mercruiser 7.4 mpi can't figure out the ratio

[Cruis]

Hi, I have a 2001 wellcraft 270 coastal with a mercruiser 7.4 MPI with a bravo 3 outdrive 22 pitch prop. Late this summer the lower shaft upright shaft snapped and I couldn't tell what ratio I had so, I took a shot buying a bravo 3 gear ratio 2.20:1 and the guy had the same engine but, his outdrive upper drive gear bearings was broken so, I used my drive gear that I had from the outdrive that was on before. Now Running the boat at 4000 rpm I am getting 18 mph but before with the old drive at the same rpm I would get 25-26 mph. Today I pulled that boat out of the water and opened the top cover of the drive and I counted the teeth and it has 23x30 and the 23 teeth it was the old drives upper drive bearing and gear. So i'm guessing the old drive that I had before was also 23x30 with means that gear ratio is 2.20:1. Does anyone have the same boat and motor and what gear ratio do you have? Do you think I have a outdrive issue or engine? Don't know what to do anything helps.
Thank you

 

[alldodge]

Your new lower may be 13x24 giving you a 2:43 ratio. The old may have been 16x27

To determine ratio without disassembly pull the kill switch (if you have one) or disconnect wires from the coil, Also need a helper

Mark the prop and the crankshaft pulley or balancer
Remove protective cover off the slave relay

Jump across the top terminals with a screw driver of remote start button while another is moving shifter into forward. Note: Bravo's should not be shifted without the motor turning

Once in FWD, helper watches prop and you watch crankshaft.
Jump the slave relay and let motor turn over 10 times then stop. Your cranking 10 times to make it easier to determine ratio

 

[alldodge]

Bravo ratios

 

[Cruis]

Your new lower may be 13x24 giving you a 2:43 ratio. The old may have been 16x27

To determine ratio without disassembly pull the kill switch (if you have one) or disconnect wires from the coil, Also need a helper

Mark the prop and the crankshaft pulley or balancer
Remove protective cover off the slave relay
View attachment 352686
Jump across the top terminals with a screw driver of remote start button while another is moving shifter into forward. Note: Bravo's should not be shifted without the motor turning

Once in FWD, helper watches prop and you watch crankshaft.
Jump the slave relay and let motor turn over 10 times then stop. Your cranking 10 times to make it easier to determine ratio

hi, I already have the outdrive off the boat and I tried that method and I got 2 and about a 1/4 more turn of the shaft that goes into the engine. looks to be 2.20:1 ratio but, someone is also saying that it can also be 1.81:1 diesel I doubt it though. Shouldn't mercruiser 7.4 MPI be running with a bravo 3 2.00:1 gear ratio ?

 

[alldodge]

Going from 2.2 to 1.8 your rpm's will be lower and speed would be higher. The 1.81 would have 29/27 gears in the upper

For the rpm's to go up and speed down using the same props, the ratio would be higher

What pitch props do you have?

 

[Scott06]

hi, I already have the outdrive off the boat and I tried that method and I got 2 and about a 1/4 more turn of the shaft that goes into the engine. looks to be 2.20:1 ratio but, someone is also saying that it can also be 1.81:1 diesel I doubt it though. Shouldn't mercruiser 7.4 MPI be running with a bravo 3 2.00:1 gear ratio ?

If you want to find out what was there originally I would contact Wellcraft, they order the power packages and the set up determination.

in the parts book for my Sea Ray it outlined all 4 engine options with outdrive ratios and riser configuration.

 

[Cruis]

Going from 2.2 to 1.8 your rpm's will be lower and speed would be higher. The 1.81 would have 29/27 gears in the upper

For the rpm's to go up and speed down using the same props, the ratio would be higher

What pitch props do you have?

Thanks I have 22 pitch props right now with the 2.20 :1 I’m getting the rpm same as before but the speed is lower. I’m not sure on the ratio of the outdrive that was on before but, I counted the drive tooth and I have 23 so,Im guessing there was also a 2.20:1 gear ratio before. But not sure why I’m not getting the same speed as before. Unless the drive before was a Diesel 1.81:1 they also have the same count of teeth

 

[Cruis]

If you want to find out what was there originally I would contact Wellcraft, they order the power packages and the set up determination.

in the parts book for my Sea Ray it outlined all 4 engine options with outdrive ratios and riser configuration.

Thanks I’m in the process of getting on to them.

 

[alldodge]

Not finding where a 1.81 tooth count for a diesel is different then the standard. Even the X drives are 29/27

With 22 props the slip is real bad

 

[Cruis]

Not finding where a 1.81 tooth count for a diesel is different then the standard. Even the X drives are 29/27

With 22 props the slip is real bad

What ratio do you think I need for the boat and engine?

 

[alldodge]

Maybe I should retract for a minute.
With the old drive was the boat on plane at 25 MPH?

With new drive at 18 MPH is the boat on plane or plowing?

Can you get WOT of 4800 rpm

 

[Cruis]

Maybe I should retract for a minute.
With the old drive was the boat on plane at 25 MPH?

With new drive at 18 MPH is the boat on plane or plowing?

Can you get WOT of 4800 rpm

Yes I would get between 25-26 mph at 4000 rpm
The new drive at 18 mph at 4000 rpm it gets me right at plane.
with the old drive max rpm and I would get 32 mph and now with the new drive I get 20 mph.

 

[alldodge]

Your getting 20 mph at WOT, meaning rpms go to 4800. You should also have to reduce the throttle because the motor will over rev, correct?

 

[Cruis]

Your getting 20 mph at WOT, meaning rpms go to 4800. You should also have to reduce the throttle because the motor will over rev, correct?

yes 4800 rpm 20 mph the motor doesn’t over rev. Compared to have the old drive everything’s runs the same except I’m not getting speed like before and this drive lashes into forward and reverse especially when I’m docking the boat changing gears fast.

 

[alldodge]

If your using the boat speedometer then it could be its most likely doing the same as before, just the issue with the pickup hole in the drive. Might be partially clogged.

Now if your using a GPS then I need to scratch my head some more

 

[Cruis]

If your using the boat speedometer then it could be its most likely doing the same as before, just the issue with the pickup hole in the drive. Might be partially clogged.

Now if your using a GPS then I need to scratch my head some more

Yes I’m using gps speed and also download an app to tell my speed to make sure. Not sure if I have a different lower ratio for the top

 

[alldodge]

I'm out of ideas what could cause it

Same 22P prop, same rpm at WOT

If the ratio was higher (2.0 or 1.8) then the prop would be spinning faster and would be going faster unless the prop has cavitation. If there is air getting to the prop from to high trim, or other method it would allow the prop to spin faster

If the ratio was lower (2.2 or 2.4) the prop spins slower, less load so the motor should over rev. Being a MPI the ECM could be keeping the rpm's down by retarding the timing, but should notice if that was happening

Putting gear ratios in the prop calculator using 22 prop, 4000 rpm, 20 mph and different drive ratios comes up as
2.0 ratio = 52% slip
2.2 ratio = 47% slip
2.4 ratio = 42% slip

If I change speed to 26 mph and ratio 2.4 slip lowers to 25%

Slip is the the amount the prop does not move forward per revolution. A 22 prop theoretically move 22 inches per revolution. Evan at 25% that is a lot of slip. I would expect to see slip closer to 20 or less. The current slip is out right bad

Are the props in ok shape?

 

[alldodge]

Double check your tach

 

[Cruis]

I'm out of ideas what could cause it

Same 22P prop, same rpm at WOT

If the ratio was higher (2.0 or 1.8) then the prop would be spinning faster and would be going faster unless the prop has cavitation. If there is air getting to the prop from to high trim, or other method it would allow the prop to spin faster

If the ratio was lower (2.2 or 2.4) the prop spins slower, less load so the motor should over rev. Being a MPI the ECM could be keeping the rpm's down by retarding the timing, but should notice if that was happening

Putting gear ratios in the prop calculator using 22 prop, 4000 rpm, 20 mph and different drive ratios comes up as
2.0 ratio = 52% slip
2.2 ratio = 47% slip
2.4 ratio = 42% slip

If I change speed to 26 mph and ratio 2.4 slip lowers to 25%

Slip is the the amount the prop does not move forward per revolution. A 22 prop theoretically move 22 inches per revolution. Evan at 25% that is a lot of slip. I would expect to see slip closer to 20 or less. The current slip is out right bad

Are the props in ok shape?

Yes prop are good I would have a vibration if they were bad. Also I since I was having this issue I tried going full throttle at once shot and the boat was more slower getting to plane but eventually reach to plane. I remember when I bought the boat and went for a test drive the previous owner was like hold on and he gave it full throttle and it made your head go back. It was faster than.

 

[Cruis]

If you want to find out what was there originally I would contact Wellcraft, they order the power packages and the set up determination.

in the parts book for my Sea Ray it outlined all 4 engine options with outdrive ratios and riser configuration.

Not getting a hold of them