I got a pair of 420's that are due for a rebuild. If the heads check out I thought about throwing a set of small blowers on them. I'm only looking to make 500HP or so. Another route could be new aluminium heads and bump the compression a touch. I've never ran a blower, I have experience with turbos but not in a boat. Which one would be more reliable? Cost? Fuel efficiency? 311 Formula. We were going upgrade to bigger newer boat but wife can't let her go. Im getting a new truck instead. Lol

price wise i think it would be cheeper to reach your goals with a couple small blowers.the merc sc525 is a 454 with a small blower and every now and then these blowers come up for sale on oso.your goal can easily be reached with better heads and raised compression or superchargers but even the small blowers look cool.

As long as they fit under the hatch...

If you are rebuilding it is pretty easy to hit 500 HP on a single carb build with some decent heads, cam and valvetrain. Off the shelf pistons are cheap and plentiful to get the compression you want.

You need (2) 177 B+M blowers that came on the 525SC Merc engines. Single carb blowers that make about 550HP on a Merc 420... They are usually on here for sale in the swap shop.

I sold two extremely low hour merc 525 sc blowers, intakes and pulleys for 3k if recall. They were mint low hour. Just for a ref.

I had a merc 420 that I made into a 525sc. I had the ebay dart hp500 knock off intake. The blower is only about an inch taller. Anyways I did 2399 pistons, stock forged rods and crank. My heads were junk, so I picked up a set of HP500 rectangle port heads, the only difference was 2.25 vs 2.19 intake valves.

Heres the full list in a better order.

Block: Stock MK IV .060 over
Pistons: Speed Pro Forged 2399
Crank: GM Forged
Rods: GM Forged
Cam: Crane 134561 Flat Cam
Heads: HP500 Rectangle port
Valves: Manley Stainless intake with Inconel Exhaust
Springs:Crane 99896-16 springs
Rockers: Comp Ultra Pro MagnumXD
Blower: Weiand 177/ Merc525
Carb: 1050 Dominator
Exhaust: EMI Thunder.

It made 572hp @5600RPM, I dont remember the torque But I want to say over 600. I remember thinking while it was on the dyno that it could break a bravo on demand.
I have over 60 hours on the setup and it has performed perfectly. It starts easy. It idles well. I am really happy. It is thirsty though. I did gain 10mph over a 420.

Are you saying the blower setup is only an inch taller than a stock 420?

Yea, maybe 2 inches. I had both the HP500 intake with a holley carb and Spark arrestor vs the stock 525sc setup side by side on my workbench. I am not talking the stock weiand 420 manifold. Those were pretty low. I compared the HP500 higher style intake to the blower. The blowers manifold is no higher than the intake ports.

Cool, i didnt think a 525sc would fit under the hatch of a 311 but maybe it will