GM Marine 420HP Cam

[Nikto]

The GM part number for the 420 HP 8.1 Marine cam

is 12558912.

Has anyone tried it??

[Tjwong]

Just be aware that GM rates their marine motors different than a standard truck engine. For instance the standard truck engine rated at 340 Hp using the testing standard for trucks is rated at 375 Hp when using the marine testing standard.

 

The main difference is that truck testing uses a exhaust restriction of 10" Hg and the maring test standard is 3" of Hg. That standard alone makes the 35 Hp difference in Hp ratings between two otherwise identical engines.

 

So that 420 Hp HP2 performance engine may only produce 375 or 390 Hp when tested at the same truck/automotive standards. In reality the actual test measurements from GM was that the HP2 produced 415 Hp @ 5000 RPM and not 420. Torque peaked at 498 ft/lbs @ 4000 RPM.

 

I know of one person that used that cam and there was lots of tuning issues associated with it. The installation was in a C5 corvette where they shoehorned a 8.1L engine in it. Once the cam was installed, and mind you for all intents and purposes this is a very mild cam. The engine had a very bad tip in hesitation, surge and got real bad fuel economy. It took several hours of dyno tuning and editing of the tables to get it to run right. In the end the dyno chart gains was not so impressive or was the time and effort worth the gains. If I remember right they gained about 18Hp on the chassis dyno after all said and done.