RPM Range

[SEGAangler]

I had to change my LU and I'm certain it has a different gearset (went to a V^ LU b/c availability & price). I know that change alone is heavily debated both ways but it's where I am at and I'm gonna have to test the prop I have and determine how big of a change a need. I've seen many different numbers/ranges stated.....can anyone confirm for me the rpm range for a 1987 140 hp Evinrude? Model is E140TXCUA

 

[boobie]

5000 to 6000 RPM.

 

[SEGAangler]

Thank you. If I hit the lower end of the range is that OK or should I strive to reach higher? (I've seen many post stating to shoot for the higher end).

 

[boobie]

With the boat lightly loaded I always shot for the top end.

 

[emdsapmgr]

Boobie nailed it. Today's thinking is to prop it to run at the top of the "normal" rpm range, or 6000. At 5000, you are "lugging" the engine. Once you get it to run at this rpm, with just you in the boat and a light load of fuel-you will maximize fuel economy and performance at all rpm's.

 

[Faztbullet]

you will most likely need to drop 2 pitchs and it will be a dog if used for towing/sking

 

[boobie]

Fatz, if you drop 2 pitches won't it be better for towing/skiing ??

 

[emdsapmgr]

Yes it should.

 

[Faztbullet]

If he has different ratio as he posted he put a V-6 unit on a V-4 looper thus it will need to drop 2 sizes to be at the top of rpm range and it will be slower out of the hole.

 

[emdsapmgr]

Any chance this V6 lower is from an 85 model V6? Isn't that the year that the V4 90 degree loopers used a V6 lower unit. The had a different ratio for that special application. (wasn't it one year only?)

 

[boobie]

Yes it was...................

 

[SEGAangler]

I have not been able to water test it yet due to work & weather. The original gears were 2.0:1 and the V6 gearcase is 1.85:1. I have, in the meantime, gotten a 15" pitch prop to test with (I was running 17" on the old setup). I'd love for it to run like a top....but as I'm merely doing some recreational fishing Neither my hole shot nor top end are critical. I just want to go out w/o fear of being stuck on the water. I'm not positive when I'll finally get the chance to test but I will certainly update with my results when I do.

 

[SEGAangler]

Emdsapmgr, I'm not sure exactly what year the V6 unit is from. All I know is it's from a 198_ model 175 HP Evinrude.

 

[emdsapmgr]

My point is that in 1985 OMC put V6 gearcases on the V4 Loopers. They changed the lower unit ratios so they'd run with normal prop pitches on those big gearcases. One year only. If you'd put a V6 gearcase on from a 1985 V4 on your 87, it should have the correct/acceptable gear ratio already. You've identified your replacement as a V6 case for sure. The year of it won't make a difference, as it is the taller ratio. You will have to play with the props more, but should be able to get it dialed in to work on your engine. A 15" prop is as good a pitch as any to start your testing with. Some of these lower pitch props are just not as efficient thru the water as say, the 17, 19 or pitch props are.

 

[SEGAangler]

The test was a failure; the (used) LU had a very horrible vibration one it was in the water under load so it never made it to open water to test prop sizes.

 

[boobie]

Is the prop shaft bent ?? Or the prop ??

 

[SEGAangler]

Turns out it's the prop shaft. It is very minor; didn't even notice until I studied it closely. I had one in the past that was visibly bent and ran smoothly (didn't run it long BC I didn't want to get stuck if/when a seal failed)..... I was surprised it vibrated so badly