I was intrigued by a recent thread where the poster showed pics of his new-to-him boat that had twin 425 hp 454 cid engines. Then I found another posting that showed Mercruiser engine power ratings from years ago. How in the world did they get those many ponies out of a 454?

All it takes is a cam, exhaust, intake and carb. I cloned my carbed 454mag to very close to 420/425 specs and picked up 400rpms. Another member closely copied my build and dyno'd his at 425hp

What year is that chart?

On it the 454/425hp is categorized with the 540/575hp in the "performance products" section........wasn't the 575 simply a 540/500 wearing a small Weiand 256? If so, could the 454/425 simply be the same thing on the smaller displacement 454?

Shortly after the HP500 came out, they came out with a HP450 which was
a 450 hp 454

Many years prior to 575SCi there was a N/A 540ci 575hp Merc Racing engine in the early 90's

The 420/425hp 454's were 454 mags that had the same cam as the 525SC, roller rockers, Holley carb, different intake manifold and Gil exhaust.

Stroke it and who knows what you will end up with.

the old 420's were great motors. made nice power and ran forever. have had many thru the years.

I wonder why Mercruiser backed off so much on the power output of the 454s in later years? Especially considering the use of modern fuel injection systems and the ability to control so much more going on in the engine. The chart shows a relatively mild 8.8:1 compression ratio. So in this pretty much un-stressed engine, it was developing damned near 1 hp per cubic inch, pretty damned good for a carbureted normally aspirated engine!

i always thought Mercs work with the 454's in the 90's was some of their best, i had a pair of 525SC's and that was one fantastic engine for what it was. After that i think marketing overshadowed engineering a bit, i cant say that was a bad thing looking at how innovative their lineup is today. Sort of a necessary evil i guess.

Emissions (clean air act) and they also wanted to concentrate on outboards. When Carl Kiekhaefer owned Merc they were pretty exclusive with Chevy and even built their Nascar motors for them. When they sold to Brunswick even though Carl stayed on for a few more years the exclusivity with Chevy went away. But even with the new competition (Ford) to buy long blocks from Merc knew putting add on's on other makers motors was not the future.