As far as the differences between Merc's 454 Mag and the 420hp---The Merc 454 Mag was a rated at 365hp. It still had all the good forged bottem end parts that the Merc 420hp and 425hp engines had as well as having the same GM rectangle heads. I can tell you that the 454mag/365hp engine only had the stock GM stamped steel rocker arms----NOT the Crane gold roller rocker arms. It also had a smaller cam (flat tappet hyd), an aluminum hi-rise dual plane intake manifold with a Rochester carb sitting on top. The 454mag/365hp also only came with the cheaper sheet metal steel valve covers vs. the more rigged finned cast aluminum valve covers that came on the 420hp and 425hp. The 454mag/365hp came from the Merc factory with stock Merc exhaust system---NOT the GIL/Merc exhausts.

Back in the late 1980's when the 454mags/365hp engines were popular, I remember a couple of guys who got very board with that engine and had a larger camshaft installed, did some minor bowl/pocket clean-up work on the heads, install a Holley 750-800cfm carb, and installed some GIL exhausts. Some didn't intall the GIL exhausts right away and still gained torque and some HP, but the stock Merc exhausts were limiting the power to a great extent. Once they installed the GIL exhaust it would really wake the engine up. We didn't have GPS available back then, but the boats really picked up some good solid speed.

I have a 365mag I will be rebuilding this winter.
I'll be changing to a merlin int and a 800cfm holley with a small hy roller cam. I'll also be changing to a stainless marine exhaust.
If I get it dynoed, I'll let you all know the benefits.

Stormrider,

Sounds like an interesting project. If you do some bowl work (nothing major) to the heads that will help a little as well---especially on the short side radius's.

Still going strong.

Hey Cuda,

Whats up? What kind of engines do you have in that pic? 454 or 420?

CUDA...WHAT IS THE BRAND OF YOUR EXHAUST AND WHERE DID U GET THEM?
THANKS,
CHUCK

Cuda/Joe,

Love them yellow coils you got there!

Can't see the manifolds, but they look like stock Merc/Gil risers. They said on this thread that they 420 came with Merc/Gils

How many quarts of oil should a 1988 420 HP engine take?