Re: Cobra horse power and gear ratio question
Howdy,
Yes, the ratios are largely based in HP/TQ of the engine AND maximum RPM. Then the prop RPM is considered for the best efficiency.
I don't remember if your 5.0L engine is a GM (305) or Ford (302) engine but
if it's GM, you can easily swap it out for a 5.7.
Looking at the parts site
BRP -- Parts Catalogs I think your engine might be a Ford 302. That would make the swap a little more problematic because some your accy's from the engine will not fit on the GM 5.7.
If you can find an engine that already has everything (engine mounts, flywheel & housing, coupler, manifolds & risers etc)..... it'll be much easier.
In general, (with Mercruisers) the ratio needed for a 5.7L engine that tops out at approx 5000 RPM, a 1.50:1 drive is used. The Drive for a 5.7L OMC is/was 1.41:1 (My old OMC 460 King Kobra used a 1.43:1 drive)
The 5.0L engine uses/used a (depending on year) 1.50:1 or 1.59:1
ref:
Boatinfo - OMC 1986-1998 Service Manual
The ratios are very close but you'll still probably need to go to a slightly higher pitch prop because of the extra HP. You'll have to experiment.
You want to the engine to run at or near the top of the recommended RPM range when operating at WOT with the boat loaded lightly.
I think that's around 4800-5000 RPM for most 5.7L engines.
If you're going to do all of this yourself, I suggest you get an OEM OMC service manual.
The drive you have will handle the HP a 5.7L engine just fine if it's in good condx. (unless you build up a 450hp stroked/blown engine)
Regards,
Rick
