Possible to tell if Merc 2.5 liter gearcase is 1.87 or 2.00 gear ratio from outside?

[Bumblebeeman]

I have one of the old 2.4 liter XR4 150 hp bass boat motors, mine is a 1991. I hit a limestone ledge and ruined my small diameter 1.78 ratio gear case. I bought supposedly a 1.87 gear ratio case off Ebay. But it is so slow I think it may be a 2.00 ratio, like they used on the 135 Opti.

With my 1.78 gearcase and a 24 inch Laser II I got 68.7 mph GPS at 5700 rpm on my Bumble Bee 178 FD.

With the used gearcase I bought off Ebay and the exact same propeller I get 58 mph GPS at the same 5700 rpm.

The numbers don't work out to it being a 1.87 gearcase, its too slow.

I looked all over the outside and can't find any identifying number. But is there a way to tell the gear ratio from the outside of the gearcase?

Thank you

[rock]

Put lower in gear. Get a 15/16 socket on the flywheel and give the motor two full turns. If the prop turns 1 full revolution it is a 2/1. If it comes up short of a full turn it is a 1.87/1.

Rock

[tlwjkw]

a 2 ta 1 is gonna be way slower than tha 1.78.... should pull a tad bit harder outta tha hole but will not top end same as 1.78 or 1.87.

its like comparin' 3.55 rear end gears ta 4.10's.....

[rock]

My statement was incorrect. This happens sometimes when you get old. The propshaft will go just past a full turn with a 1.87/1 gear not come up short of a full turn. Thanks for the correction Chris.

Rock

[gotboostedvr6]

That^

[Merc 2.5]

I have one of the old 2.4 liter XR4 150 hp bass boat motors, mine is a 1991. I hit a limestone ledge and ruined my small diameter 1.78 ratio gear case. I bought supposedly a 1.87 gear ratio case off Ebay. But it is so slow I think it may be a 2.00 ratio, like they used on the 135 Opti.

With my 1.78 gearcase and a 24 inch Laser II I got 68.7 mph GPS at 5700 rpm on my Bumble Bee 178 FD.

With the used gearcase I bought off Ebay and the exact same propeller I get 58 mph GPS at the same 5700 rpm.

The numbers don't work out to it being a 1.87 gearcase, its too slow.

I looked all over the outside and can't find any identifying number. But is there a way to tell the gear ratio from the outside of the gearcase?

Thank you

Seems to me if you lost 10mph by goin to a lower gear wouldn't the rpm go up on top? With the same prop? Or is that on the limiter

[Bumblebeeman]

Reply to Merc 2.5,

Yes the rpm went up. I quoted the same rpm I had with the 1.78 gear case in the speed comparison to keep things simple. But its the first time I ever saw over 6000 rpm on this engine. With the 1.78 case and a 24 inch Laser II the highest I would see is 5600 rpm summer and about 5850 winter. I have one of those little Tiny Tach digital mini tachometers and I think they are very accurate.

[Merc 2.5]

Ok that sounds better. Yep u got a higher gear

[FMP]

Lower

[tlwjkw]

maybe he's sayin' "higher numerically"... :reddevil:

[Merc 2.5]

maybe he's sayin' "higher numerically"... :reddevil:

Naw I just screwed up and had it backards. Forgot which 1 he had b4 he bought the 2.00. 1

[tlwjkw]

dammit!.. I covered ya butt and ya gotta spoil it.... :p

[FMP]

He must be saying higher amount of gear reduction:thumbsup:

[Merc 2.5]

He must be saying higher amount of gear reduction:thumbsup:

Ya that's what I tryin say!!

[FMP]

I just needed a higher amount of listing

[Zoomba]

How fast is the boat at the new top rpm. Sometimes letting the engine spool to 6500-7000 will fre up some horespower. What is the WFO rpm?