How to verify OMC cobra gear ratio

[jtheitkamp]

How can I verify the gear ratio of my 1991 OMC cobra outdrive? I've lost about 10mph at wot. Have not changed props. The last work to be done was late last summer my local boat repair shop replaced my pinion gear and a couple of bearings. I'm suspicious that the wrong gear ratio was re-installed. Is there a way to easily verify the ratio.

 

[mkast]

Re: How to verify OMC cobra gear ratio

Remove the drive, turn the input shaft one revolution, count the number of turns the propeller makes.

 

[jtheitkamp]

Re: How to verify OMC cobra gear ratio

This is probably an absurd idea, but can I start the engine, put it in gear, shut off the engine, pull the spark plugs and rotate the engine 1 rev and count prop turns?

 

[mkast]

Re: How to verify OMC cobra gear ratio

Why would you start the engine?

 

[HT32BSX115]

Re: How to verify OMC cobra gear ratio

Remove the drive, turn the input shaft one revolution, count the number of turns the propeller makes.

You'll get a far more accurate number by turning the input shaft 20 turns and counting the prop turns.


The 460 King Cobra was 1.41:1, the 5.7L models were 1.43:1 etc.

Someone here has an OMC service manual and can probably look this up, Most of the drives were rated by ratio and by input turnsutput turns. (For example, 1.41:1 is also 24:17.......24 eng revs to 17 prop turns.)


It's much easier to count 24 turns of the input shaft and 17 turns of the prop vs 1.41 turns to 1 turn (unless you can set up a protractor that reads in degrees)


Unless you have a drive that has an exactly 2.00:1 or other even ratio it's going to be hard to measure it unless you can set up something that measures in fractions of a circle.

Even if you can't remove the drive, you can pull the spark plugs and turn the crankshaft with a socket wrench etc. if it's in gear, you can have a helper count blades on the outside while you count turns on the damper. And then get out your sliderule!!! (or calculator!)


Cheers,


Rick

 

[jtheitkamp]

Re: How to verify OMC cobra gear ratio

I assumed it would be better to put it into gear while the engine is running. I've got the 4.3L V-6. My manual indicates upper ratio at 21:19 and lower ratio 14:26 with overall 1.68:1. I'll manually rotate the engine 10 or 20 times to get a closer estimate. Is it possible to have put in the wrong ratio gear?

 

[HT32BSX115]

Re: How to verify OMC cobra gear ratio

Well,

Anything is possible. Some parts for Cobras are scarce. Some dealers may want to use "in-stock" parts that they have on-hand instead of locating new-old stock or aftermarket parts.

On the other hand you may have other problems with the engine that is preventing you from reaching max WOT RPM.

http://forums.iboats.com/showthread.php?t=295070

I would check the above link and rule out those other things before you assume you have the wrong ratio drive.

You can still turn the engine by hand and count input-turns/prop-turns to calculate the ratio

 

[jtheitkamp]

Re: How to verify OMC cobra gear ratio

Thankyou for that link and all the other info you folks have left. Looks like I have some work to do now.

 

[jtheitkamp]

Re: How to verify OMC cobra gear ratio

I just did the crank the engine by hand and count the prop turns and I all is good. RPM's are about 4600 to 4800. Looks like the reconditioned prop that is stamped 21 is more like 19. Will be getting a new prop 21 or 22 deg. prop. Thanks for all the help from you folks.