New owner of 195ID 250 HP - Have RPM question.

[Solraider]

First off a big thank you to everyone that's been contributing information to this site. We took delivery of our boat several weeks ago and based off issues other members posted I had similar ones such as loose intake grate bolts, shavings etc under the engine, and an issue with the engine hatch closure stud. We have 15 hours on it and have had no fault codes, water leaks, oil leaks etc. It has been running good. But the motor will not go over 7730 RPM. Based on ScarabMikes postings I have been using 91 octane etc. Once at the 6 hour mark the RPM hit 8100 and then we backed down. I called my dealer this morning and he is going to look into it. Has anybody had this problem and corrected it to get the max RPM?

 

[ScarabMike]

Hey @Solraider , welcome to our community!!

The engine will spike sometimes and then settle down to the proper RPMs. Its normal for it to happen, especially after a tight turn while the pump is gaining pressure.

 

[Solraider]

Thanks Mike, I would just like to get it to run at the rated RPM of 8100. Last night it hit 7800. Unless humidity really makes a difference, the higher the humidity then performance drops.

 

[ScarabMike]

Thanks Mike, I would just like to get it to run at the rated RPM of 8100. Last night it hit 7800. Unless humidity really makes a difference, the higher the humidity then performance drops.

Ive never seen 8100 on my boat consistently. 7800 is the norm.

 

[wango tango]

8100 is normal, I got it for a month before it warmed up. there is a difference between fresh water and salt water performance, so important to state the water your using your boat in, salt or fresh.
I run in fresh water and will get 8100rpm 42 mph with a clean bottom of boat, 1/4 tank of gas and no more than 2 people on board. Now that is when the outside temperature is below 84f, water temperature is below 78 and humidity is below 50. I also boat on a lake that has weeds and sometimes the speed or rpms drop I check for weeds in pump. As I'm responding to this article I'm getting 7800 to 7900 rpm and only 38 top speed because of temps. and humidity stated above.

 

[James R Whitten]

If I run high octane I can get 8100 but 87 octane get about 7800.

 

[Compatico]

If I run high octane I can get 8100 but 87 octane get about 7800.

I thought the supercharged models (200/250 hp) required 91 octane fuel?

We're just looking at buying a new Scarab and I've been trying to get as much info ahead of time - the dealer said they all work better on high octane. But I already know that octane is dependent upon compression ratio and ignition timing - running 91 octane in a boat engine that only requires 87 octane gets you nothing but higher fuel costs. Octane is a combustion delay rating, not a power output rating. More octane does not mean more power unless the engine is designed to use it. Marketing tends to indicate that octane is what makes power in your engine, but that's not exactly correct, and it makes consumers believe that running 89 or 91 octane fuel is somehow better for their boat/vehicle. Some fuel companies even go as far to suggest that higher octane is better for your engine!

My tow vehicle uses 87 octane. Putting in higher octane fuel does nothing but hurt my wallet. Fuel economy and max power output is not impacted at all.

I don't mind if my new boat uses higher octane though, if it's designed to use it. My current boat, an older 4.3 liter mercruiser engine, uses 87 octane and premium does nothing for it.

 

[James R Whitten]

It's my understanding that the rotax engine had a octane sensor. When it senses lower octane it pulls timing to prevent detonation. The higher the octane the more timing you can run without detonation, which in turn is more power.

 

[Solraider]

Using 91 octane, running in fresh water, air temps from the mid 70's to upper 80's, and the humidity has been between 60% and 70% (typical central MN weather) RPM mainly 7700 and may hit 7800 occasionally.

Will have to wait till fall for lower temp and humidity levels to see if it effects RPM.

 

[wango tango]

Cooling supercharger air is key, and lower humidity.