4.3 LX gen+ performance with 22"

[keijonen123]

Hi all,

I have a -97 4.3 LX gen+ engine with 22 " prop on a sea ray 175BR. I took it on a test drive and the outcome was 46.6 mph at WOT and the RPM value was as low as 3600. According to my math skills this speed is not possible even in theory at this RPM value. I assume that outdrive gear ratio is 1.84. Could someone confirm the gear ratio with the following serial numbers?

engine serial: 0K1970930
outdrive: 0K073798

1. What can be causing this low RPM value compared to speed?
2. What is the typical performance of this engine with a boat like mine? (engine RPM with certain pitch etc...)

Any suggestions, opinions??

Thanks in advance...

 

[alldodge]

Re: 4.3 LX gen+ performance with 22"

Hi all,

I have a -97 4.3 LX gen+ engine with 22 " prop on a sea ray 175BR. I took it on a test drive and the outcome was 46.6 mph at WOT and the RPM value was as low as 3600. According to my math skills this speed is not possible even in theory at this RPM value. I assume that outdrive gear ratio is 1.84. Could someone confirm the gear ratio with the following serial numbers?

engine serial: 0K1970930
outdrive: 0K073798

1. What can be causing this low RPM value compared to speed?
2. What is the typical performance of this engine with a boat like mine? (engine RPM with certain pitch etc...)

Any suggestions, opinions??

Thanks in advance...

Your serial number shows it is a 1:81
You can look it up here at the Merc site Parts Catalog | Mercury Marine

Your doing 46.6 at WOT at 3600 RPM, no I don't think so. I'm guessing something is providing erroneous information. Either your tach or speed-o

 

[keijonen123]

Re: 4.3 LX gen+ performance with 22"

Thanks,

The speed was gps speed so I really think that tachometer reading is incorrect then. What would be the easiest way to confirm is the meter faulty? Is it usually meter itself or some sending unit?

BR

 

[funk6294]

Re: 4.3 LX gen+ performance with 22"

Do you have timing light with the built in inductive tach? You could verify the tach with it, although we already know it's not reading right. You may pull the tach and check the selector switch on the back is set for six cylinders. Even if it is set right rotate the selector a couple of times to help ensure there is no dirt or poor connections causing problems.

 

[alldodge]

Re: 4.3 LX gen+ performance with 22"

Thanks,

The speed was gps speed so I really think that tachometer reading is incorrect then. What would be the easiest way to confirm is the meter faulty? Is it usually meter itself or some sending unit?

BR

Agree with funk, it's the tach and there are no sensors. The tach receives pulses from the coil, gray wire. You could pick up a hand tach to compare with but the hand tach will cost as much as a new one.

 

[achris]

Re: 4.3 LX gen+ performance with 22"

Dash tachos are NOT accurate enough for testing purposes. You need to use a 'shop tacho', or one built into the more advanced timing lights.

As already said, your drive ratio is 1.81:1 (close enough to the 1.84:1 to not make any difference)...

I had a slightly older 4.3LX (Gen II engine, not to be confused with the Gen II drive) in my 20' cuddy and I was running a 21" prop and making just on 40 knots at about 4900 rpm... If the other figures are correct (speed and prop pitch), then I make that your engine is probably closer to 3900rpm, which is still too low. Recommended for those engines is 4400-4800... and I prefer to prop towards the top end of that...

Chris......

 

[keijonen123]

Re: 4.3 LX gen+ performance with 22"

Hi guys and thanks for the comments!
Today I checked the back of the tacho and the setup or screw was pointing to either 3P/6C or 6P (I am not sure because I was not able to recognize which end of the pointer was actually the tip! and these two selections were separated by half circle!) . I changed the setup by half circle but do you guys know which one is correct?

And another question is that why my outdrive port side there is serial: 0D696374 which is different from what is labeled on the boat? Can I still be sure with this serial that the ratio is 1.81?

But yes I think I need to get some inductive handheld tacho..

Thanks...

 

[alldodge]

Re: 4.3 LX gen+ performance with 22"

Hi guys and thanks for the comments!
Today I checked the back of the tacho and the setup or screw was pointing to either 3P/6C or 6P (I am not sure because I was not able to recognize which end of the pointer was actually the tip! and these two selections were separated by half circle!) . I changed the setup by half circle but do you guys know which one is correct?

And another question is that why my outdrive port side there is serial: 0D696374 which is different from what is labeled on the boat? Can I still be sure with this serial that the ratio is 1.81?

But yes I think I need to get some inductive handheld tacho..

Thanks...

Yours is set correctly, it should be on the 3P/6C which is for a 6 cylinder. The 3P is also the same setting for an outboart with 3-poles.

Your serial number 0D696374 is for a 1:50 ration Alpha Gen II. Looks like your drive has been changed. Don't know if it is a 1:50 or not but probably is. You could find out by pulling the drive and counting the Revolutions

 

[keijonen123]

Re: 4.3 LX gen+ performance with 22"

Ok! I will perform the manual check but if the ratio is as it seems Is it a problem that I have this lower (1.5) ratio in terms of performance or can I fully compensate this with lower propeller pitch?

 

[alldodge]

Re: 4.3 LX gen+ performance with 22"

Ok! I will perform the manual check but if the ratio is as it seems Is it a problem that I have this lower (1.5) ratio in terms of performance or can I fully compensate this with lower propeller pitch?

You can compensate with a lower pitch prop which will take care most the issues but will not perform as well as if it had the correct ratio drive. I would keep what you have the difference would not be that much, mostly hole shot.

 

[keijonen123]

Re: 4.3 LX gen+ performance with 22"

Ok thanks! By the way, is that serial number also saying that my outdrive is LH meaning that propeller would be spinning counter clockwise when looking from behind the boat? Because that is not the case..

 

[achris]

Re: 4.3 LX gen+ performance with 22"

The 'counter rotation' is done in the lower housing. If just the top housing has been replaced, it could have come from a LH drive.

You need to find out what the ratio actually is, then we can stay getting into the prop side of things, and get an accurate tacho hooked up....