Using the Rotax 670 in a Rans S-9 or S-10
As an introduction, in 1996, Scott Hillam built a Rotax 670 (click for view) for three different Rans S-9s. One for Paul Ordini (The Great Ordini, air show pilot), one for himself, and one for Johncy Walden, who sold his to Paul Wells of BlueMax ULs, who sold it to Bob Widmer, me. Scott sold his to someone that bought the farm when his engine quit due to fiberglass in the fuel system. He tried to make it to the field and hit some trees, or something, a real bad situation.
I have talked at length to both Scott, and to Paul Ordini once, and to Johncy who build my S-9 (CLICK). When I bought it, it had the expansion chamber that Scott Hillam had made, and it also had MRD mechanical fuel injection (CLICK). Apparently, Johncy had experimented with carbs in various configurations to no avail. He must have been working with Scott and came up with the MRD setup. To this day, Paul Ordini is using the MRD FI, and says it works just great as long as you don't pull the throttle into the danger area which is defined as about 2/3 throttle with a light load on the engine because it goes rather lean, and about four to ten seconds later it melts a piston. At least that is what happened to my plane three times now.
In my attempt to iron out the fuel and incuction problems, it is one of the main reasons Johncy sold his immaculate S-9 in the first place, I made several modifications all at the same time. Not really a smart idea. This move can really mess up your program to use a 2 stroke engine that is not familiar to most flyers. Before I start with what I have come up with as a workable engine, let me give a few comments about what the 670 is, and how it differs from both the 582, and the 618.
Ski Doo snowmobiles began using the 670 in their faster and more powerful sleds in the early 1990s. It is similar in many ways to the 618, which has RAVE valves. This stands for Rotax Automatic Variable Exhaust. What that means is that the exhaust port height can be varied, or raised and lowered by a diaphragm system operated by exhaust port pressure opposing a spring. They raise the ports at around 4500 or so RPM, but like the logo says, they are variable depending on the setting of the spring. They may raise at 4000, or at 6500 depending.
Other than that, the 670 is probably the meanest, toughest of the rotary valve engines that are similar to the 582. One other main difference is that the output shaft is 17mm longer, and has a larger shaft than that of the 582/618. The "C" gearbox will bolt up to these engines (CLICK) with some modifications. Other than that, and the fact that it has a single ignition, those are the main differences. Instead of 65 HP, the 670 will eaisly crank out over 100+ HP in the same size package. It looks like a 582 with a couple strange round things over the exhaust ports; the RAVE valves.
My engine uses the JPL starter. Weight is very important to aircraft. My S-9 is really heavy at over 500 lbs. with fuel and water and chute. For some reason, Scott did his development work using a 2.62 to one "C" gearbox (the "C" box is necessary for the 670 power), but somehow, Paul Ordini ended up with a 3 to one gearbox on his. Paul swore to me that I needed to simply dump the 2.62, and go with the 3 to one. He said he uses a 72" Warp drive with square tips on his 350 pound screamer. Since mine is heavier, I chose a 70 inch, taper tip, Warp three blade and a 3:1 "C" gearbox. Looking back at this point, I wish I had kept the 2:62 as my tip speed would have been higher and prop efficency would be better, but this is before I learned about taper tip Warpdrive props. See below.
I can't tell you how many experiments, tests, adjustments, rejetting, and fiddling I have done over the last two years to come up with what I have now.
Here is what work
I met Rob Rayford of R&D Aerosports, who is one of the few suppliers of these 670s, and introduced me to them at Sun-N-Fun. For some reason, not many flyers listen to him, because I don't think they believe he knows what works. I visited his place in late May of '03, and he does have it together. He races snowmobiles when he can't fly in Wisconsin. You should see the trophys! Rob built a dyno tested pipe for me (CLICK), and it made enormous torque at 6500 RPM. Having worked with my 670 with Scott's Hillam's pipe that came with the plane, I realized the engine was begging to be run at a higher RPM, so I modified Rob's pipe in the center section to raise the RPM, and slightly reduced the power band. This pipe along with the dual 44mm Mikuni Super BN carbs made some awsome power. My problem was the 2.62 to one gearbox would cavitate the origional two blade, 71 inch square tip to the point that it was starting to crack the aluminum hubs like the 912-S prop recall was doing. A three blade is so much smoother, but the 2.62 box would not spool up the engine with enough pitch to fly fast (120 MPH), so I installed the 3 to one gears and ended that mess. Later I came to realize that likely the 2.62 gearbox may just be the best by using a higher twist wooden two blade. To late now.
Now the engine runs right up to 7000 RPM which is 900 RPM under redline for the 670, and the pipe causes it to go rich (that means the pipe is past it's useful power RPM), and acts like a rev limiter. All tuned pipes have upper and lower operating limits. It is not uncommon for race snowmobiles of this displacement to crank out 150 HP at 9000+ RPM and stay together a fairly long time.
I use two carburetors that are 44mm, and the (factory Rans setup (CLICK) ) uses two 38mm Super BNs. The 670 likes a bunch more air, but the 38s will do if you jet them for the higher displacement engine and power.
One issue I want to address is the piston silica content. Under the dome of the piston you will notice a letter with a circle around it. If it has a "B" in the circle, don't use it. It must have a designation "D" which is a much higher silica content piston that is used in the '96 and up engines. This higher content will drastically reduce the likelyhood of a seisure if the engine happens to run hot. I guess they learned about this when they started racing snowmobles on grass in the summertime.
The next issue is one that I think is is virtually a necessity. Rob of R&D tried to tell me that these engines work better with a clutch like a RK400 by Air Tech, or similar. Remember the crankshaft is not the same as the 582, 532, or 618. As mentioned above, those engines have a different taper, and the 670 has a longer shaft for a bigger snowmobile drive clutch. The RK400, or equivilant such as the one Rob uses on his MV gearboxes can be modified by Rob to fit the 670 shaft. Spend the extra $500 - $600 for this clutch, and you won't have to worry about all of your guage needles falling off from rough idle below 2100 RPM.
Now that you are using a clutch, you will need a starter for a 618 such as the GPL which has a larger motor option for the 618. The wire terminal bolt comes out the end of the motor, rather than the side. The other option is to use the manual pull start.
The prop. As mentioned above, the 70 inch, taper tip three blade Warp Drive is best. I think this is ideal for most aplications using the 3.0 to one gearbox. Two blades tend to go faster, but three blades tend to pull harder. My results were very good. I could curise at 95 MPH at 5600 RPM, 100 at 5700, 115 at 6200, and 120 max at 6950 RPM. The climb was a steady 1950 on a good day. Since I want to keep my operating RPMs fairly constant while doing aerobatics, I had my Warp Drive blade tips tapered. The tips are thin, and they stall, and allow the RPM to stay the same while the airplane slows down in the verticle, therefore the engine stays in the power band.
The carburetors. After a lot of trial and error, the carb jetting is correct on the dual 44mm carbs. There are six adjustments on the Mikuni 44mm Super BN.
1. Pop-off pressure. I used the 1.5mm needle and seat with the 95 gram spring. The "Pop-off" pressure is 30 psi measured according to the manual.
2. Low speed mixture adjustment. 1 1/4 turns out.
3. Low speed jet. .775mm, or what they call a 77.5
4. High speed Jet. 1.40mm, or what they call a 140
5. High speed mixture. Two and one half turns out
6. Idle speed with a clutch is around 1400 RPM
While on the subject of carburetors, the carbs MUST be installed in the right position. This means that the throttle shafts are paralell, and require a link rod and joints to open the butterflies. Do not place the throttle shafts end to end, here is the reason. The diaphrams are affected by G forces. G force should only be applied with the diaphrams on edge, and not flat (horizontal). Why, you ask? Because if they are mounted flat, when you pull up, the engine will go lean, and when you push up (inverted) the engine will go very rich.
You will find that when the carburetors are installed, there is not enough room for the fuel pumps that are installed on the carbs from the factory. You must purchace fuel pump block off plates, remove the fuel pump, and install the block off plate. This means that you will now need to obtain a dual outlet fuel pump to feed both carbs that now have no pumps. In addition, you need to make a plate that will actually connect the two carbs together at the air filter end to keep them from vibrating in opposition to each other.
These settings had the EGTs both at between 1000 degrees loaded, or 1150 at cruise and decent. Start rich and work twoard lean.
This engine likes to run between 130 and 180 degrees water temp, so provide plenty of radiator air flow.
How much does building a Rotax 670 cost? My answer is; less than a 582 with all the same equipment. Here again is a list of components needed to build a good, strong 670:
The basic engine can cost from $600 to around $3000 depending upon the condition.
A 3 to 1 Rotax "C" gearbox or equivlant
The inverted carb setup.
An expansion chamber.
A strong starter.
A clutch for the C gearbox.
A Warp Drive Prop.
By the way, the Rotax 583 is a great engine option too, and is the RAVE valve cousin to the weaker 582. It belts out about 90 HP where the 670 is good for about 115 HP.
What should you expect from your Rans S-9 0r S-10 with this engine? Expect to get a kick in the ass when you hit the throttle. I don't care if you are in the air, or on the ground this engine cranks it out if it is set up correctly. I think it is the perfect engine at this time for a well built plane to perform aerobatics. Expect to burn a lot of fuel if you have the throttle open and are exceeding 115 MPH. About 10 gallons per hour is what I estimate. At cruise you burn just about the same as a 582. Fuel and air make horsepower. I reccomend Penzoil Air Cooled 2 Cycle Engine oil, Lucas semi-synthetic, Redline, or just about any oil that is of similar quality. What I use is Lucas simi-synthetic. In addition, I burn 100 octane av gas on reccomendation from someone with Rotax. You should be the envy of all of your aerobatic friends because they are going to be spending about ten times what you are for their plane with their IO-360. Expect the sound of your annoying little "model airplane" that you sit in to drive the big boys nuts when you can do a proper lomcov`ak on ten bucks worth of gas.