In The Beginning, There Was Rotax

Rotax engine technology


ARTICLE DATE: November 1993

In the very early days of ultralight and very-light aviation, there were many powerplant companies competing in the U.S. market, including names like Cuyuna, Kawasaki, and Konig, which have now passed into virtual obscurity. There was, and is, a good reason for the disappearance of so many light engine manufacturers from the scene. That reason: Rotax.

In recent years Rotax has come to be the dominant name in ultralight and very light aviation. Some argue that without Rotax, sport aviation could not have achieved its present status. They may well be right about that.

But how has Rotax achieved such a prominent place in the industry? Simply by offering a complete line of lightweight, reliable engines for everything from the smallest, slowest ultralights to 150-mph fiberglass homebuilts.

The blueprint for their dominance of the market is clear: offer high quality products, make parts readily available, and sell the product at a reasonable price. That kind of formula spells success in any ball game. Rotax is one company which isn’t content to rest on its laurels, however.

The firm, in fact, has just introduced to the United States one brand new design: the much-rumored high-powered two-stroke 618, with RAVE (adjustable variable exhaust); and it is ready to release another in the near future, the long-awaited turbocharged version of the 912, the 914. These two new engines are aimed at taking advantage of homebuilders’ desire for more power.

The two, however, represent two distinctly different approaches to accomplishing that goal: With the 914, Rotax has sought to improve upon the performance of the 912 by incorporating a turbocharger into its design; and in the 618, they’ve attempted to offer 912-like power at a 582-like price by making some improvements in two-stroke design.


EDITOR: If there is one thing that Rotax did right and did well, it was the Rotax “9” series four stoke engines.

The root cause of all the excitement over the release of the 914 is the phenomenal and continued success of the 912 model. For those of you not familiar with the engine, the 912 was Rotax’s first attempt at a four-stroke engine.

They seem to have gotten it exactly right. With the 912, Rotax took proven four-stroke design and ran with it. The 912 is a conventional four-cylinder opposed-configuration engine of 1211 cc. But the engine design incorporates some rather unusual approaches as well.

For instance, it combines air and liquid cooling (the cylinders are air-cooled and the heads are liquid-cooled) for more efficient thermal discharge. At 5800 rpm, the engine produces 80 hp. At 5000 rpm it turns out 65 hp and uses less than 4 gallons of fuel per hour.

914 engine

When you combine this kind of solid performance with a four-cylinder, four-stroke engine (like those many long-time pilots are used to flying behind), it’s no surprise that the 912 has taken the world light aircraft market by storm.

In fact, Rotax has sold more than 600 of the units in North America in little more than a year. The engine’s only real downside is its relatively high price of about $8500. Of course, this is only the acquisition price.

With a factory TBO of 1000 hours (expected to go up to 1200 by next spring) and very efficient fuel usage (no oil injection required), Rotax believes the 912 will pay for itself over time with better fuel efficiency and reliability than any available two-strokes.

Though it is nearly identical in most regards to the 912, the 914, for all the performance it offers, might as well be a brand new engine. Like the 912, the 914 is a four-cylinder, four-stroke engine which utilizes both liquid and air for cooling. The 914 features dual electronic CDI and dry sump forced lubrication.

In essence it, like the 912, is a sophisticated, modernized aircraft-type engine. The 914 differs from the 912 in that it is turbocharged. The turbo unit is made by Garrett, an industry leader in the production of aviation turbochargers.

The Garrett name alone should be reassuring to potential purchasers who might be a little anxious about the use of a turbocharger on a small engine like the 912. As one does on any engine, the turbocharger on the 914, in very basic terms, boosts the performance of the plant by increasing the engine’s air intake.

It does this by harnessing the exhaust gases to spin an impeller which in turn produces an increased air flow into the intake side. An engine getting more air creates more power than a conventionally aspirated one.

In order to prevent accidental over-boost, the turbo is operated by a computer-controlled servo to automatically adjust the amount of boost the engine gets, so turbocharger control need be no concern of the pilot. Whether operating at sea level or at 10,000 ft. msl, the engine is designed to put out 100 hp at rated horsepower.

Of course, at altitudes above 10,000 feet, performance of the engine will decrease. Still, it will surely put out very usable performance figures even well above the 10,000-foot level.

And the ability to fly with more power at higher altitudes will surely be a welcome benefit to the many pilots whose high performance very light aircraft could take advantage of higher cruising altitudes.

Rotax’s Director of Technical Development, Arnie Lepp, believes the 914 represents a real step up from the 912, citing increased performance with an increase in weight of no more than 10 pounds. The 914, Lepp says, with the “same amount of iron and aluminum (as the 912) is putting out 20 percent more power.” That’s real achievement.

What customers will the 914 appeal to? It will likely be a long list. The engine is bound to look awfully good to any builder seeking an engine which promises high speeds and increased power but which doesn’t penalize him with a lot of extra weight.

Builders of everything from Kitfox Vixens to Pulsars, from Murphy Renegades to Ultravia Pelicans, should take a keen interest in the 914. The 914 is projected to cost about $12,000 by the time it is released early next year.

Rotax 618 914 turbo engine

LEFT: The Rotax 618 is a liquid-cooled two-stroke engine which will put out 75-hp thanks in large part to the engine’s new RAVE technology.
RIGHT: The 914 is a turbocharged version of Rotax’s incredibly popular liquid/air-cooled four-stroke 912. The company claims the 914 will deliver 100-hp up to 10,000 feet.

While that kind of sticker price will probably keep many interested builders from exploring the possibilities the 914 has to offer, many others will doubtless be willing to part with a little more of their hard earned cash for the chance to incorporate this little screamer into their homebuilt projects.


At a time when airplanes are getting fatter and fatter, pilots are looking for an affordable engine which provides more power without adding a lot of extra weight. The 618 may bet he answer to their prayers.

Though it looks much like the company’s legendary 582, the 618 is a really a whole new product, one which uses brand new ways to produce more power than previous two-stroke aircraft engines.

The 618’s major claim to fame is that it’s powerful, producing nearly 75 horsepower at full rated power. That’s 10 horsepower more than the 582. Thanks to RAVE it does that without adding much weight or displacement.

But the 618 incorporates a brand new feature intended to help it produce more horsepower at lower fuel consumption. This revolution is RAVE (Rotax Adjustable Variable Exhaust). The idea behind RAVE is a simple but ingenious one which, according to Lepp, was developed for use in motorcycles or racing many years ago.

Whereas a four-stroke engine uses a cam to time the opening and closing of its valves, a two-stroke engine’s timing is determined by its porting. On a standard two-stroke engine, the exhaust timing determines the torque curve.

618 engine

Depending on how the timing is set, you get maximum performance in only one part of the engine’s power curve. You could get good torque at high power settings, or you could get good torque somewhere in the middle, but you’re not going to get maximum performance at all power settings.

Not so with the RAVE system, according to Lepp. RAVE is designed to automatically adjust the exhaust setting to coincide with the engine’s power to provide the best power and fuel consumption at any given rpm range. If it works out as planned, RAVE will surely represent a quantum leap in two-stroke development.

Moreover, the 618 features several other improvements in addition to RAVE. To handle the increased power, it has a new, heavier duty crank and a beefier flywheel. Also, to allow for faster engine warm up, the 618 incorporates a redesigned head and thermostat bypass system.

The weight penalty for all this? A mere ten pounds. The 618 has been extensively tested in several airframes, including a SeaRay, Kitfox, and Chinook, and it has reportedly performed very well in all three.

Total Weights
Standard Components
Engine 68.3 pounds
2 Carbs 4.0 pounds
Intake Silencer 2.4 pounds
Electric Starter 7.7 pounds
"C" Gearbox 17.6 pounds
Exhaust System 13.2 pounds
Total 113.2 pounds
Optional Items
"E" Gearbox 24.7 pounds
Generator 230 W DC 2.4 pounds
2 Radiators (small) 4.2 pounds
2 Radiators (big) 4.9 pounds
Oil Tank 3.5 pounds
HAC kit 0.4 pounds

I spoke with Patrick Rediker, Chief Pilot for SkyStar Aircraft, one of the companies which received pre-production copies of the 618 for testing. Rediker told me that for the test period, SkyStar replaced the Rotax 582 in the factory Kitfox Model IV demonstrator with the 618 and the result was impressive.

In Patrick’s words: “The airplane nearly levitated itself!” He also told me that during the ten hours he flew the 618-powered Kitfox, he often found himself heading back to the field to shoot touch and goes…just to experience the sensation of the extra power.

According to Rediker, because of its extra boost, SkyStar definitely plans to make room for the 618 under the pretty round cowls of their Kitfoxes. The best part of the deal might be the attractive price of the unit. At only $5500, including C-type gearbox, carburetors, electric starter, and exhaust system, the engine is sure to be a hot seller.

Rotax, in fact, sold out of the engine before Oshkosh was half over! The 618 is back in stock and is now available from your nearest Rotax Service Center. Adjustable Variable Exhaust (see photo) which improves upon the fuel efficiency of the motor by varying the exhaust timing without the back pressure influence.

The engine comes stock with oil injection, dual Ducati CDI ignition, rotary valve inlet, thermostat, dual carburetors, intake silencer, electric starter, Type “C” gearbox, and exhaust system.

An option which will be available this fall is the Type “E” gearbox, which incorporates an electric starter, adding 42 mm to the engine length on the prop-shaft side but saving you approximately 5 inches on the mag side. This also allows for your pull starter to remain in place. Approximate retail price of the Rotax 618 is $5398.

Rotax 618 Rotax 914 engine


rotax c gearbox RK 400 clutch assembly

New from CPS, and available for the first time is an all-new retrofitable clutch for Rotax Model “C” and “E” gear boxes. A simple bolt-on conversion, the RK400 engages the propeller only above 2500 rpm through the use of a spring-loaded centrifugal clutch mechanism. Once engaged, the propeller responds in the normal manner at higher rpms.

What are the advantages of the RK400?

Starting is accomplished without prop load, which means vastly improved cranking ease. If you have tried to start the new Ducati Ignition engines with a short, inside-the-cockpit pull-rope, you know how frustrating this can be.

Ducati engines must have at least 300 rpm to spark. With the RK400 Clutch installed, all you are cranking against is simple piston compression. Recoil starting effort is greatly reduced.

No prop load at idle means a smooth quiet idle to as low as 1000 rpm. Gone are the days of high 2000+ rpm rough idles which burn excessive fuel, shakey airframes, and annoyingly extended landing approaches and run-outs.

With the RK400 Clutch installed, the prop free-wheels below 2500 rpm. Larger diameter, high-inertia props run smoothly, even at the higher 3.4 to 1 and 4.0 to 1 ratios. Because the prop is no longer “hard-mounted” to the engine, wear and shock loads to crankshaft, gear box, and other drive train components is greatly reduced.

Also, with the RK400, there’s no spinning propeller during starting and idling, adding a measure of safety. The RK400 Clutch takes the place of the Flywheel #1 (Rotax #958-980) and the Rubber Hardy Disk #6 (Rotax #958-961).

The new Hub slides on the Crank taper and secures with the original crank end bolt and washer. The outer Clutch ring bolts to the existing Coup-ling Flange #4 (Rotax 958-970) with three alien bolts provided with the conversion kit.

Reinstall the gear box as usual and the installation is complete. The RK400 is precision-machined from solid steel, and Cad II finish-plated for corrosion resistance. The four clutch shoes are retracted by dual circular springs for rpms below 2500.

As the rpm increases, the centrifugal force allows the shoes to engage the outer clutch ring, starting the prop to turn. The higher the rpm, the harder the shoes engage the outer ring, preventing slippage.

Shoes, springs, machined parts, and hardware can be ordered as replacement parts. The RK400 Clutch weighs seven pounds total, while replacing three pounds of factory parts, for a total installed weight of only four pounds additional.

The RK400 Clutch Kit sells for $475 and is available from: California Power Systems, Inc., the exclusive US distributor.

Article Name
In The Beginning, There Was Rotax
Rotax IS the leader in ultralight engine technology. Looking for a two stroke or four stroke for your gyrocopter, consider the Rotax. Expensive - YES - Reliable - YES, whats your requirements?

Be the first to comment on "In The Beginning, There Was Rotax"

Leave a comment

Your email address will not be published.