1990 OMC 2.3l vs 3.0l Drive Ratio Question

[JJBoatr]

Morning Y’all,

My brother recently acquired a nice Four Winns 170 Freedom with an OMC 2.3Lthat had a roached upper gear set. He replaced the entire lower unit with a small case driv from a boat equipped with a 3.0L. His boat is running great except for one thing...he cannot reach recommended WOT (5200 - 5600) with a 13.25 x 17P propeller, best we got was 4500 at 37mph and that was trimmed to the ragged edge of venting. We were both told by a local mechanic that the 2.3L & 3.0L shared the same identical drive ratio.

What I think I’ve discovered is that the two engines did not share drive ratios except for a couple years in the mid 80’s. According to an old post by the greatly missed Don S., the 2.3L had a drive ratio of 2.0 and the 3.0L had a ratio of 1.71. I also have other evidence in my 1992 OMC factory brochure showing a ratio of 1.84 for the 3.0L engine.

Now, OEM parts fiche shows the same upper gear set is used for both engines, however the lower gear set part numbers are different and this makes sense to me because the lower pinion is where the final ratio reduction takes place too my knowledge.

So the big questions...can anyone confirm the ratios were in fact different? If they are different: are we fighting a losing battle by trying to overcome the ratio with a prop change? I think we might be. We want to try a 13.5x15 but we will at best gain 400 rpm which still puts us below the lowest recommended WOT of 5200.

Background on what we have done to the boat to rule out other problems that would cause the seemingly low power.

Verified anti-siphon valve was good, verified unobstructed fuel pickup, set the points, verified advance, rebuilt and tuned carb (Holley), verified no exhaust obstruction with bore scope. We even gave the carb an extra shot of fuel carefully with a windex bottle while at top speed, engine bogged a little and didn’t pick up rpm, fuel supply is good.

We really appreciate any experience based assistance anyone may provide. If anyone has this engine in a similar sized boat it would be really awesome if you could share your top speed and other performance characteristics.

link to the thread Don S. Posted in

https://forums.iboats.com/forum/eng...nes-outdrives/463666-3-0l-cobra-vs-2-3l-cobra

 

[JJBoatr]

Here’s a video of how it was running yesterday. Boat was empty except for some tools and mandatory equipment, about 14 gallons of fuel and my brother and I tip the scales together at about 475lbs...we’re not small lol. It’s not running bad for a small engine in a 2100lb boat. If you listen you can here a knock when he puts the throttle down. We’re worried that it’s detonation from lugging but neither of us have ever heard detonation to know what it is for sure.

https://youtu.be/048fGeFZWgM

 

[JJBoatr]

I decided to go do a spin test to determine the ratio, somebody please correct me if I’m wrong on this procedure. Starting with marks aligned at TDC i had my son turning the crank clockwise calling out every time the timing mark passed the TC mark on the timing belt cover and I recorded it. I in turn had marked the propeller hub and the gear case and was marking a tally every time the prop mark completed a revolution. In the end he turned the crank 20 complete revolutions and the prop had completed 12 and just under a half revolutions. So doing the math of Crank revolutions / Prop revolutions his ratio came out to be 1.61:1. If my procedure is correct thats geared like my boat that has a 5.0HO in it!!! Sound like I did it right guys & gals???

Big question...can we drop pitch to compensate and have the boat perform to it’s potential? I doubt it being wed have to go down to about a 12P, but worth asking. We still have his original drive unit and will be tearing down the lower gear case to see if his pinion and forward gear are usable after the upper gears grenaded. Best case scenario is we can swap the correct gears in

 

[Scott Danforth]

first, there is no way a 2.3 is going to spin that fast. 5200 RPM is about max and they dont live that long at that RPM

and no, there is no prop jump between a 1.6 and 2.0 drive

 

[JJBoatr]

first, there is no way a 2.3 is going to spin that fast. 5200 RPM is about max and they dont live that long at that RPM

and no, there is no prop jump between a 1.6 and 2.0 drive

We are not trying to run it at that rpm all the time. No boat engine lasts if it’s always at max. We are simply trying to properly prop and gear his boat so the engine is operating in ALL its normal rpm ranges given the load on the engine. Thank you for answering the question about the prop.

To address your comment about it spinning that fast: why would that be the factory spec if it wasn’t cabable? OHC motors are capable of much higher rpm than a push rod motor in stock form...and they live plenty long. You currently have a built big block in your boat based on your signature. What experience do you have with the 2.3L? I am asking genuinely, not going for a snarky tone.

 

[Scott Danforth]

Used to wrench on a bunch of 2.3's over the years

The pinto motor in stock config likes to float valves much above 5200

Btw, my BBC bottom end was built to spin about 8k, induction runs out of steam at about 6k for now

 

[JJBoatr]

Used to wrench on a bunch of 2.3's over the years

The pinto motor in stock config likes to float valves much above 5200

Btw, my BBC bottom end was built to spin about 8k, induction runs out of steam at about 6k for now

Thanks for sharing your experience. My goal however still remains to get that motor spinning to OMC marine recommended rpm to avoid any chance of doing serious damage from detonation by way of over lugging the motor all the time.

I found your response interesting because you might be the first person to ever recommend keeping an engine rpm below the factory spec. I always read that marine and automobile engines aren’t built to the same spec and shouldn’t be compared.

Your big block sounds like a fun motor, but I don’t care how many rpm it can turn because it’s not a 2.3L Ford which is the topic of his thread.

 

[Scott Danforth]

The 2.3 can spin to about .5200 rpm befor the valves start to float. At 6000, the stock springs are useless. Factory rev limitor at ford was 6000 RPM and auto trans upshifted at 5600 with the tv cable pulled tight. However the motor doesnt last there very long. Drive doesnt like RPMs above 5500 either.

 

[JJBoatr]

Thanks for that very useful information. We will keep those numbers in mind while dialing this driveline back in to where it is supposed to be. With any luck we’ll report results within a week, but there’s some bad weather coming. Good time to tear a drive apart though.

 

[jimmbo]

Taken from the 1990 OMC brochure
http://162.144.28.33/lib/omc/brochures/1990omccobra.html#/10
The 2.3 had a WOT range of 5200 - 5600
the gears on a 2.3 were 2:1
the gears on a 3.0 were 1.86:1

 

[JJBoatr]

Taken from the 1990 OMC brochure
http://162.144.28.33/lib/omc/brochures/1990omccobra.html#/10
The 2.3 had a WOT range of 5200 - 5600
the gears on a 2.3 were 2:1
the gears on a 3.0 were 1.86:1

Thanks for sharing your brochure info! Now the question is why did I did I get a ratio of 1.61:1 doing a spin count. My brother and I are going to do one more spin check before tear down to be sure neither me or my son added or missed a rotation yesterday. I don’t think we did but worth double checking.

 

[JJBoatr]

Just pulled the top cover on the drove that’s on his boat and I’ll be dammed, the upper is geared wrong. Tooth count is 21/17, supposed to be 21/21. This sucks, we don’t have a spare set of those.