines for a guy....8.8cr, 750 Hollys, GM rectangle port heads...bone stock no port work or anything, GM DUAL PLANE intake manifolds, and some of RMBUILDER's easy going hydraulic roller cams.....dyno'd at 481hp @5300rpm and somewhere around 500 lbs of torque----dyno'd with water cooled marine tubular headers, alternators, all belts & pullies, etc. Jetted a little rich too!

I'd say not too shaby (I thought maybe 450-460hp at the most) for bone stock heads and dual plane intakes with an easy does it roller cam. Engines were some really old Merc Cyclone 400hp engines.

What about these cams
http://www.summitracing.com/parts/HRS-CL120405/
http://www.summitracing.com/parts/HRS-CL120255/

Do yourself a favor and call "RMBuilder"/Bob Madara of Marine Kinetics in New York. 585-654-8583 He will get you the best cam for your application.

Just think what would have happened if he would have pumped up the comp to around 9.5??

mrfixxall
 

Buy this one and swap the motors,then off the boat,i know this boat vary well and it will go into the 80's,tom spent around 30 k and everything is rebuilt,engines are stroked to 496,i know for a fact they dynoed a little 600 hp,trannys rebuilt with hd clutches and the drives were gone through..its been forsale for a whyle and i think she (his widow)will take 20k for the boat,,

Dont let the looks of the engines fool you they are new with app 80 hrs on them..

http://www.offshoreonly.com/classifi...o30340-en.html

The stock Merc 420 or Merc/Gil replacement exhaust on a Merc 420 HP with GM heads will not likely make 550 HP NA. If the exhaust is still wet, not likely even 500 HP. As others have said, on a 454-470 CI block with GM heads your best bet is a small blower. Besides the heads, the exhaust is also a big HP hurdle.

Merc made a 525 SC, would it make sense to copy this format? The 525 SC should also be the low cost upgrade.

Were they stock longblocks with the cam, carb, and intake change alone? I have 454 mags that were freshwater engines and only have a few hundred hours on them, and was possibly looking to bump them up to around 500hp without getting too crazy cost wise.