After-market Turbocharging Considerations

May 2000

This month’s ShopTalk will be the final one in a series on turbocharged Mooney aircraft. I admit that some notable models have been omitted because of the unavailability of parts or support by the manufacturer. The Mooney Mustang and Bullet are two that come to mind.

I have tried to cover the more popular turbocharged Mooneys flying today and we wrap up the series with turbo normalizing. Currently, there are two normalizing kits available to Mooney owners. The original kit was produced by Rajay during the 1960s and 70s. New on the market is the M-20 Turbos kit, which was certified just over a year ago. This article will first discuss the basic idea and then the pros and cons of this engine modification.

Normalization, in this context, means to operate the engine at the equivalent of sea level pressure and standard temperature, i.e. 29.92” Hg at 15°C. This, of course, is what we refer to as sea level density altitude or STP – standard temperature & pressure. At this altitude, an engine will be able to produce 100% of its rated power. Indeed, it is at or near STP that most engines’ horsepower is measured. A normalizing turbocharger will be able to provide an engine with that equivalent pressure well above an operating altitude of 10,000 feet. Since most aircraft are cruised and cruise climbed at less than 80% power, needed engine performance may be available up to 18,000 feet or more.

With this range of altitudes available at usable power levels, a pilot has more options when deciding to top weather and terrain rather than skirt around them. Compared to a normally aspirated aircraft, operations at high-density airports are not as critical because full rated engine power is available.

The first normalization kits were developed by Rajay Manufacturing in the early 1960s with STCs for dozens of aircraft. They also produced turbochargers for the automotive aftermarket industry. The company floundered in the 1980s and ultimately failed. Airesearch purchased the assets, manufacturing and marketing a small subset of the original Rajay line. Recently, Consolidated Fuels purchased the Rajay assets from Airesearch, building replacement parts on an as-needed basis.

With respect to Mooneys, the original Rajay STC was for models M20A through J. The M20E & F kits (the most popular) mounted the turbocharger on the right side of the engine, next to the oil sump and the oil filter was relocated to the left firewall. Utilizing a completely new exhaust system, a vernier cockpit control allowed the pilot to manually adjust the wastegate to achieve a maximum manifold pressure of 27” Hg (through 20,000 feet). Turbo Lycoming fuel nozzles replaced original nozzles and upper deck manifold pressure lines connected the nozzles ensuring proper fuel atomization during turbocharger operation. More fuel pressure was required and subsequently the engine driven pump was replaced. Modification to the fuel pump vent system was also needed. Between the vacuum pump and its drive pad, a turbo oil scavenge pump was installed. This installation was not intercooled but still required a completely new exhaust system. All this made for a very crowded engine compartment.

To not adversely affect the longevity and reliability of the IO-360 engine certain operational limitations are imposed. Turbocharger boost should not be applied until the manifold pressure drops below 27” Hg. At 27” maximum MP, maximum continuous RPM is 2500 with higher (up to 2700) limited to three minutes. At this power, CHT will be 350C to 450C. Since the wastegate control is manual, it is possible to over boost the engine. The pilot must carefully adjust the wastegate control and monitor the manifold pressure at high power settings.

New Rajay turbocharging kits are not being offered for sale but used kits may occasionally become available. Buyer beware! The General Aviation Revitalization Act of 1996 affects the installation of a used Rajay kit as well as any STC modification of your aircraft. The Act stipulates that written permission must be obtained from the STC holder to install their product. When your mechanic submits the Aircraft Alteration Form (337) to the local FSDO, the FAA inspector should ask to see the STC release from the STC holder. Without the release, the 337 form is not accepted and your aircraft no longer meets its Type Design. Your aircraft is no longer legally airworthy … insurance void!

In May of 1998, the FAA issued to M-20 Turbos a new STC for an improved turbo normalizing kit for all 200HP Mooneys (IO-360). The developer and owner of M-20 Turbos, Bill Sandman utilized his M20J for the research, development, and certification of this kit. The kit uses a standard Rajay turbocharger located under the engine near the firewall. A fixed cowl flap is installed at the turbo location. This provides airflow through the engine compartment from over the cylinders down past the turbo, exiting at the bottom rear. Unlike the original Rajay turbo kit, M-20 Turbos provides an intercooler and new engine baffling. The baffling enables the engine to operate in the flight levels without overheating. The intercooler allows for efficient engine operation by lowering the compressor discharge temperature. Unlike the Rajay system, M-20 Turbos utilizes a fixed exhaust wastegate. By operating the throttle (not adjusting a vernier control) the pilot controls MP up to 30” Hg. maximum. Close attention must be paid as the engine can be over-boosted at lower altitudes. An intake system overboost valve will limit MP to 32.5” Hg.

M-20 Turbos did extensive flight-testing using computerized monitoring of engine parameters. Under extreme conditions of high altitude and power settings, turbine inlet and compressor discharge temperature, cylinder head, and oil temperature and predetonation conditions were measured to determine critical altitude as 20,000 feet and the service ceiling of 22,000 feet. This testing proved the engine would run cool enough to allow the mixture to be set to only 25°C rich of peak TIT at cruise power settings. The prototype currently has over 1000 hours of flight with no unusual problems.

M-20 Turbos estimates 40 to 50 man-hours to install their kit on a well maintained 201 or MSE aircraft. More time is needed for the E or F model as 201 style cowlings must be installed. Items not included, but needed for certification are a TIT gauge and a fuel flow meter. As of this date, the quoted price for the kit is $23,389. Aside from the flight-testing, this kit does not utilize new technology, but provides a good solution to turbo normalizing and addresses the problems of high altitude engine cooling.

Neither turbo kit utilizes pressurized magnetos, so high altitude ignition flashover could be experienced on poorly maintained or high time magnetos. The Lycoming IO-360 engine is not stressed or designed (in my opinion) to be boosted to over 30” Hg. of manifold pressure. A turbo normalized IO-360, when operated properly, should last as long as one normally aspirated. When converting your aircraft to high altitude operation, pilot and passenger oxygen must be provided. Another consideration for proper aircraft operation is the avoidance of engine shock-cooling during descent. Adequate flight planning is necessary; speed brakes are a valuable tool.

Like other aftermarket add-ons, resale value will lag behind your investment. However, improved performance and utilization are the main rewards of turbocharging. Neither of these kits will make your 200 HP Mooney into a TLS, but your performance envelope will increase dramatically as will your flying options and enjoyment.

The question to turbo normalize or not comes down to, “Do I need to use altitude to avoid mountains, convective weather, icing or gain true airspeed? Or do I need to operate from high-density altitude airports?” Here in Evanston, Wyoming (EVW) the field elevation is 7,163. During the summer, the density altitude often exceeds 10,000 feet. A normally aspirated engine will draw only 21” Hg manifold pressure. For a well-tuned new IO-360, you will have only 140 HP available to accelerate to 82 knots TAS (70 KTS IAS). How do you think this affects your useful load and climb performance? Turbocharge your Mooney and gain back 60 HP (a 42% increase). Now you won’t have to leave your significant other and baggage when you travel to Evanston.

The original Rajay turbo kit was ahead of its time in the 60s and 70s. With all the computer testing and the intercooler installation, I’m sure that the M-20 Turbos kit is setting a new standard for turbo normalizing in the year 2000.

As always, if you have a question about this article, you may contact me at my aircraft repair shop, 307-789-6866 or via e-mail. Until the next ShopTalk, enjoy flying your Mooney.