So I’ve been digging around a lot and am having trouble finding parts for my specific engine. It’s a 1990 OMC Cobra 5.0, the boat is a 1990 Four Winn’s 205 Sundowner
Howdy,
Since you could have a drive that was originally intended for a 2.3L 4-cyl engine, you may have to remove the top cap and count teeth on the gears to determine for sure the ratio (but that won't tell you the lower ratio)
It may be easier to simply remove the spark plugs and turn the engine (in FWD gear) enough times to determine the over all drive ratio.
(2.00:1 is 2 engine revolutions for every 1 prop rotation, 1.5:1 is 3 engine turns is 2 prop turns etc)
When you know that, you'll know how you should proceed with engine tune up(if required) , prop selection etc.
In any case, if the boat runs "ok", take it out and run it at WOT (wide open throttle) trim the drive for maximum speed and note the max speed (phone GPS etc) and MAX RPM
Here's where there is sometimes a problem since many boat tachometers are WAY off at the top of the range. If you can either find another tach, or borrow a shop tach, you'll need to definitely verify your tach.
According to the OMC manual, (search for Boatinfo manuals page 2-58 be patient... their site is snail-slow to load)
A 1989 5.0 has a recommended 4000-4400 WOT RPM
A 1990 5.0 has a recommended 4200-4600 WOT RPM
Both years used a 1.59:1 drive
To get the best performance, you need to select a drive ratio + prop pitch to obtain a WOT RPM at or near the max recommended RPM when operating with the boat loaded at the typical load (weight) you'll normally run it AND at the pressure altitude where you normally boat. (this is industry "Standard" for most all planing type boats I/O Outboard and Inboard)
if you use the wrong prop or the wrong drive ratio, your performance will probably be less than max. I.E, if you use a markedly higher ratio drive that requires you to use a markedly LOW pitch prop, (Or a low pitch drive with a very high pitch prop to compensate) it may not perform as good as it could with the recommended ratio
Ratios are listed in the Boatinfo manual on page 12-73
If yours performed ok (originally) with a 1.59:1 drive and a 17p prop, and you now have a 2.00:1 drive, you may have to use a 21p prop etc
You can use a prop-slip calculator to get in the ballpark if you don't have a decent starting point.
http://www.go-fast.com/Prop_Slip_Calculator.htm
It's imperative to know the WOT SPEED, RPM, ratio and prop pitch to even know if you're in the ballpark!
Cheers,
Rick
