replace 5.0 with 4.3 omc stern drive

[boating birdman]

I have a 5.0L 305 chevy with a bad 5.0 omc cobra stern drive. A friend has offered a omc out drive for a 4.3 . Will it work?

 

[dubs283]

Re: replace 5.0 with 4.3 omc stern drive

probably has a different gear ratio and won't work

the 5.0 v-8 will destroy the gears in the drive over time

you will have to find a drive that has the same gear ratio for it to work properly

 

[rs2k]

Re: replace 5.0 with 4.3 omc stern drive

What year is the out drive and what year is the 4.3 engine? Gear ratio may not be an issue.

 

[boating birdman]

Re: replace 5.0 with 4.3 omc stern drive

The 5.0 is a 1988 not sure what the 4.3 is. Ill have to find out. If it will work, It will save me $$ and alot of headache. thanks for the info.

 

[danond]

Re: replace 5.0 with 4.3 omc stern drive

Sigh.

It will not "destroy the gears in the drive over time" but if the ratio is different, it will definitely make your propeller selection very challenging as it'll be driving the prop at the wrong speed for the engine's HP/torque curve.

 

[fabrimacator21]

Re: replace 5.0 with 4.3 omc stern drive

Sigh.

It will not "destroy the gears in the drive over time" but if the ratio is different, it will definitely make your propeller selection very challenging as it'll be driving the prop at the wrong speed for the engine's HP/torque curve.

My thoughts exactly...... the v8 units are usually a higher gear ratio anyways so that makes no sense.

There should be a tag on the I/O itself.... should have a model # and a serial # on it... write both down then start looking for an online manual to confirm your gear ratio....

You will need to do the same to the 4.3 I/O....you'll just need a different prop if the ratios aren't close.

Once you know your gear ratios just use an online prop calculator to determine what pitch the new prop needs to be.

 

[rs2k]

Re: replace 5.0 with 4.3 omc stern drive

The 4.3 may work just fine there. A lot of 4.3's are just as if not more powerful than 5.0s. The 4.3's breath much better than the 5.0 and are usually in the same torque and horse power range as the 5.0. The 4.3 is basically a shortened 5.7 and is superior to the 5.0 in many ways IMHO.

It really depends on the power and torque range of both motors.

 

[fabrimacator21]

Re: replace 5.0 with 4.3 omc stern drive

The 4.3 may work just fine there. A lot of 4.3's are just as if not more powerful than 5.0s. The 4.3's breath much better than the 5.0 and are usually in the same torque and horse power range as the 5.0. The 4.3 is basically a shortened 5.7 and is superior to the 5.0 in many ways IMHO.

It really depends on the power and torque range of both motors.

Dont know about the marine 4.3's but the auto 4.3's will make less tq and depending on which model 5.0 it is (assuming it's a chevy 305) 10-40 less hp. Tq is what you need to worry about though.... I'd be suprised if the 4.3 is within 20 ft lbs of the 5.0 off idle and at peak.

Was this a 4bbl 5.0? If you can come up with the serial and model numbers on your 5.0 and the I/O as well as the 4.3 I/O we could you alot better.

 

[Bigdave196]

Re: replace 5.0 with 4.3 omc stern drive

Quit worring about anything until you know what you have to worry about.
Put it on, change the fluids, and take her out. You will never know until you try.

A while back I had to re-gear a 72 chevy of mine from 4.56:1 to 3.73:1 because I was driving it across country. The cost of the gears ( $50 used) saved me $300 in fuel. Everyone told me to buy shim kits before I attempt to do anything. Funny thing was when the new gears were installed, they lined up perfectly, and didnt need to be shimmed at all.

So put it on, take it out and see where your WOT rpms are and your top speed at WOT is. Then go from there. If everytihng is close enough or barely noticeable then IMHO why bother. If its not comming up on plane or your doing 30mph at 5500rpm's then we go from there.

Have fun, and good luck.

 

[fabrimacator21]

Re: replace 5.0 with 4.3 omc stern drive

Quit worring about anything until you know what you have to worry about.
Put it on, change the fluids, and take her out. You will never know until you try.

A while back I had to re-gear a 72 chevy of mine from 4.56:1 to 3.73:1 because I was driving it across country. The cost of the gears ( $50 used) saved me $300 in fuel. Everyone told me to buy shim kits before I attempt to do anything. Funny thing was when the new gears were installed, they lined up perfectly, and didnt need to be shimmed at all.

So put it on, take it out and see where your WOT rpms are and your top speed at WOT is. Then go from there. If everytihng is close enough or barely noticeable then IMHO why bother. If its not comming up on plane or your doing 30mph at 5500rpm's then we go from there.

Have fun, and good luck.

Thats the point... he doesn't know if he can put it on.... and why wouldn't you check the gear ratio's before hand?... theres plenty of calculators to nail down what prop he'll need and then theres no messing with it.

 

[a70eliminator]

Re: replace 5.0 with 4.3 omc stern drive

Birdman, 4.3 upper unit is 21:19
5.0 upper unit is 21:18

The lowers are the same for 4.3-5.0, gearing is done in the upper.


With the 4.3 upper you get 1.68:1

What you have now is 1.59:1

I think you'll be good probably get a better holeshot