454 magnum vs 420 engine
 

What are the differences between 1988 454 magnum and 420 engine both from mercruiser? I was told the 420 is better??? Please advise?

Re: 454 magnum vs 420 engine
 

From what I have been told they are two very different motors. The 420 will make more HP, and they have better parts in them. I believe same crank and pistons but the 420 I was told has roller rockers and so on. The Mags came I think in a few different flavors from 365hp-385hp? The 420 is obviously more.... I am sure that you will get some much more definative info....

Jon

Re: 454 magnum vs 420 engine
 

Pretty much what Audio said. Roller rockers, cam, intake, carb. Not sure about CR, but I can check. Bottom end/block is about the same.

Re: 454 magnum vs 420 engine
 

I believe the HP420, HP425, HP465 etc were the early "HP" motors. better parts for the High Perf engines.

Re: 454 magnum vs 420 engine
 

Holley 800 carb, Wiend single plane intake, hotter cam, Crane gold roller rockers, high perf. exhaust system.

Re: 454 magnum vs 420 engine
 

Does anybody know anything else about a 420? Are they still available? Please advise.

Thanks

Re: 454 magnum vs 420 engine
 

Other than the exhaust. looks like it would take just over $1100 to convert a 365 Mag into a 420. using Summit racings catalog.
Then another $1719 for some Aluminum Edelbrock heads. How much more hp would the heads give you?
$2000 for Brodix heads
$2200 for Merlin and just a little more for Dart heads

Re: 454 magnum vs 420 engine
 

The 420 also has the "188" casting rectangular port heads & matching Team G single plane intake manifold..... no, the 420 is no longer made, but can be duplicated... better cams are available today for it. Bare head castings now are about 700.00 each here (new) and used are not much cheaper, so aluminum heads are a good & comparitive replacement.

Re: 454 magnum vs 420 engine
 

Get a crate 502.My 2 cents.

Re: 454 magnum vs 420 engine
 

the 420 cam is the crane #h-228/312-2s-14t1.2 or part number#132561 in the crane cataloge 228/236 duration at .050 114 lobe seperation 530/551 lift.i would upgrade to a rollerfor not much more money but a lot more torque and horsepower from th same bottom end.use a dart single plane with an 800 holley jetted up alittle

Re: 454 magnum vs 420 engine
 

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.

Re: 454 magnum vs 420 engine
 

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.

Re: 454 magnum vs 420 engine
 

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.

Re: 454 magnum vs 420 engine
 

Still going strong.

Re: 454 magnum vs 420 engine
 

Hey Cuda,

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

Re: 454 magnum vs 420 engine
 

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

Re: 454 magnum vs 420 engine
 

Cuda/Joe,

Love them yellow coils you got there! :D

Re: 454 magnum vs 420 engine
 

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

Re: 454 magnum vs 420 engine
 

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