OMC 350 and 1.65 gear ratio

[ajkolar]

1988 OMC Cobra w/ 350 GM SBC<br /><br />I made one of those drive stands with casters on the bottom, man what a back saver! Thanks to everyone on the pics and direction there. However I pulled the drive today and checked the gear ratio. The drive itself has a tag on it that says 5.0 (but I thought when the previous owner swapped the in the 350, he may have swapped gears as well, guess not) So I have a boat with a "FRESH" 350 (only 25 hrs), but I have 1.65 gear ratio (I believe this is correct, as I had to turn the drive shaft rigth inbetween 1.50 and 1.75 turns to get the prop shaft to spin 1 time). I think this explains why I can not open it up with a 14.25 x 21 SS prop (it will walk right past 4600, probably all the way to at least 5000 I would guess, but I don't run it that hard) So I believe I need to get a higher pitched prop, will this solve my problem, or are other things I need to be worried about with this setup? This is a 20 ft VIP vixen cuddy cabin (~3000 lb dry wieght).<br /><br />Just to make sure, how hard is it to get to the teeth on the gear to count them. The drive is already off the boat, what esle would I have to remove to get a good view of this?

 

[Don S]

Re: OMC 350 and 1.65 gear ratio

The 5.7 should have a 1.41:1 ratio, the 1.65:1 is for a V6. Possibly the gears were changed in the drive without changing the tag ????????? Hard to say why, but for effeciency, you need the proper gear ratio and prop. Trying to fix the ratio by changing props will cost you effeciency but may get you the correct WOT rpm.

 

[KaGee]

Re: OMC 350 and 1.65 gear ratio

See what others say, but I don't think there's much if any difference. Where gear ratio's really matter is upgrading to a V8 from a V6.<br /><br />I know in the Stringer series, the gear ratio only varies between 4 cyl, 6 cyl, and 8 cyl. All 8 cylinder use the same. I don't have a Cobra book, so i can't say for sure.<br /><br />If you are pushing 5000 WOT, perhaps a prop change is in order.

 

[ajkolar]

Re: OMC 350 and 1.65 gear ratio

Don S., I thought is some applications the 5.7 used a 1.5 ratio? The only engines in that boat were the origninal 5.0, and the new 5.7. I can't imagine it would have ever been switched to a different ratio that is for a v6? Is there a possiblility that the original 5.0 used 1.65 gears? Again, how hard to count the teeth to confirm that I have that ratio? <br /><br />Also, what kind of $$ am I looking at to replace the 1.65's (if that is indeed what I have) with the correct 1.41's? Thanks for the help?

 

[KaGee]

Re: OMC 350 and 1.65 gear ratio

AJ,<br /><br />Last year we had several cases of units being overhauled, and the shop carelessly swapped the upper or installed the wrong gear set. It drove the guys crazy because they either over-rev'd or could not obtain enough WOT rpm's. While you are at it, wouldn't hurt to settle any questions before you put it back together.

 

[ajkolar]

Re: OMC 350 and 1.65 gear ratio

I agree, but would really like to know if I am facing a real problem with my ratio to HP? I have put A LOT of work into this boat (new engine, two new axles (trailer obviously), two new batteries, new stereo system, new lighting system, new sundeck support, complete gelcoat restoration, new trim sender, and am doing all routine maintenance this year (gimbal bearing, u-joints, impeller, anodes, bellows, and other TLC). (sorry to go on and on, but just wanted to show that I am willing to put in the work, but....) The fact is the boat is 18 years old now (and in pristiene condition for that age), but if I am facing drive issues, I will probably use for one more year and sell, get something newer... If I can throw a 22" 4 blade on her and be good, I'll probably use the boat for another 5+ years (Note: i am only 23 and don't have a huge amount of discretionary cash flow Sorry for "venting" but that gives you an idea of what I am dealing with, and I really respect your guys' opinion because it is obvious you REALLY know what your talking about, just trying to get a little direction.... Thanks again!

 

[Lou C]

Re: OMC 350 and 1.65 gear ratio

OK this is what I have in my 1988 OMC manual:<br />5.7 overall: 1.41:1<br />5.0 overall: 1.50:1<br />4.3 overall: 1.68:1

 

[Don S]

Re: OMC 350 and 1.65 gear ratio

Let me repeat:<br /><br />

Originally posted by Don S:<br /> The 5.7 should have a 1.41:1 ratio, the 1.65:1 is for a V6. Possibly the gears were changed in the drive without changing the tag ????????? Hard to say why, but for effeciency, you need the proper gear ratio and prop. Trying to fix the ratio by changing props will cost you effeciency but may get you the correct WOT rpm.

Like I said EFFENCIENCY is the name of the game. You may get the proper rpm, IF you ever get on step with a high pitched prop, or if you get on step at all. You will probably also loose top speed, and who knows, you may not be able to find a prop that will get into the proper range.<br />This is why drive manufacturers use different ratio's. The props can't do it all.

 

[Bondo]

Re: OMC 350 and 1.65 gear ratio

and who knows, you may not be able to find a prop that will get into the proper range.<br />

Ayuh,......<br /><br />We've had some discussion about this in the last year of so,......<br /><br />There's About 6"s of Prop Pitch difference between the 1.65/1 gears,+ the 1.50/1 gears,.......<br /><br />That would mean you'll need a 27-28" prop to get to the overall ratio of the 1.50/1........Seeing that you're Over Reving the 21" prop.......<br /><br />That's well above the "Normal" range of Pitches Available...........<br />And,... The " EFFENCIENCY " is going to Suck.........

 

[ajkolar]

Re: OMC 350 and 1.65 gear ratio

Thanks for the info. How do I access the teeth to verify that I have a 1.65 ratio?<br /><br />What kind of $$ to switch ratios?<br /><br />Right now I get our of the hole pretty decent, and do ~50 mph at 4600 rpm, so I guess things can't be too messed up (that is why I want to count the teeth just to make sure) Thanks again!

 

[ajkolar]

Re: OMC 350 and 1.65 gear ratio

How many teeth should an OMC Cobra with a 1.50 gear ratio have on the gear directly below the top plate (gear is vertical)? How many teeth on a 1.65, and how many on a 1.41? Thanks for the information.