2000 90hp RPM change w/different prop

[Clint37]

Thanks to everyone who have helped me with this so far. For those who aren't familiar with my situation, I'm in the process of trying out some different props to replace my my slightly pitted aluminum 14x19 prop on my 2000 Johnson 90hp. My first (and only so far)test prop is a 14x21 stainless. <br />With this new prop, I noticed a considerable difference. After reading some more posts, I'm not sure if what I noticed was a good thing. <br />My WOT RPM's dropped to 4800 (from 5300)with me/wife/3year old and 5gals of fresh gas. THe hole-shot is still good and my top speed picked up about 4mph to 45-46mph.(17ft fish/ski). The motor seems to run flawlessly. The mechanic at the Marina says the WOT RPM range is 4500-5500. So, everything is great...right? Some posts that I've read say 4800 could actually damage the engine by lugging it. Could someone explain this to me. If this is the case, why does Johnson publish the 4500-5500 range? Thank you again!

 

[JB]

Re: 2000 90hp RPM change w/different prop

Hi, Clint.<br /><br />True, your WOT is in the range. How about your load?<br /><br />Is that at the top of your load range or at the bottom?<br /><br />You should be able to reach 5500 with your lightest load and exceed 4500 with your heaviest load.<br /><br />If that is your "Normal" load I would drop back to a 19" prop.<br /><br />The less you torque stress your engine the longer it will last.<br /><br />Good luck.

 

[Clint37]

Re: 2000 90hp RPM change w/different prop

JB,<br />I would say the 4800RPM's was with a fairly normal load. With just me aboard it may turn 5000rpms at WOT but not the max of 5500. I guess I'll start looking for another 19 or possibly even a 17 pitch to raise the RPMs back up. I'm afraid the top speed will suffer but that's much easier to live with than a damaged engine. Thank you for the valuable advice!!

 

[Dhadley]

Re: 2000 90hp RPM change w/different prop

Before you throw money at another prop lets look at the set up. If you havent played with the X dimension -- you should before trying another prop. Raising the motor on the transom will improve the overall performance, especially the top rpms. Going from an aluminum prop to a stainless should allow for a higher X dimension. What brand props are we working with?<br /><br />Yes, 4800 is hard on the motor. The combustion temps will be higher than the motor likes. Coking will occour quite rapidly. You havent mentioned which 90 hp you have but we assume you have the carb'd model. The 60 degree motors are really happy at 5800 rpm. <br /><br />Lets raise the motor and see how much we gain.

 

[DJ]

Re: 2000 90hp RPM change w/different prop

Clint37,<br /><br />You just received some of the best advice available, at any cost. I would follow it.

 

[Clint37]

Re: 2000 90hp RPM change w/different prop

Dhadley,Djohns and JB,<br />Thank you very much for your help once again. Sorry I didn't specify the motor type but it is a 60 degree, carbureted 2000 90hp Johnson. When you say "coking" the engine, does that mean overheating? <br />As far as the SS prop I'm using, I bought it used ($80) and there is no name and no numbers anywhere to be found on it. I'm told by the original owner it's a 14x21 off of a 79 Johnson 115. While this SS performs much better than the aluminum, it still ventilates in normal turns when the motor is trimmed up some. I'll keep looking for a lower pitched SS to increase RPMs. Any recommendations on a particular brand? Thank you all for being so helpful...and patient!

 

[ob]

Re: 2000 90hp RPM change w/different prop

Clint,Coking is a term used to refer to the accumulation of carbon deposits ,particularly in the ring grooves of the pistons.There are a wide variety of propellers from which to choose for your setup.I tend to go with manufacturers recommendations and use OMC props on my John/Rude engines as they are designed to more closely match the torque curves of your engine.Not to say that others would not but your engine was originally engineer tested with OMC propellers.SSTII props are widely used as a multi-purpose stainless.

 

[Clint37]

Re: 2000 90hp RPM change w/different prop

ob,<br />Thank you for taking the time to explain that to me. You guys have really helped me out tremendously. I'll be sure to post how my next prop test turns out. Thank you again!<br /><br />Clint

 

[vissertrades]

Re: 2000 90hp RPM change w/different prop

Having just evaluated 6 different props on my '95 50 hp looper (2 cyl) I can tell you that when propped above 5500 rpm my boat responds much better throughout the entire rpm range. Johnson specs 4500-5500 rpm for this motor and I've had it set up that way, it was a dog everywhere but WOT rpm. I think the older 50 hp Johnsons were 6k rpm max and my motor seems like it has plenty of power beyond 5500 rpm. I doubt Johnson has reduced the durability of the internal engine parts. I'll bet they spec rpm ranges based on power curves on a dyno and set range to peak hp +/- 500rpm with a std test prop. Once the peak hp is hit power drops off but I wonder if the boat really cares that much, ie., on plane WOT trimmed out is absolute max hp needed? Does a dyno with some sort of std test prop really match the real world "system" (boat + motor + load). Anyone care to expand on this?<br /><br />Currently set up with OMC SST 11-1/4x19" prop pushing 5800-5900 rpm with med/light load!