How many hours does a 454 motor generally handle before it is need of a rebuild? For everyones informaTion please state the magnum
and regular 454

I had a 1987 454 Magnum 330 go 1600 hours and was still running strong when I pulled it cause I had extra money for a more powerfull engine. I got some money for it from a guy that needed a cheap running engine. This engine is not really considered a true "Magnum" although Merc called it as such. 2 bolt main, cast pistons and plain rocker arms. It was in a 23' Velocity. At normal cruise the engine was loafing at about 3000-3200 rpm. I bought the boat with 150 hours on it and I still have it today. It always had either Mobil 1 15w-50 or Amsoil 20w-50 (mostly Amsoil after the first 2 years). Now that I have a larger boat, my brother is the principle user.

454 mags= top end refresh around 500-600hrs and full rebuilds in the 1000range

7.4's= leave them alone until the break. Probably somehwere in the 800-1500hr range.

My 1992 330HP went south at 600 hours. I was never very nice to it when driving although it did get frequent oil changes with Mobil 1.

The 92 outdrive is still going strong with a 496 Mag HO driving it.

my buddys untouched 365 mag has 1500hr on it and still runs strong

If you maintain them regularly they'll go 1000-1400 hours from what I'm told. I've heard to watch out for the manifolds because once they go you're finished! I'm swapping mine out this winter.

I have a 97 454 mag mpi with 550 hours and it doesn't burn a drop of oil!

That's a FACT!

Ask me how I know that?

I think you're the one who gave me this good advice.

what exactly should be watched for in the manifolds, im getting steam out of the port side exhaust tailpipe, and the water on that side is quite a bit warmer on that side, does that mean cloged manifold? is that the kinda stuff ur talking about

you;re watching for them to get heavily corroded on the inside - as in flakes that will both restrict water flow, and indicate that you're headed for danger of a leak. If they leak into the exhaust.... unless you're lucky... you'll dump water into the engine... rust if slow and don't catch, hydrolock and break stuff if fast and don't catch... either way, manifolds are cheaper than engine builds...

on lasting... depends a lot on how you treat it - how hard you run it, how often you change and check the high quality lube you use in it, keep it in tune, etc...