1970's 120 mercruiser outdrive 1.98 gear ratio vs. 1.50

[Lowchopr]

hey guys, hope everyone is having a great weekend. i took my 18 ft mercruiser out today after changing out the outdrive. got to the lake and it would not plane, found that the gear ratio is 1.50 instead of the 1.98 i took off (because it would not shift). my question is, is there any prop size that will work with this combination??. thanks in advance, appreciate it.

 

[WesNewell]

Going from 1.98gr to 1.50 I'd think you'd want to go down in pitch by 25% at least. So if you had a 19P prop with the 1.98 gr drive, you'll probably want a 14P prop with the 1.50 gr drive.

 

[Faztbullet]

Still be a dog on hole shot.....

 

[Bondo]

my question is, is there any prop size that will work with this combination??.

Nope,......

 

[WesNewell]

Sure there is. You just need to lower the pitch of your current prop by 25%. If original was 20p, get a 15P. Should operate about the same as before changing the outdrive.

 

[Lowchopr]

thanks guys! just got back from the lake! 10 miles an hour is better than nothing!! anyways, when i got this beast it obviously had the wrong 1.50 drive and a 19p prop. after the 120 engine replacement, (cracked block of course) i took it out with that combination but never got to throttle it up because the prop hub smoked. being a mercruiser rookie, i thought it was the drive. went to the guy who sold me the engine and he sold me the outdrive for it, a 1.98 ratio. by this time i found out about prop hubs, so i put the 19p back on the 1.98 and would not plane.The local boat shop told me to get down to at least a 15p, so i did,,and ...walla.....i got my rpms and 30 mph...but the 15P on the 1.50 still wouldnt go today,,, just a boggin down,,,.this is an 1985 19 ft w/cuddy. so with that being said, im having trouble gripping the math on the previous posts, but i really appreciate all of your input..any further education on this would be awesome! thanks again!!!

 

[QBhoy]

Basically it’s not going to work effectively. Even if you went down to the 15”...it’s still not going to ever perform right. Sorry to break the bad news. You either need to change the ratio out to lower ratio, or throw at least a 4.3...but preferably a 305 or 350 in there. That’s not without issue either..perhaps for another day.
change the ratio or whole outdrive is the answer.

 

[Lowchopr]

Right on thanks for your input I'm going to stick with plan A and get that 198 back on there with my15-pitch....

 

[Lowchopr]

hey guys, wanted to give an update.got the 1.98 and the 15p on, took her out today, 4000 rpms @26 mph. still took about 15 seconds to plane, but ran great. got a little warm under extended full power,,almost 240 degrees, but cooled back down cruising..running about 180 degrees. is that the normal operating temp? on the garden hose, all water pumping out from passenger side of the drive and no water out of the prop. has a new water pump in the lower....

 

[jimmbo]

240 is more than a Little Warm(water boils at 212), even 180 is too hot for a Raw Water cooling system,

 

[Lowchopr]

Gonna pull the thermostat and try it.

 

[Lowchopr]

Shouldn't i have water coming out of the prop?

 

[Lowchopr]

I also don't know the condition of the engine water pump going to check that tomorrow

 

[QBhoy]

You’ve got something going on. With a 15” you should have been way up the rpm I’d think. Hope you get it sorted.

 

[achris]

If you have already done the drive pump impeller, I'd be looking for a blocked elbow and/or manifold. If it's not that, then go for the water pocket cover test... -> https://forums.iboats.com/forum/eng...0645-i-m-running-hot-help?t=556903&viewfull=1

Chris.........

 

[Faztbullet]

I remember there was a service note about using a Alpha unit on a MC1 housing that had something about the cooling but just cannot find my dang notes..

 

[Lowchopr]

Yep!! Pulled the riser and the flapper is gone!! I cant see it,,,,what am i going to have to pull of this time?!!

 

[Lowchopr]

Thanks faztbullet

 

[achris]

Yep!! Pulled the riser and the flapper is gone!! I cant see it,,,,what am i going to have to pull of this time?!!

If the flapper is gone, it means the engine was run for a while with no water in the exhaust, so there's a very high likelihood of water pocket cover meltdown.

The flapper itself could be lodged somewhere in the exhaust passage, or it could have been shot out the centre of the prop.

I'd pull the drive and then pass a stiff wire down through the exhaust pipe to the exhaust passage in the bell housing. That will tell you if it's in the passage somewhere. If it's not, and you can't see it in the drive itself, consider it gone. Get a new one, and while the drive is off, do that water pocket cover test... (I'm betting it's leaking)...

Chris.......

EDIT: Also, how do the water passages in the manifold and elbow look?

 

[Lowchopr]

Thanks chris!! I have to pull the drive for that? I was thing maybe some comressed air back up through hole in the housing?,,, the riser and all other passages are good. I do need to research the water pocket but this drive is off a known good running boat, only been out on the water once,, (last weekend)