omc 6hp vs 8hp...same gear ratio?

[nwcove]

from what i can see, the omc 6 and 8 hp have the same gear ratio.....or am i missing something? is the only difference between these motors the prop? one is a 1985 6hp, and the other a 1990 8 hp. (in reguards to the lu of course!)

 

[Texasmark]

Re: omc 6hp vs 8hp...same gear ratio?

I just looked in a '94 sales brochure and the ratio for both is 2:1. The difference is the rpm. The 8 is rated 500 rpm higher than the 6. Well since hp = torque, rpm, and a constant, that translates to more rpm. Could be done with just a carb or some other engine tweaking. I would bet on that small an engine, they just swap carbs to give the 8 more gas so it can put out more rpm's.

Mark

 

[Faztbullet]

Re: omc 6hp vs 8hp...same gear ratio?

The differences are: raised exhaust port,intake manifold,cylinder head,carb body and jetting....

 

[steelespike]

Re: omc 6hp vs 8hp...same gear ratio?

Looked up props for both they use the same numbers.
Like the 9.9 and 15 hp motors do.

 

[Sea Rider]

Re: omc 6hp vs 8hp...same gear ratio?

The differences are: raised exhaust port,intake manifold,cylinder head,carb body and jetting....

Will have my doubts if you need to change all those mentioned parts, the 6 is the rstricted version of the 8, same as 10 being the restricted version of the 15, both shares same props and the only 3 parts you need to change are : reed valve assembly, related gaskets and carb, so assume same for the 6 & 8. A birdie that works at OMC told me so after vaily attempting to pass my 10 to 15 HP with horrible results playing with jets diameters...

Happy Boating

 

[nwcove]

Re: omc 6hp vs 8hp...same gear ratio?

off on a bit of a tangent.....would it be hard to reseal one of those lu's with basic basement tools? my teenage son went out and bought the 6 hp....thought he could do it with his buddies, but the seller wasnt quite as honest as he should have been. the motor leaked all its gear oil on the basement floor overnight.

any advice? ......other than tell my kid its a lesson learned!!!

 

[Faztbullet]

Re: omc 6hp vs 8hp...same gear ratio?

ill have my doubts if you need to change all those mentioned parts

The blocks of the same year have 2 different part #'s due to exhaust timing differences, the head is different as it has about 10psi more than a 6,the reed plate has 1 more reed than a 6 and the carb venturi is larger in carb body along with a larger main jet to match carb. Just about the same parts on a 9.9/1515 conversion but you also need to modify the tuner on the 9.9/15 to make it work.

 

[pecheux]

Re: omc 6hp vs 8hp...same gear ratio?

off on a bit of a tangent.....would it be hard to reseal one of those lu's with basic basement tools? my teenage son went out and bought the 6 hp....thought he could do it with his buddies, but the seller wasnt quite as honest as he should have been. the motor leaked all its gear oil on the basement floor overnight.

any advice? ......other than tell my kid its a lesson learned!!!

I beleve it's illegal in Canada to do a business transaction with a teenager. So considering the seller was dishonest As a teenager's father I would have a tendency to pay the seller a visit and insist on voiding the transaction with my son ... leading to your son getting his money back. On the other hand if the seller mentioned the LU needed repair and the price paid for the motor was dirt cheap ... then the seller was not dishonest.

 

[Texasmark]

Re: omc 6hp vs 8hp...same gear ratio?

The differences are: raised exhaust port,intake manifold,cylinder head,carb body and jetting....

I can't believe that they would spend that much engineering and production costs to put that much into a 2 hp difference engine at that hp level......absolutely mind boggling. Buttttt you should know; I don't.

Mark

 

[nwcove]

Re: omc 6hp vs 8hp...same gear ratio?

I beleve it's illegal in Canada to do a business transaction with a teenager. So considering the seller was dishonest As a teenager's father I would have a tendency to pay the seller a visit and insist on voiding the transaction with my son ... leading to your son getting his money back. On the other hand if the seller mentioned the LU needed repair and the price paid for the motor was dirt cheap ... then the seller was not dishonest.

as far a i know, its not illegal to sell a teenager an item, but a teenager under 18 cannot sign a contract ie cell phone. the seller didnt mention the problem with the lower unit tho, but the motor was cheap and starts first pull. i think it will be a lesson learned for my son ( ive tought him to be honest, and i would never sell an item of any kind , that had issues known to me, without telling the buyer).
i guess it will also be a mutual mechanical lesson....in resealing a one piece lu, hopefully it goes well. and i would still appreciate anyone who done one chiming in and passing along any advice!!

 

[pecheux]

Re: omc 6hp vs 8hp...same gear ratio?

Have you noticed where the oil is coming out for the LU ? seals or the filling screw or vent screw ? Reason for asking is I own a Force that kept on taking water in the LU and pressure checks showed nothing wrong ... until I noticed upon a LU oil change that the tread inside the filling screw was damaged. Merc dealer replaced the inner part treads for $ 25. and that was the end of water infiltrating the LU. Water in or oil out could be the same type of problem. Nevertheless I do realise that emptying in one night is a BIG leak. Cheers.

 

[nwcove]

Re: omc 6hp vs 8hp...same gear ratio?

thanks pecheux, its definitely leaking bad at the prop shaft seal(s), but if i can figure out how to pull the seals without causing damage to the lu, i hope to replace the drive shaft seal(s) also. looking like i might have to drill and use my big hammer!! lol ( would like to buy a manual, but it would cost almost as much as my son paid for the motor)

 

[Chinewalker]

Re: omc 6hp vs 8hp...same gear ratio?

The differences in the intake go beyond just the extra pair of reeds. In order to accommodate the extra reeds, the plate is larger, and by extension, the crankcase casting is different. Therefore, there is no easy way to convert a 6hp to an 8hp without having most of an 8hp to work with...

 

[pecheux]

Re: omc 6hp vs 8hp...same gear ratio?

thanks pecheux, its definitely leaking bad at the prop shaft seal(s), but if i can figure out how to pull the seals without causing damage to the lu, i hope to replace the drive shaft seal(s) also. looking like i might have to drill and use my big hammer!! lol ( would like to buy a manual, but it would cost almost as much as my son paid for the motor)

Well it's winter here anyway so you have time on your side .. LOL Prior to proceeding with the repair I would try to fill the LU with an automatic transmission sealer additive. (probably avail at Canadian Tire) and let it sit for long enough. These sealers additive act in a way to make the automatic transmission seals become softer and slighly bigger. Might work with a LU seal .. who knows and cant hurt to try. You can also have a look at this article about using grease in a LU as a patch.

http://www.theboatshopmotorparts.com/information

I Obviously agree that fixing by doing a reseal job is best ... but I am lazy ... LOL

Good luck

 

[nwcove]

Re: omc 6hp vs 8hp...same gear ratio?

thanks pecheux, i will have to look into filling the lu with lubriplate as a fix. the drive shaft seals look like an easy fix, but the prop shaft seal might be a different story without the proper tool. i do have the broken lu "cap" with a seal in it to experiment on!

thanks again
tim