Cobra gear ratio

[jajin2004]

First time to post- I will try to do it right. I bought a couple of outdrives to rebuild for spares, I was told the both are from V-8's. One says 5.7L on the plate and it has 21:16 gears. No problem. The other I have questions about is what I believe to be a 1987 Cobra outdrive- Model # 983842, Serial # T0449244, with a gear ratio of 21:19. My questions are: Is the year 1987 right? Is this gear ratio for a v-8 or for the 4.3 v-6? If for the 4.3 v-6 can it be used on a v-8 (5.0) with a different prop? What size prop? Can it be used on a 5.7 with a different prop and what size prop? Thank you for your help.

 

[HT32BSX115]

Re: Cobra gear ratio

Howdy,

I have an 87 service manual and there is no drive model 983XXX in it. They're all 984XXX/985XXX

The 21:19 drive is indicated for the 4.3L V-6.

All the cobras will accept the same "size" prop. The pitch is another matter. Unless you can find someone with the same approx size boat and engine/drive etc... you'll have to experiment. You can get in the ball park with a good prop slip calculator


regards,


Rick

 

[Don S]

Re: Cobra gear ratio

The 983842 drive is from 1986, and as mentioned is for a 4.3.
You may change props and get the correct RPM, buy you will be out of the props effeciency range. That is why gear ratios change for different engines.
Another point on boats with twins, you MUSH have the ratios and props the same on both engines or you will have all kinds of sync, pulsing, and vibration problems.