Something about torque falling off at increased rpms on a 2 cycle OB

[Texasmark]

I was out with my sweetie today and observed a couple of things. Some guys on here talk about engine torque falling off when you get out of the recommended rpm range. I didn't know one way or the other.

So, today I disproved the myth. I have this thread and a similar related thread in the prop section and they are related. Just so happens this is related to engine torque above rated rpm range and the other has to do with prop performance.

So, on engine torque above the recommended rpm range, which for my engine is a 5500 upper limit:

I usually run a 24XP Ballistic prop on the engine in the attachment.....sorry for the side view but I couldn't find out how to rotate it up right and I am retired and tired of having to think. Forgive me!

That prop runs my little boat at 5700 rpms right at 50 mph. Bam prop slip calculator says that 5700 rpms with a 24P prop with a 2.33 gear box at 50 mph results in an 11% slip. That's a good number for a light boat with a stepped hull....see Avatar.

So I take the 24 XP off and install a Ballistic 21P....both props shown in the prop thread. Today I took some data:

I cut the throttle back to a reasonable rpm and running at 40 my rpms were 5800. Running the BAM prop slip again that's a whopping 20% slip. But the point here is that the engine is still putting out ponies.

So I firewall the throttle and the rpms go up to 6600. My little Merc just loves it. Purrs like a kitten. Every time I firewalled it the little darlin just said "yes sir HP at your command". Back to the story. At 6600 I am running 47.5mph. BAM prop slip calculator says slip is now 15%.

The slip in my opinion is due to the hull efficiency. The point here is that the engine continued to put out HP at the increased rpms, and took the additional rpms, and did it at a 145 vs 138 loss ratio meaning that it was more efficient at the 6600 rpms than it was at the 5800 rpms.

So sirs, if your engine craps out when you "give her the reins" maybe you need to invest in a Mercury.

My humble 2c and worth every penny you paid for it.

Mark

 

[metriccrescentwrench]

Re: Something about torque falling off at increased rpms on a 2 cycle OB

Bam prop slip calculator says that 5700 rpms with a 24P prop with a 2.33 gear box at 50 mph results in an 11% slip

I've always had an issue with comparing the slip of one prop to another..... The calculated slip numbers depend on the stated prop pitches being dead nuts on the money for both props, and I don't believe that's the case. I'd be interested in hearing what some prop designers have to say about that.

 

[Frank Acampora]

Re: Something about torque falling off at increased rpms on a 2 cycle OB

Mark: ALL internal combustion engines have a horsepower and torque curve that RISES with RPM while at some arbitrary RPM defined by engine design, torque will begin to fall off. This is NOT necessarily at the upper RPM range as stated by the manufacturer.

The decreased torque does NOT mean decreased horsepower so running the engine faster that the Recommended RPM range will not necessarily mean less speed. Prop slip is independent of the horsepower and torque curves and dependent upon prop design. THUS: The need to experiment with props to get the best performance for your particular engine and application. Again, obviously, a slow cruiser would need to be propped differently than a race boat.

And by the way: mercury is a crap engine! Go back to Force. LOL

 

[Texasmark]

Re: Something about torque falling off at increased rpms on a 2 cycle OB

And by the way: mercury is a crap engine! Go back to Force. LOL

Frank, only from you could I appreciate this comment. Grin! I had a couple of Force engines before they were "Let the Force Be with you".....Star Wars! Had a '71 55 and 85. Never had a problem with either. The 55 didn't have enough power for my Chrysler 16' Sport Fury open bow tri-hull and all I needed to do with it, and as far as the 85, my buddies with their loop charged OMC's cold go the same distance on one tank with it taking me two and for the difference in the two engines being 20 to 25 hp, I couldn't out run them.

My comment was based upon someone back over the years talking about if you got above the recommended rpm range the engine would fall on it's face and your speed would go to crap. I know the equation for HP and according to them the torque would crap out faster than the rpms would rise. Well so much for their educated know not.

Mark

 

[metriccrescentwrench]

Re: Something about torque falling off at increased rpms on a 2 cycle OB

...according to them the torque would crap out faster than the rpms would rise. Well so much for their educated know not.

That's gonna' depend on several factors related to engine design. How well the intake breathes, porting, exhaust tuning, etc. etc. A tuned exhaust can be very "peaky". I used to race 2-stroke go-carts, there was a lot of art involved in setting those things up for a particular track. With certain expansion pipes you could really feel the acceleration as the engine came "on the pipe", but above that rpm band it had nothing.

 

[jimmbo]

Re: Something about torque falling off at increased rpms on a 2 cycle OB

On the 90hp merc inline 6 and the OMC 99.6 cu inch 90hp v4s, they peaked power between 4500 - 5000 rpm and really did make less power if revd higher than 5500. those engine did have tiny ports compared to the 115s and higher

 

[dingbat]

Re: Something about torque falling off at increased rpms on a 2 cycle OB