dereknkathy

i see a lot of builds that don't seem to be that much more than stock merc builds and are running 200+ hp more than the stock 365 mag is rated. and if the mag IS that weak, what are the worst components to upgrade first?

79formula

I was thinking the same thing. I have a merc 420 and see guys with the same longblock, a single plane intake and roller cam advertising 500hp. 80hp gain from a cam + intake seems like a bit much.

Rookie

Cam, lifters, heads, intake, carb and exhaust. So about $6000 to get 565HP.

Don't ask me how I know.

79formula

The 330-365-420s are all very different motors. More than HP alone.

330s are all cast parts, peanut port heads, iron manifolds, dual plane intake and Qjet.
365s are forged lower end, rectangle port heads, iron manifolds, dual plane and Qjet.
420s are forged lower end with more compression, little more cam, gil manifolds, some had single or dual plane intake with Holley double pumpers.

So to get 500-600hp out of a 330 is going to take more parts/$$ than say a 420. They are very different.

TomZ

I thought the 420 was the same compression as the 365. Blueprinted, hotter cam, better intake (depending on year), and more carburetor. Gil exhaust.

Getting another 100-150 out of the 365 isn't that hard of a stretch. Depends on where you put your money. The short block is GOOD. Well suited and over-built for what's expected. Heads (with a compression bump) and a good hydraulic roller cam plus the exhaust upgrade (a must as it's the absolute bottleneck for the whole package) will make for a very stout engine.

These engines are getting old so count on the fact that they may need at least a basic refresh. Just my .02.

airjunky

The original berkeley packajet offered the olds 442 motor , basically, at 10.25 to one .
with a hideous exaust that makes center risers look like stellings .didnt last long .
the 365 i believe is pretty much the post high compression pre catalyst egr tune chevelle ss motor from 71
With a different exhaust and distributor tune ect ect
But that extra 100 hp they found on the peanut port with a cam swap i would like to see on a dyno
not that anyone would ever dyno a stock merc 330 with cast manis. I wouldnt . but i would believe there is more of a jump from the peanut motor to the mag than the 35to 50 hp suggests

TomZ

Actually, the 365 pretty much IS the '71 LS6 but with iron heads. Factory rated at 9.0:1 compression with aluminum heads, it made 425 HP. Swap the heads to iron rectangular ports (with a very slightly larger combustion chamber) and change the cam... presto... GM/Mercruiser 454/365 Magnum. In the hot rod world, this is one of the absolute best GM/Chevy big block platforms to build from... if you can find one now (automotive).

I can see getting good, strong numbers out of a 330 with heads, cam, exhaust. I dont see the longevity/reliability outside of something for a street/maybe strip car... definitely not in a marine application.

Unlimited jd

Pretty sure aluminum heads on those 70-72 motors were pretty rare cross orders between l89's etc. the standard ls5,6,7 were all iron heads.

79formula

The 365s were 8.6 compression while the 420s were 8.8 compression.

"MILD THUNDER
03-23-2014, 11:34 PM
Wonder where the hp420 gets its comp bump from .I'd swear they were built like " mini hp500's' been a while since the '80s though .going to have to check da books tomarrow now curious

Like I was saying earlier, the 420HP engines were assembled at mercury hi performance back then. One of the other things done to them, was larger intake valves. The 420's had 2.25 intakes, 365 had 2.19. You are correct that they state the static compression ratio of a 454 mag at 8.6:1 in my book, and the 420 at 8.8:1. My guess is maybe they installed larger valves, custom valve job, and a little head milling.

Most guys assume a merc rect port head is the same from a 454 mag, to a 800sc, but they were assembled with different components per application."

These engines are getting old so count on the fact that they may need at least a basic refresh. Just my .02.

Greatguy66

I had dyno done on a new 454 gen 5 magnum rectangular port/roller hyd cam/ motor.Dyno was over 420hp!Remember big difference between magnum motor vrs 330/454