Max RPM for Rotax 618

[Chuck Irby]

My 618 Operator's Manual gives me the following information:

Power output: 55kW (73,8 hp SAE) at 6750 1/min.
Match propeller to achieve above indicated full load r.p.m. as per engine version. (this is copied exactly like it is in my manual)

I have been told by one CFI that this is too many RPM for a 618, that I need to prop it back to 6500.

Another instructor told me to turn it up to 6800.

Does anyone know, for sure, what setting I should use for the maximum RPM?

My sincere thanks,

Chuck

[GyroRon]

6750 will give you the most horsepower and best rate of climb performance. But the cruise power setting will need to be higher, so this rpm is best for climb but worse for fuel econ.

6250 will give you a great cruise. You can maintain a given airspeed at a lower engine rpm and use a little less fuel when pitched to this rpm. But the trade off is you will not have as good of a rate of climb.

6500 is a good in between number to pitch the prop to. This gives you a good rate of climb and a good cruise and best all around fuel econ.

I pitch all my rotax stuff to 6450-6500 as I like to horse around and want the climb but I also like to go places and need a reasonable cruise.

[Aussie_Paul]

Good advice Ron. Is that rpm when tied to a tree or at climb out air speed?

Aussie Paul.

[Chuck Irby]

Gyro Ron and Aussie Paul,

Thank you Ron for that advice. Based on what you've stated, I'll start off at 6500, then based on how it climbs, I will either add or take away some pitch. Since it's on a Dominator single with a 3 blade Warp Drive and 24' DW's, I am hoping that I can prop it back to somewhere between 6200 & 6400 and still climb quickly enough.

Paul, thank you for the added question and your concern.

Chuck

[GyroRon]

It will give you plenty of performance at 6500 Chuck. I know how big you are, I was the guy you got the RFI-150 from a year or so ago.

I could throttle mine back 200 rpm with a 582 and 23 foot Dragons and still climb out okay, you got a lot more power and more blade and we weight about the same.


Paul those numbers are tied to a tree or on climb out, however you want to do it. In my experience with gyros and Rotax engines, you won't see much gain in RPM in climb from what you had in static rpm, maybe 100 rpms. to be completely accurate I had my dominators pitch set to give me 6430 +or- 25 rpms static on the ground, but only because I was too lazy to set the pitch one more time to get it right on 6500 static.

[Chuck Irby]

Thanks Ron,

Actually I've lost about 15 pounds since I saw you a little over a year ago. I weigh 185 now and plan not to gain any more. Actually, I would like to loose some more...

Thanx again,

Chuck

[Chuck Irby]

Ron,

BTW, where did you get that picture (the pilot)? I think it's great!!!

[GyroRon]

I hijacked that off another forum I go to where someone else was using him. Not sure if it is a pilot, a skydiver, a guy on a motorcycle or what, but it is kinda funny looking. Can you see the bugs in his teeth?

[Chuck Irby]

Hey Ron,
Yes, the bugs show up well. It reminds me of my motorcycle riding days when I had nothing in front of my mouth. I rode from the mid fifty's up until 2000 (mostly Sportsters), and actually got quite a few bugs in the teeth. Some are rather tasty (but not a good taste).

[Chuck Irby]

Ron,

I added more pitch today. My max RPM during climb out was 6450.

3/4 of the way around a real tight pattern, without really pushing it, I had climbed to 1700 feet. However, it was only 62 degrees when I took off about 4:30 this afternoon.

Do you think I might be able to keep this same setting this summer?

Thanx again for your input.

Chuck

[GyroRon]

I think if you wanted to you could leave it alone and never mess with it again. 6450 is a good all around Rpm for a Rotax. If you like to fiddle with stuff you might could add or take away a little bit of pitch for the different seasons and temps but I never did. 6450 now, and in Hot summer temps I bet it won't change any noticeable amount.

The only reason I would change it is if you want a tad more climb out, and then it would be to lessen the pitch a hair to see 6700 or so on climb out. But remember your cruise speed will suffer.

Like I said I would leave it as is.

[GyroRon]

Chuck also a side note. The rotax Bing carbs run a bit richer in mixture at wide open throttle than at slightly reduced from full open settings. The reason I bring this up is on climb out it would be better for your engine to leave the throttle wide open till your ready to really reduce power once your ready to go into cruise mode.

If you got a good egt gauge, go up and experiment with it and see if this doesn't happen with your engine too. It should run cooler egts at wide open and pulling power slightly in from wide open should show a higher - many times much higher - egt readings.

I say pitch the prop for 6400-6500 rpm measured either with the gyro tied to a tree or taken on climb out at 50 mph or so. And while climbing out leave it at full wide open throttle till you reach your desired altitude then go to a cruise throttle setting - 5500-5800. It is not going to cause excessive wear and tear on the engine to climb out wide open throttle. Watch those EGTS!!!

[Chuck Irby]

Ron,

I really appreciate your input. I know that what you just stated is good advice.

One more question Ron:

As long as my EGT is okay, how long could I run at WOT, propped to 6450, without hurting anything?

Thanx again,

Chuck

[GyroRon]

As long as your temps are in check, you could leave it there till your out of gas! It isn't going to hurt that engine to run it hard. Babying it around isn't going to do much with a Rotax.

just keep good fuel, good oil, fresh plugs, and keep a eye on the carbon build up inside the ring groves and on top of the piston etc....,

Keep the muffler painted or coated to keep away rust, change gearbox oil once a year, check the carb sumps every couple flights for water and debris, and make sure to warm it up before taking off and don't run it real hard - say a prolonged climbout - then just chop power to idle for a prolonged time.... make sure you don't cause sudden major temp changes in the engine, and last watch the instruments to make sure the coolant and egt temps are good and that is it.

Run it like you stole it but follow the above and you should get long long service life out of the engine.

[Chuck Irby]

Thanx again Ron,

Have you ever heard of a product called Sea Foam? Mike Solano recommended it for removing carbon deposits. It's a gasoline additive.

If it really works, maybe a can every 20 to 30 hours would keep carbon from building.

[GyroRon]

Never heard of it. I think if you use the right oil and good gas and your Egt temps are high enough - but not too high! - you won't have too much of a Carbon problem.

I use mainly Havoline Tcw3 two stroke oil. I know it is cheap and isn't what Rotzx reccomends, But I have used it in my boats, jet skis, Rotax ultralight and gyro engines, and especially in all my two stroke lawn equipment like the power blowers edgers weedeater etc... for my lawn service business. In all these years I have never had to scrape Carbon when using this oil.

In yeaars past I used many different oils in my lawn stuff and found that about every three months I had to take the mufflers off all my two stroke stuff and scrape the exhaust port out as the carbon would have had it nearly closed off. After using the Havoline I have not had to scrape the port clean in years and I mean Years! I had a base gasket go out on one of my backpack blowers a year or so ago and at the time that blower was 3 or 4 years old and had a lot of miles on it, I went ahead and took off the muffler expecting to need to clean a little carbon while I had everything all apart and I couldn't believe it, not hardly a speck of carbon. All using Havoline tcw3 and the cheapest 87 octane fuel I can find.

So long story short. I think finding and using the right oil is key to low carbon deposits.

[Chuck Irby]

Ron, that makes sense to me. Have you ever run synthetic oils?

[GyroRon]

Nope. I got good service out of 2$ a quart oil. I don't think the added cost of the synthetic oils would really be worth it. Others may disagree and others may have a money tree in their backyard ;) I don't , so if 2$ a quart oil works good then that is what I will use.

[Chuck Irby]

Ron, I completely understand. I don't have a money tree either. That would be nice though.