Does anyone have any idea what kind of horsepower these merc engines actually produce? I have read some threads that say these engines are actually a lot lower HP than what they state. I have a set of 1989 454/420HP , are they really 420 hp from the factory?

As far as I know, all of Mercs' horsepower numbers are lower than their engines actually produce. I guess they do this to cover themselves. If you were to put one of their motors on a dyno and get less power than they sell it as, I would think you would have a valid law suit.

Tyler Crocket in one of the builds he did on a stock 98 502. He dyno'd it before the build and it only made 393hp.

http://www.hotboat.net/members/artic.../BOLT-ON_s.htm

Russ

Russ Let me try this again... I assume this was a 502/502 merc engine?

Our federal Gov. allows OEM's a variable of +/- 10% on production engines. I have not seen this in writing, I have tried to find it, but my info comes from a couple of reliable sources. Bob, RMbuilder, due to his extensive background working in or for GM, may be able to shed some light on this.

Chris

I beleave it was a 502 mag. From Merc it was rated at 415HP.

393 HP is 5% low, but there could be that much variation from dyno to dyno, let alone from operator to operator and run to run. Statistics is a b*tch. Your 420s are great engines. If compression and tune is good, you make that much HP.

Thanks for the info...although I was hoping I had like 300hp and that explained why I only go 60mph.

formula377

What boat are your 420's pushing? I have the same problem with my resin bucket 357.