Lower units??

[Ron G]

I was wondering how do you tell the gear ratio of the lower unit you have??and how do you know what gear ratio you need for speed or torque?Mine was replaced by the previouse owner.these maybe silly question.

 

[JB]

Re: Lower units??

Whatever gear ratio you actually have is really immaterial, Ron, so long as your prop is selected to reach top operating rpm at WOT.<br /><br />You can estimate the ratio by turning the flywheel and counting prop revolutions vs. crankshaft revolutions.<br /><br />Good luck. <br /><br />(240)

 

[Dhadley]

Re: Lower units??

It's easiest checked with a little help from someone. Pull the plugs out so you can turn the flywhell easily. Mark a spot on the shaft and flywheel. Turn the flywheel (clockwise only) until the propshaft has rotated 10 times. Then divide 10 into how many times the flywheel rotated.<br /><br />If you're talking about your 150 the flywheel should have rotated a little more than 18 1/2 (18.6 to be exact) times. Hence the 1.86:1 ratio. It doesnt have to be an exact deal because there are only so many ratios that bolt up to that motor -- 1.86:, 2:1, 2.12:1 etc.<br /><br />BTW -- if we are talking about your 150 you'll have to do some digging to find the flywheel.<br /><br />As far as which ratio we need, we really don't have a lot of choices. On most recreational boats any of the production ratios seem to work fine. You could put a V4 case on yours to get the 2:1 for better acceleration but you'll be stuck using V4 props with smaller diameter. And it probably wont last long. Those gears weren't meant to take the torque your motor makes.<br /><br />Race boats are a different story. We fool around with ratios depending on what type of race and what conditions.

 

[Ron G]

Re: Lower units??

Thanks Dhadly i was just wondering,thats a new lower unit on the 175 now,i was just curious,so for more acceleration a lower gear ratio is what ya want?and for more top end a higher ratio?is this correct.

 

[Dhadley]

Re: Lower units??

Basically that is correct. On recreational boats it doesn't make a ton of difference because like JB said, we use the prop to get to a given rpm. If we use a high ratio, we use a lower pitch. Low ratio, higher pitch.<br /><br />The factorys have it pretty well nailed as far as overall recreational performance. And keep in mind that different ratios put different stress on gears.