Questions about lower unit

[RetNav]

I have two 90 model 200 hp evinrudes, set up one standard and one counter rotation.

The 200 with the standard rotation will turn 6000 rpm at wot no problems. For reasons not stated, I put the counter rotation lower unit on the same motor and it will only turn 4800 rpm at wot. The two props are from the same mfg, dia and pitch the same. Only difference is one is standard rotation and the other is counter rotating.

Today I pulled both lower units off and checked the ratio by putting them in gear and rotating the prop while counting the turns of the shaft. For one turn of the prop the standard rotating unit turns 1.8/1.9 times. For one turn of the prop the counter rotating unit turns 1.75 times.

Both units turns freely, no binding, no water in oil, shifts good, etc.

Could the difference in ratio cause a 1200 drop in rpms at wot?

I have been fighting this difference in rpms for about two years and hope this is the answer.

 

[RetNav]

Re: Questions about lower unit

bump

 

[tashasdaddy]

Re: Questions about lower unit

sure it could, you need to reprop the counter rotating one, so the engine is not lugging. or change the gear ratio.

 

[RetNav]

Re: Questions about lower unit

Appreciate the response.

I guess I will have to go to the dealer where I bought the motor and apologize for raising cain all this time. I thought it was the motor and they kept telling me that the motor was good. What bothered me was they had no suggestions as to what was keeping it from turning higher at wot.

 

[reload]

Re: Questions about lower unit

tashasdaddy, please explain how you come up with 1200 rpm difference. I don't have any experience with dual engines. I think I understand the lugging because the drag from the 6000 rpm motor holding back the one trying to rev. Doing some quick calcs I don't see 1200 rpm. I may be doing it all wrong.

 

[RetNav]

Re: Questions about lower unit

reload,

Lets look at it a little different. The starboard motor turns 6000 rpms, with a 1.9 to 1 ratio that would put the prop turning at 3157 rpms. This motor can do that for a long time (well not to long as the boat only has 120 gals of gas).

The port motor has a 1.7 to 1 ratio. It is struggling to get the rpms up, but doesn't turn more than say a max of 5000. That would put the prop turning at 2941 rpms.

As you can see the port is trying to get there but due to the ratio it does not have the power to get the r's up. By my calculations, if the motor could reach 5375 the props would be turning about the same.

Hope this doesn't muddy the waters to much.

 

[reload]

Re: Questions about lower unit

RetNav, that is about what I came up with. I was trying to come up with 1200 rpm difference and was getting more like 500. I was atleast on the right track.

Thanks