Gear Ratio for 2004 Mercruiser 5.0MPI (21' Boat)

[yamhoss86]

Hello all,

Looking to purchase an Alpha one for my engine (2004 Merc 5.0 MPI).
Not sure about what gear ratio.
Some sites say 1:62 some say 1:47.
Not sure which one to get, looking for some guidance.
The boat is 21' weighs about 3500lbs.

Thanks.

 

[Bt Doctur]

1.62 , the 1.47 is for the 5.7 pushing atleast 260Hp

 

[yamhoss86]

1.62 , the 1.47 is for the 5.7 pushing atleast 260Hp

Thanks, the HP rating for my 5.0 MPI is 260HP, should i go to 1:47 or stay at 1:62?

 

[Bt Doctur]

what gear ratio was the old drive and what prop were you using , Also what was the wide open throttle reading

 

[yamhoss86]

what gear ratio was the old drive and what prop were you using , Also what was the wide open throttle reading

Never ran this engine on this boat. Dont any any info about the old OD.
The mercruiser manual says WOT is anywhere from 4600-4800 Rpms.

 

[Fun Times]

Is/are there any old serial numbers that are still legible on top of the engine? Typically there are (should be) some stickers found on the flame arrestor cover that have the engine, transom and stern drive serial numbers. Problem is they do wipe away very easily during cleaning. If they are there and legible, post all the numbers you can. If you can't read them then you should be able to find the engine and transom assembly number. Having those numbers, we might be able to tell what the boat originally had. If not, you could also call/email the boat manufacturers with he boat hull ID number and they should be able to tell you what gear ratio the boat originally came with.

SERIAL NUMBER LOCATION

The Engine Serial number is located near the
starter at the Flywheel housing, the Rocker
Arm or Flame Arrestor Cover.

The Sterndrive Serial number is located on
the Upper Driveshaft housing Starboard side
or on the Back of the housing.

The Transom Serial number is located on the
Upper part of the Transom Assembly

 

[yamhoss86]

Serial: 0W041535

 

[yamhoss86]

Bump

 

[thumpar]

With an MPI and not an overly heavy boat I would think 1.47 unless it is high altitude.

 

[yamhoss86]

With an MPI and not an overly heavy boat I would think 1.47 unless it is high altitude.

Thanks, Would a 1:5 work?
I have a deal lined up for an Alpha One Gen II (1:5).
If not, i can always go for a 1:47.

Thanks.

 

[thumpar]

It would work. They only made the Gen II in 1.5 until 1995 so it is an older drive.

 

[yamhoss86]

It would work. They only made the Gen II in 1.5 until 1995 so it is an older drive.

Thanks, any idea on prop size?
I'm thinking 14.5x19.

 

[Fun Times]

Serial: 0W041535

Unfortunately the engine serial number posted didn't seem to get registered with Mercruiser properly when sold new so the Stern Drive and Transom assembly numbers didn't seem to pull up so verifying the actual gear ratio from new through Mercruiser isn't available. That mentioned, with your boat size/weight, a 1:47/1:50.1 ratio would work fine at/around sea level.

5.0 MPI - 260 HP Specifications: Drive/Gear Ratios (primary/optional/high altitude): For Alpha: 1.62/1.47/1.81:1
Drive/Gear Ratios (primary/optional/high altitude): For Bravo One: 1.65/1.5/-:1 Drive/Gear Ratios (primary/optional/high altitude): Bravo Two: 2.2/2.0/-:1 Drive/Gear Ratios (primary/optional/high altitude): Bravo Three: 2.2/2.0/2.43:1
5.0 MPI - 260 HP - Ebbtide Boats

Was there any other numbers found on the flame arrestor cover?

Gear Ratios A very common question raised when replacing the Mercruiser Alpha sterndrives involves the gear ratio. NOTE: Mercruiser made the following gear ratio changes to the Alpha Model Sterndrives several years ago in effort to develop s stronger gear combination that would offer customers better overall Sterndrive durability. Be aware that these New Gear Ratios are a direct replacement for the older gear ratios. There is no noticeable difference in performance. What was a 1.50:1 thru 1995, in 1996 changed over to a 1.47:1 What was a 1.65:1 thru 1995, in 1996 changed over to a 1.62:1 What was a 1.84:1 thru 1995, in 1996 changed over to a 1.81:1 What was a 1.98:1 thru 1995, in 1996 changed over to a 1.94:1 In 1998 Mercruiser introduced the 2.0:1 gear ratio to the Alpha Generation 2 Sterndrives which has since replaced the 1.94:1

Also be sure to inspect the orange wire on the backside of the alternator to ensure it's not rubbing on anything to where the wire would cut into a sharp object as there was an old open service bulletin saying it needs looked at. Here's the scenario of the situation.

Possible Chafing of Orange Alternator Wire on 5.0L/5.7L/6.2L MPI Engines.
Situation
On the units identified above, an orange wire attached to the output terminal of the
alternator (the orange alternator wire) was routed by MerCruiser in one of two ways. One
way was correct, the other was incorrect.
The incorrect routing places the orange alternator wire between the oil dipstick tube and
the alternator. With that incorrect routing, it is possible that the insulating material will be
chafed by the dipstick tube or the alternator, which could cause the wire to fail. If the wire
fails, it could become an ignition source. See the "incorrect routing" photographs below.
The correct routing passes the orange alternator wire beneath the alternator, not in contact
with the dipstick tube, which eliminates the opportunity for chafing and failure identified
above. See the "correct routing" photograph following.
In addition to the two orange wire routings described above, many boatbuilders modify or
disconnect the orange wire to facilitate the installation of battery isolators. Boats that have
been so modified will not require the corrective action outlined below.

ENG: 0W041535 4M4122MRS MERCRUISER 5.0L ALPHA MPI ​
Serial/Model:		4M4122MRS
Manufacture Date:	06/30/2004
Comments:	5.0L MPI ALPHA FWC DR ENG ONLY

 

[achris]

Thanks, any idea on prop size?
I'm thinking 14.5x19.

The only way to get the right propeller is to take the boat out and check the engine revs at wide open throttle using a proper calibrated shop tacho (dash tachos are notoriously inaccurate)....

Chris.........

 

[yamhoss86]

Thanks guys, info helped allot.
Have a few wiring questions but i think I'll just start a new thread. Thanks.