Help me understand outdrive gear ratio/engine logic-gctid352760

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Ok I'm thick headed. I have a 1988 2160 trophy. Stock gm 305 5.0 was pulled and replaced with gm 350 5.7. Omc cobra outdrive gear ratios were different for each engine, maybe 21/14 upper and 1.42 lower for the 5.7, and 21/13 upper and 1.5 lower for the 5.0.

So I'm running a 15x15 four blade stainless. That got me to 4,200 rpm wot w/ the tired 305. I know I may have to re-prop to adjust for the gear ratio difference. But I don't comprehend the mechanics of the gear ratio difference.

I presume I may have to increase pitch. Is this right? What about size? Three blades perhaps?

Help me understand the dynamics...all I get is the 5.0 cobra was geared for more torque than the 5.7 cobra and I know I have a prop which seems to increase the torque for the standard 5.0 cobra.

I run heavy with gear and souls. &@$! bodies weigh more than the souls.

Thanks for any insight.

Jarrett

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I have a spec book that says 5lt 1.50:1. 5.7 lt 1.41:1.

[Chief_Alen Guru Find all posts View Profile Close Chief_Alen]

http://bluewatermarinesvc.com/html/propeller.html

Here read this maybe it might help.

The motor you had the 5.0 and the motor you have the 5.7 gave you a increase of maybe 60 hp.

The drive you have is just fine and the prop will be just fine " if " you reach the recommended rpm for wot " wide open throttle.

Speed does not enter the equation but what does enter the equation is the amount of weight on the boat. You should have a sticker that says how many pounds and people the boat should have aboard while under way.

The way most boats are measured is with 2 people, half a tank of gas, and empty water tanks.

Now run the boat with a gps and see if you get between the recommended wot for your motor with the drive and prop you have now.

Say you do and it is in the upper range say 4600 rpm the new 5.7 is around 5000 rpm. Say your there, and the speed is whatever it is now you know your dialed in. Keep in mind your tach may not be correct. You never want to run for a very long time at wot. 15 minutes is more then i ever run at wot.

Once at the beginning of the season and once at the end. Just so i know all is well with the motor.

Say now you want to bring onboard more gear and beer " hey that rimes " You need to go down in pitch in regard to the prop.

Your setup as it stands a 5.7 with the drive you have and the prop you have tested as i described maybe will do for just cruising around at 3 /4 of wot around 3500 rpm. That by the way is recommended for running all day every day. 3 / 4 throttle.

Maybe this is a little to long a post but it's a start when others chime in more info will be added to maybe help you.

Good luck !

And if anyone has not welcomed you aboard to BOC - i will be the first.

Welcome aboard !

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Reel Power wrote:
Ok I'm thick headed. I have a 1988 2160 trophy. Stock gm 305 5.0 was pulled and replaced with gm 350 5.7. Omc cobra outdrive gear ratios were different for each engine, maybe 21/14 upper and 1.42 lower for the 5.7, and 21/13 upper and 1.5 lower for the 5.0.

So I'm running a 15x15 four blade stainless. That got me to 4,200 rpm wot w/ the tired 305.

I know I may have to re-prop to adjust for the gear ratio difference. But I don't comprehend the mechanics of the gear ratio difference.

I presume I may have to increase pitch. Is this right? What about size? Three blades perhaps?

[COLOR]#0000ff wrote:
Try to not go there yet!

After your installation, you'll want to first perform the OEM spec'd WOT RPM test! [/COLOR]

Help me understand the dynamics...all I get is the 5.0 cobra was geared for more torque than the 5.7 cobra and I know I have a prop which seems to increase the torque for the standard 5.0 cobra.

[COLOR]#0000ff wrote:
The OEM 5.0L engine gearing was to actually aid in the less torque and hp of the 5.0L by reducing over-all ratio from that of a "would-be" 5.7L engine.

[/COLOR]

I run heavy with gear and souls. &@$! bodies weigh more than the souls.

[COLOR]#0000ff wrote:
Again, do the WOT RPM test with the boat loaded as you would with all of these guys on board.

Then perhaps do another WOT RPM test with the boat loaded otherwise.

Think of this like you would a multi purpose pleasure/ski boat.

These guys will sometimes have one prop for towing tubers/skiers, and another prop for just cruising. [/COLOR]

Jarrett, I doubt that you're anymore thick-headed than most of of us re; this topic. It can be rather difficult to wrap our heads around, unlike auto/truck gearing and tire size.

With Marine Propulsion, everything that makes sense in the auto world pretty much goes out the window. It apparently has everything to do with prop pitch, prop RPM and potential prop slip. The engineers select the over-all ratio for the engine, not the other way around!

The 5.0L engine w/ the higher numerical number ratio (lower in terms of reduction) of 1.5:1 is to help the less torque and hp of the 5.0L engine to turn a prop that is best suited for the hull.

The 5.7L engine w/ the lower numerical number ratio (taller in terms of reduction) of 1.42:1 is to work better with the increased torque and hp of the 5.7L engine to turn a prop that is best suited for the hull.

Believe me.... while my words may make sense, I don't claim to fully understand Hydro-Propulsion, but I think that Chief and I are both saying basically the same thing here.

Bottom line.... you'll have to accept that the engineering suggests the 1.42:1 over-all ratio for 5.7L engine.

Then perform the WOT RPM test just as you would with any engine/drive to see if the combination is correct.

The two ratios are only .08 revolutions apart from one another, so you may even find that the same prop works with the 5.7L w/ the new taller reduction.

.

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Slightly less confused gents, thanks. Am I correct that the manufacturer's recommended WOT on a 1988 gm 5.7 is 4,400-4,800?

Any reason to stay on the low or high side of recommended WOT diven the ratio difference in the drives?

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Reel Power wrote:
Slightly less confused gents, thanks. Am I correct that the manufacturer's recommended WOT on a 1988 gm 5.7 is 4,400-4,800?

Any reason to stay on the low or high side of recommended WOT diven the ratio difference in the drives?

Jarrett, the one thing that I prefer to avoid is calling out specific torque #'s, timing #'s, WOT PRM #'s, etc.

One misinterpretation and/or mistake on behalf of either party, and ............................

IMO, it's always best to see your own OEM work shop manual, .... but you are probably very close!

For your engine break-in purposes, I think that many of us would suggest that you be on the high side of your WOT RPM.

If in the future (and the likihood of loading the boat heavily) again, I'd stay on the high side of the WOT RPM.

To do otherwise, you're entering the area of over-burdening the engine.

.

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Roger that 2850 Bounty. Makes sense to me, especially not burdening the engine at the heavy loads I run and not making pronouncements regarding exact specifications (I'm an attorney don't shoot).

Still waiting for Michigan Motorz to get me the engine paperwork they forgot to throw into my boat when I met their employee at MI Cabelas. Need to send them an email. Kudos to their business for a smooth transaction and a great purchase/install price.

Got a two-stroke remote kicker going on the swim mount to keep the burden and hours off the new 350 base engine.

Thanks for all your input guys. Two months till splash in NE Ohio.

[Chief_Alen Guru Find all posts View Profile Close Chief_Alen]

The motor should have a sticker on with the wot.

If not a quick phone call to them will resolve the issue i have dealt with them and highly recommend them.

After you run the boat get back to us with how it went and pictures we love pictures.

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From the OEM Manual - WOT for 5.0 and 5.7 is 4200 - 4600.

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Thanks Orca.

Chief I'll post some pics in the projects or owners department this Spring once I get 'er shined up. Michigan Motorz was great to work with, no complaints here.

Just checked out the kicker install yesterday, nice work by the mech. Given the age of the factory plastic swim platform I think the two stroke option was prudent. He claimed to have seen a few bust the plastic platform with a four stroke hangin' out back there. The entire slightly used 9.9 remote kicker/controls/cable/remote tank/hose/ez steer purchase and install cost less than purchase price of teh slighly used Mercury four stroke pro kicker w/ controls I had lined up for the project. The pro kicker weighed over 125 lbs. Factory spec says 114, but I call bs. 2003 Mercury two stroke is appx. 75. Manual tilt on the two stroke is a piece of cake from the transom. I'll run a low-profile 7.5 gal. external tank strapped opposite the kicker on the platform, removable for filling.