Drive question?

[Chassisman]

Hello all,

I recently bought a 90 convincor gtx with a 330hp engine and it wont run well at all (no speed) come to find out it has a 1.65 ratio drive on it which i believe is incorrect any idea how much that would slow it down over the 1.50 ratio . Any info greatly appreciated Thanks.

Brian

 

[JimK]

Assuming the same prop and same rpm for both, there would ~10% difference ... but in reality there should be little difference in performance since the proper pitch prop for the ratio lower unit used should be nearly the same. Example: the 1.65 might use a 23 pitch prop where the 1.50 uses a 21 pitch and get the same engine rpms.

 

[keith4216]

Just curious what kind of speeds are you seeing. I have a 91 gtx magnum bravo set up but have never gpsed it. Just installed one the other day so Im interested in seeing what its really doing. The speed lier I mean odomator is a joke so i cant wait to see how slow Im really going. lol

 

[mkhammer]

Hello all,

I recently bought a 90 convincor gtx with a 330hp engine and it wont run well at all (no speed) come to find out it has a 1.65 ratio drive on it which i believe is incorrect any idea how much that would slow it down over the 1.50 ratio . Any info greatly appreciated Thanks.

Brian

Sounds like you have a Alpha drive with that gear ratio, What was said is correct you could swing a bigger pitch prop with the 1.65 the 1.50 would not hit the same rpm limit using the same prop. So you should be proped to hit about 4500 to 4800 rpm with your setup.
If you are not seeing that rpm you need to go down a size in pitch to bring your rpm up. Then you may get a little more speed out of it. If you are seeing that rpm you need more power and a taller gear ratio and possibly a larger prop to go faster.

 

[Chassisman]

Thanks MK i really appreciate the info. I am pretty sure it is a bravo i , just had it repaired and they didnt say any different (broke a u joint) they spun it for me there and came up with the ratio maybe it was like 1.63?. It currently has a 22 pitch on it and runs about 45-4600 rpm but im only seeing like 48 mph on gps (horrible). I thought i read on here people with similar boats with the same power running in the 60s , maybe i need to look elsewhere timing etc. Thanks again .

 

[wca_tim]

all other things being equal... ie. assuming you've got the right prop on it, the boat should be faster with the higher gear ratio...

generally props moving toward the 27-29" pitch range are going to be better at translating torque into thrust for the speeds you want to be running and so you should be at least 3-5 mph faster with the higher gear ratio...

How much have you played with trim? are you sure the engine is in good tune? etc.. all the normal stuff.

 

[Chassisman]

all other things being equal... ie. assuming you've got the right prop on it, the boat should be faster with the higher gear ratio...

generally props moving toward the 27-29" pitch range are going to be better at translating torque into thrust for the speeds you want to be running and so you should be at least 3-5 mph faster with the higher gear ratio...

How much have you played with trim? are you sure the engine is in good tune? etc.. all the normal stuff.

I am not sure of much yet ! lol I just bought it and only had it in the water once and broke a u-joint , I played with the trim a bit and while it definitely picked it up , nothing like my 88 but i had more pwer in that boat. I am going to go over everything soon tune etc. but it seemed to run fine for the most part just no speed .

 

[Big Red]

That boat needs alot of trim and it will run faster on 1-2 foot of chop. Trim it up till you crap your pants then bring it down a hair

4600 rpms, 22p prop, with a 1.65 gear, at 48mph gives you 17% slip,, the slip rate needs to be closer to 10% on that boat 17% is not good.

 

[mkhammer]

Thanks MK i really appreciate the info. I am pretty sure it is a bravo i , just had it repaired and they didnt say any different (broke a u joint) they spun it for me there and came up with the ratio maybe it was like 1.63?. It currently has a 22 pitch on it and runs about 45-4600 rpm but im only seeing like 48 mph on gps (horrible). I thought i read on here people with similar boats with the same power running in the 60s , maybe i need to look elsewhere timing etc. Thanks again .

They made bravos with that gear ratio but it was more prevalent in a Alpha and they did put some Alpha drive behind big blocks.

you are down on rpm and should be able to spin a bigger pitch than a 22 with a 1.65
It's a power issue, do a compression check to see if the engine is tired. if compression is good, yes check your timing. a big block won't make HP if the timing is backed down I would start with 34 deg. 36 max total advance and should be all in by 3000 rpm. Just make sure it does not detonate. Also you could have an ignition problem. sometimes a distributor will hold an engine back, I sell DUI Marine HEI distributors $319.00 custom curved and will make at least 15 more HP over stock thunderbolt. This may be what you need to put it over. I have seen 200 rpm gains with these distributors without changing anything else.

Or this could be the problem

As mentioned something does not sound right hgh slip and below redline rpm.
so its pushing the prop as high rpm as the engine can which is not on the rev limiter and it has a high slip factor. hhmm. The engine is working hard to move the boat, and if it could get to a faster speed the prop would not slip as much

Sounds like your hull is water logged. something is bogging it down