4.3L vs 5.7L Outdrive Gearing (16T/19T)

[triscadek209]

Hello all, I'm new to the forum and looking forward to gain some knowledge.

I currently have a 1990 Four Winns 200 Horizon with a 5.7L Cobra I purchased about 2 months ago and decided to save some cash when it came to oil changes... yes you know where this is going. Needless to say, during the test drive the upper unit seized and the next day I realized I only filled it half way with gear oil. Luckily for me a buddy works at a boat shop and is able to get me a used outdrive theyve been wanting to get rid of for $350 to just bolt right back on the boat. Hes putting new seals in it for me. Bad part is that its out of a 4.3L V6 which has a 21-19 tooth vs my original outdrive with has a 21-16 tooth setup. My question is how many RPMs will 3 teeth throw me off? I should have more low end torque and less top speed with the V6 drive but Im thinking a 21 or 23 pitch prop should overcome this and get me back to normal? They say for every 2" of pitch change is 300-400 RPM at WOT.

Thanks, Cale

 

[Howard Sterndrive]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

for $350 I suppose you can try a 23P prop and see. But another problem may arise with the amount of torque transmitted to the lower unit by 21/19 gears and a 5.7 V8. The lower may not handle the over spec torque too well.

 

[triscadek209]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

Yeah it's that or $700+ for just a used upper online plus labor. Good news is my 5.7 only rates at 220hp... In 1990 lol.

 

[triscadek209]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

Of all the research I've been doing the past couple days it seems I read somewhere that they're all equally strong and compatible from the 4cyl up to the V8 however it's just the gearing that will throw the RPMS off some. The only thing I can't find is how much they'll be thrown off...?

 

[funk6294]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

From what I can find the v8 had a 1.43 ratio and the v6 has a 1.68. If you compare that to what merc does, thier v8 is a 1.5 and the v6 is a 1.81. So putting that into perspective omcs v6 is somewhere in between what merc specs for thier respective drives. For 350 bucks I would take a stab at running it. Only thing you will have to go through is repropping the boat. The v6 lower will spin about 15 percent slower so if your currently spinning a 19 pitch prop, a 15 percent larger one would be around a 22, although I am sure some trial and error will be neccassry to determine the final size.

 

[Howard Sterndrive]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

Of all the research I've been doing the past couple days it seems I read somewhere that they're all equally strong and compatible from the 4cyl up to the V8 however it's just the gearing that will throw the RPMS off some. The only thing I can't find is how much they'll be thrown off...?

but you'll be giving the lower more torque than it even a 260HP V8 gives it.

Power is the product of rpm and torque. When the RPM to the prop shaft is lowered by using V6 gearing, and then you compensate by up-pitching the prop, you are increasing the torque component of the power in the vertical shaft of the drive.

However, it should be alright if you are an adult driver and not popping the throttle on and off as you jump swells or something. You'd prolly have to try to to break it.

 

[Howard Sterndrive]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

some rough math says your vertical shaft was carrying 285 ft. lbs of torque with 220HP with the engine doing 4500 rpm and 21:16 gearset
with the new drive, repropped to recreate same speed, the vertical shaft will be carrying 336 ft. lbs. of torque.

If you had a 260HP engine with your original drive, the vertical shaft would carry 303 ft. lbs.
A 260HP engine through the V6 drive would make 398 ft. lbs. (wow)

I'd probably avoid that 336 scenario. more of an increase than I thought. 336 is a hell of a twisting force on that shaft and vertical gear going into the lower. (more than OMC ever expected it to carry - assuming the big King Cobra units had different vertical shafts and lowers which I'm pretty sure they did))

And 23P props are less efficient under hole-shot. A Raker might be required to get a planing time you're happy with and they're expensive.

 

[triscadek209]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

Thanks for all the input guys, Im thinking Ill lose close to 10 mph on my top speed but we will see. For $350 its kinda hard to say no. If it lasts me this summer and maybe next, Ill be happy. The boat is in great condition and I worked the previous owner down to $3000 with a tandem trailer, boat cover and life jacketsl. For my first boat this is a great starter and Ill upgrade in the next year or so.
I was running around 42mph with a borrowed 17p aluminum 3 blade with a 25mph head wind the day it blew so Im hoping the V6 drive and a 21p or 23p 4blade will get me where I need to be but Ill start with the 19p 4blade that came in Monday. Good news is that my parents have a 1989 Bayliner 2855 with the same 5.7L cobra setup as me and theyve had hell over the last couple years trying to get it out of the water and any speed out of it. SO, maybe with how easy the drives are to swap, we may swap and put the V6 drive in their boat for a better chance of getting out of the water and get my top speed back. Hes not worried about top speed so much but his boat weights 7200lbs vs mine that weighs 3200lbs so that would be a lot more torque/stress on the V6 unit.

 

[Walt T]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

Give it a shot, I probably would. Let us know how it works out.

 

[Idlespeedonly]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

Man that stinks. Nothing worse than breaking a new toy. I think with a change in prop you will be pretty happy with it. FWIW, I have a 5.0 drive behind a 350. Cant really tell much of a difference.

 

[triscadek209]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

Man that stinks. Nothing worse than breaking a new toy. I think with a change in prop you will be pretty happy with it. FWIW, I have a 5.0 drive behind a 350. Cant really tell much of a difference.

Just FYI incase you didnt know, according to the chart I found, if you have a Ford 5.0L 302CID drive, then its a 21/18T, if you have the Chevy 5.0L 305CID Drive then you have a 21/17T. 18t giving you less top end but better whole shot.

 

[triscadek209]

Gears in upper or lower?

Gears in upper or lower?

So, from my understanding the different gear ratios are in the upper of the OMCs, right? Or are they in the lowers? What I mean is the different ratios for the V6 vs V8 outdrives.

 

[triscadek209]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

So results are in, I was able to go for a test run on Saturday with the family. Had 4 adults, cooler and 30 gal of fuel and the boat popped right out of the water! Yay! ...but didn't do much right after :-/
Too speed is 42-43 at 4600 RPMS @ WOT (if my tach is right) and it was SCREAMING! The humm of the engine I was happy with for a cruising speed was around 32 at 3500 RPMS. This is with a 19pitch 4 blade and the 21/19 set up gears. Two weeks ago with the 21/16 setup and 17 pitch 3 blade I was at about 3800-4000 RPMS running 43mph and still had plenty to go. Will I be ok with jumping to a 23 pitch prop? If so, 3 or 4 blade? Or should I consider swapping gears from my old case provided I can get the seized bearings off of them?

 

[coolbri70]

Re: Gears in upper or lower?

Re: Gears in upper or lower?

gears are in the lower unit, consist of forward and reverse

 

[triscadek209]

Re: Gears in upper or lower?

Re: Gears in upper or lower?

It's in a 90, blew my upper in my 5.7L and swapped with a 4.3L whole stern unit for $350. Well, lost about 10mph. So if I mate my old lower up with the 4.3 upper I'll be good to go?
The 4.3 is 21/19 and my 5.7 was 21/16 tooth.

 

[southkogs]

Re: Gears in upper or lower?

Re: Gears in upper or lower?

As a rule, "yes, gear ratios are different on both upper and lower gear case between different HP ratings."

I'm not sure about the other models, but I think on the electric shifts a few of them shared the same upper ratio ... but it's pretty specific. Dunno' about the Cobras.

 

[triscadek209]

Re: Gears in upper or lower?

Re: Gears in upper or lower?

Here's kinda what I was going off of.
OMCVolvoPenta_Catalog_Flipbook

 

[funk6294]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

If you can borrow a larger prop, I would go for it and see how it affects rpms and performance. What was your previous wot rpm and speed? Also was the prop you swapped to the same make and model prop (ie mirage) or a different make/model.

 

[triscadek209]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

Um I'm not too sure, was just a cheap aluminum prop with a couple slight bends in it. Never got to hit top speed seeing as how we were 5 min in to the test run when it locked up. But I was around 42-43mph at like 3800-4000 RPMS.

 

[funk6294]

Re: 4.3L vs 5.7L Outdrive Gearing (16T/19T)

If you can get your hands on a 23, try it. If it's too large you will know quickly and help make the decision as to which way to go. Rational is below....