The Rotax 912 is one of the most modern aircraft engines available today, but it’s not exactly new. I remember attending one of the first schools for the then-new 912 engine in 1989—that’s more than 15 years ago!
Since then, Rotax has made many improvements. Factory testing cannot anticipate the special demands of every engine application, some of which are quite unique. Today, thousands of Rotax 912, 912S, and 914 engines are flying in hundreds of different aircraft types. Experience with the engine, and its varied installations, has pointed the way to many improvements for both the engines and their installation and operating procedures. These changes have resulted in significant refinements and increased time between overhauls (TBO) for all three 912 engines.
Here are a few tips I have picked up over the past 15 years of working with these engines. Starting with a good installation will get you on your way toward years of trouble-free flying, so it’s good to get it right from the beginning.
Which 912 Should You Use?
First, let’s talk about the differences between the various versions of the 9-series engines. I receive a lot of questions from people who are ready to make their engine installation but are having a difficult time deciding which engine to install.
The 912S (100-hp) and 912F (81-hp) engines are certificated to FAR Part 33 requirements; standard or primary category certificated airframes require these more expensive engines to meet FAA regulations.
The 912ULS is not certificated, and this is the 912 engine most owners use. Experimental aircraft use this uncertificated version, and so do the new factory-built special light-sport aircraft (S-LSA). S-LSA can use the uncertificated (UL) engine because the Rotax 912UL and 912ULS comply with the new ASTM LSA engine design standard. This will save buyers several thousands of dollars and give them more freedom in maintenance. The two engines are almost identical. (It is unlikely any S-LSA will use the 914 engine as it requires a constant-speed propeller to make full use of its potential, which is not allowed under the LSA rule.)
Some people ponder whether to install the 81-hp 912UL or the 100-hp 912ULS; others drift between the 912ULS and the turbocharged 115-hp 914UL. There are only a few pounds difference in weight between the 912UL and the 912ULS, and they are virtually the same size, so why would anyone buy the 81-hp version? Few do, but there are some good reasons to choose the 81-hp engine. First, the lower compression ratio of the 912UL allows it to operate safely on regular 87-octane autogas, while the 912ULS requires 91-octane or higher. (Recall, the United States uses MON plus RON divided by 2 to determine octane ratings.) This gives those operating the 81-hp version a little more flexibility, especially when operating outside of Europe and North America where premium fuel is not always easy to find and fuel octane ratings can sometimes be questionable. Regular 87-octane fuel is also less expensive as is the 912UL engine, to the tune of about $1,500 (U.S.). Good, low-time, used 912UL engines can often be found in the $7,000 to $9,000 dollar range, while used 912ULS engines are just about impossible to find.
All 9-series engines will run on 100LL (low lead) avgas, the pros and cons of which I will discuss in a later article. For some aircraft, including many trikes, the 912ULS is just too powerful, producing an excessive amount of torque on takeoff and dangerous climb angles. For these lighter aircraft, the 912UL is the perfect choice. The 912ULS also produces more heat and thus may require additional cooling.
The 912ULS develops its additional 19 hp through a combination of increased displacement with a larger cylinder bore, increased compression, and a more aggressive camshaft.
The 914UL is actually a turbocharged version of the 81-hp 912UL. (No, you can’t turbocharge the 912ULS because its higher compression ratio is not suitable for that.) The 914UL offers excellent high-altitude performance with 115 hp available at 10,000 feet and 100 hp available up to approximately 16,000 feet. To make full use of the 914, you need to add a constant-speed propeller (and its added expense). As you climb, the 914’s computer automatically adjusts the turbo waste gate to maintain power, so the prop pitch must be increased to avoid over revving.
For those who often fly at high altitudes the 914UL is a marvelous engine, but if you spend most of your time at low altitudes, then the less complex 912ULS may be a better choice.
If you’re installing an older engine, no matter which one, make sure all required service bulletins have been complied with. You can access the most current versions of all Rotax manuals and service information at www.flyrotax.com
This article complements the Rotax operator’s and installation manuals. Those who are engineering all-new installations and not using an OEM-supplied mounting kit will need further information.
Understanding the Required Instrument Markings and Engine Limits:
To get started, you will need an accurate tachometer with a redline at 5800 rpm and a yellow arc from 5500 to 5800 rpm. These are the minimum markings; all 9-series engines have a maximum continuous speed of 5500 rpm, with a maximum operating speed of 5800 rpm providing takeoff power for up to five minutes. If you want to get fancy, you can have a redline at 1400 rpm, which is the absolute minimum allowable idle speed, followed by a yellow arc from 1400 to 1800 rpm, followed by a green arc from 1800 to 5500 rpm.
When idling below 1800 rpm, the normal oscillating torsional loads on the gearbox are increased, so minimizing the time spent idling below 1800 rpm will increase the life of your gearbox. The weight of your propeller blades will determine the speed at which you should be idling. Some of the lightweight, composite props, like the German two-blade Neuform, will idle well down to 1400 rpm, while heavier, three-blade props like the rugged Warp Drive require higher minimum idle speeds in the 1600 rpm to 1800 rpm range. Note: Do not set your idle speed above 1800 rpm or your starting carburetor circuit (choke) won’t work well.
It’s not a bad idea to check the accuracy of your tachometer with a handheld optical tach before beginning this marking procedure. Many tachometers are off by 100 to 300 rpm. Some can be easily adjusted in the field.
Synchronizing the carburetors using a dual vacuum-gauge kit, like the one pictured on page 41, is an important part of setting up any of the dual-carb, 9-series engines and achieving a smooth idle. (See the two-part article in the April and May 2004 issues EAA Sport Pilot & Light-Sport Aircraft for a detailed description of the carb synchronizing procedure.)
You also need to monitor your oil pressure and temperature. Rotax suggests that the ideal oil temperature range is 190°F to 230°F. Installations that are capable of keeping the oil temperature cooler than 230°F, under the most extreme conditions, will normally run too cool when the ambient temperature dips in the winter months. For this reason, most 912s can benefit from the installation of an external oil thermostat. The Permacool thermostat (Lockwood part number OILCOOLTH) has been working well in this application and sells for just $60. It comes equipped with a 180°F thermostat, so on cold days your oil temperature will be maintained at 180°F or warmer, which seems to be sufficient. Based on our positive experiences with this thermostat, I would suggest marking your oil temperature with a green arc from 180°F to 230°F, followed by a yellow arc from 230°F to 265°F, and a redline at 266°F. The oil should reach 120°F before applying takeoff power. This lower limit is often ignored but should be noted with a placard, a note in your pre-takeoff checklist, or by a half-width green arc from 120°F to 180°F.
The oil pressure gauge on the 9-series engines should be marked with a green arc from 29 psi to 73 psi. This is what Rotax recommends as the normal safe range. The absolute maximum is 100 psi for a short time following a cold-weather start; the absolute minimum is 12 psi (below 3500 rpm), so both of these warrant a redline.
A healthy 912 engine will produce oil pressure between 45 psi and 60 psi once the engine has warmed up. If you are indicating more than 60 psi or less than 45 psi (on a warm engine), this is cause for concern and could indicate an erroneous gauge reading or a restriction in the oil line feeding the oil pump.
You can check the accuracy of your oil and cylinder temp gauges by removing the probes and immersing them in a pan of boiling water. The Rotax-supplied VDO cylinder head and oil temperature probes are interchangeable.
Because you get two cylinder head probes and only need to monitor the hotter of the two, you will have a spare. You will not lose any coolant when removing the cylinder head temp probes.
Many of the digital engine information systems (EIS) will allow you to monitor both cylinder heads simultaneously, making it easy to figure out which one is hotter. If you only have one gauge, the airframe manufacturer should be able to tell you which cylinder head runs higher, otherwise you will have to make back-to-back flights switching the probes on the ground to determine which one is hotter.
The type of coolant being used will determine the maximum cylinder-head temperature, which is measured at the hotter of the two heads supplied with VDO senders. Those using a conventional 50/50 mix of coolant and distilled water may operate with the following maximum redlines:
.9 bar/13 psi pressure caps will yield a maximum redline of 239°F.
1.2 bar/17.5 psi pressure caps will yield a maximum redline of 248°F.
Engine owners who use Evans NPG+ waterless coolant can mark their cylinder head temp gauge with a maximum redline of 275°F for the 912ULS and 914ULS, and 300°F for the 81-hp 912UL. (See “Keeping Cool” in the January 2005 issue of EAA Sport Pilot for more information on Evans coolant.)
If your cooling system can prevent your cylinder head temps from exceeding 240°F under the worst conditions, then stick with the conventional 50/50 coolant water mix. If not, then you will need to either improve your cooling system or switch to Evans NPG+ waterless coolant. Expect to see at least a 10-degree temperature increase with the Evans, in part because of its higher viscosity. The higher boiling point of the Evans coolant is what allows the greater limits.
One More Essential Instrument
The last engine-related instrument that I feel is absolutely essential is a simple mechanical hour meter. Include this in your panel even if you are using an all-in-one EIS. Almost all EIS have the ability to track engine hours, but like anything electronic they can malfunction; you do not want to risk losing an accurate record of your engine time.
It is best to wire the hour meter to a pressure switch that can be mounted in the oil pump on the 912UL and 912ULS. These engines have an unused, threaded hole adjacent to the oil pump and the oil pressure sending unit that is used to supply the turbocharger on the 914 with its oil supply.
You will need an adapter because the available pressure switches are ¼-inch NPT and the oil pump threads are metric; both are available through Lockwood Aviation Supply and RANS.
We have discussed the minimum engine gauges needed to begin a Rotax 9-series engine installation and the parameters they display. Once again, these include:
■ a tachometer
■ a cylinder head temp gauge
■ an oil temp gauge
■ an oil pressure gauge
■ an hour meter
Optional gauges that would also be valuable include:
■ a fuel pressure with a green arc from 2.2 psi to 5.8 psi
■ a volt meter
■ a dual exhaust gas temperature (EGT) gauge
Next time, we’ll begin discussing the actual installation.