Operational hints and tips for the M20K

SHOP TALK – January 2000

by Kerry McIntyre


This ShopTalk is the first in a series of articles about the proper operation of your turbocharged Mooney. We will start with the M20K (231 & 252) series. The second article will be on the TLS (M20M) and the final article will be on aftermarket turbo-normalized M20 aircraft.

A Little History:

In 1979, when the M20K was first introduced, it ushered in new operating procedures far different than previous Mooney aircraft. The M20K used the TSIO-360 engine, a 210 horsepower engine built in many different versions and installed in many aircraft such as the Cessna Skymaster, the Piper Turbo Arrow and the Seneca (II to V). The Mooney versions, in ascending order, are the TSIO-360-GB, -LB and -MB series. All these engines develop well above standard day manifold pressure (29.92") to obtain their full torque and 210 horsepower. On the -GB and -LB, 40" of manifold pressure on a standard day is 100% power, which is 210 brake horsepower. On the -MB (which is intercooled standard from the factory), 36" produces 210 horsepower.

Hot Running Engines on Early Model 231 Airplanes:

All 231 owners who operate -GB and -LB engines know you can't use full throttle without exceeding red-line manifold pressure below 14,500' pressure altitude. This is where the main engine problem is on the 231. When a turbocharger spins, it produces compressed air and as it spins faster it compresses air more. But the more the air is compressed, the hotter it gets. When you deliver hot air to the cylinders it affects atomization of the fuel and raises the cylinder head temperature (CHT) on those cylinders.

Neither the 231 nor the 252 have a density altitude controller, so you must adjust manifold pressure up or down based on actual OAT (compared to standard day conditions) to get exact power settings as published in your POH. Basically, you reduce manifold pressure if the OAT is below standard day at the pressure altitude you are flying or you increase the manifold pressure if OAT is above standard day. Your POH will delineate these corrections. Or, you can purchase a TLS and the engine will do it for you.

The TSIO-360 engines all use an engine driven fuel pump to deliver unmetered fuel to the metering plate which is fixed to the throttle plate. As the throttle is advanced, more metered fuel is delivered to the manifold valve on top of the engine and then distributed to the individual fuel injection nozzles at each intake port. But there is no adjustment to change the rigging on the metering plate at full throttle to allow more fuel to get to the engine to help cool it down. When operating at near red-line manifold pressure, the pilot only has three options to cool down the 231 engine: 1) open the cowl flaps, 2) lower the nose of the airplane (speed up) to allow more air to flow over the cylinders, 3) reduce power.

How to Lower These Temperatures:

Unfortunately, this won't help the high compressor discharge temperatures (CDT). That problem must be solved by changing the engine configuration. The two most common ways to lower CDT is to add an intercooler to the intake system and/or install an automatic wastegate system. This is what Mooney and Teledyne Continental Motors (TCM) did on the -MB engine used in the 252. But for 231 operators, let's look at your options.

Aftermarket Intercoolers:

First, there are a couple of aftermarket intercooler manufacturers to choose from. What will an intercooler installation do for your airplane? All intercoolers basically work the same way. An intercooler is just a radiator for the hot compressed air before it gets to the cylinders. Cooling this induction air is better for the engine, resulting in overall cooler running temperatures.

But be careful! You can't use the original POH power charts if you install an aftermarket intercooler. At the higher power settings, the POH charts may lead you to over-boost the engine, using more horsepower than the engine was approved for. You must use the power (manifold pressure) reduction formula or chart developed by the intercooler manufacturer for correct engine settings. The new power settings take into account ambient temperature and how much cooling the intercooler is delivering. This then provides a corrected manifold pressure to produce the percent of engine power you are trying to achieve.

Many owners with aftermarket intercoolers are confused on this manifold pressure correction formula, Often, a 231 owner, with an intercooler, will not be running the correct power settings and may be over-boosting and ruining their engines.

A negative aspect of an aftermarket intercooler is that it adds a restriction in the intake system. This restriction requires the turbo to spin faster (produce more hot air) to compensate. Don't get me wrong, I'm not against aftermarket intercoolers. They do reduce the CDT (compressor discharge temperatures) going into the engine and that's a good thing.

Automatic Wastegates - Better Than Intercoolers?

A better way to help the engine in the 231 is to lower the compressor turbine speed. A lower turbine speed will result in both lower CDT and lower TIT (turbine inlet temperatures). Lower turbine speeds and inlet temperatures will prolong the life of the turbocharger and all the exhaust components. Lowering the turbo speed also reduces back pressure and temperature in the entire exhaust system, including lower EGTs.

Remember, heat is the enemy of your engine. The fixed wastegate is a compromise on your 231 engine. It does not allow the use of full throttle at takeoff (maximum-metered fuel delivered to the cylinders) below 14,500 feet. Above 14,500 feet you start to lose manifold pressure, even with throttle control full forward, often restricting operation at higher altitudes.

Having the ability to fly at the flight levels gets you to the smooth cool air for those long cross country flights. That is why most turbocharged aircraft are purchased in the first place.

So how do you get the throttle to open more for increased metered fuel flow without over‑boosting while reducing turbine speed? Install a Merlyn automatic wastegate system. The automatic wastegate allows more exhaust gas to bypass the turbo below 14,500, thus reducing the CDT and TIT and allowing the pilot to use more throttle to obtain forty inches of manifold pressure. When you get to 14,500 feet the automatic wastegate is now restricting the bypass and allows the 231 to climb to 19,500 feet before it will no longer maintain 40" of manifold pressure. This allows you to carry the full 210 horsepower to 19,500 feet. It will also allow you to obtain 75% power for cruise at much higher altitudes. With the Merlyn system, at 14,500', the turbo is turning less RPM to maintain the same BHP that the fixed wastegate ran at. Remember lower compressor speed equals lower CDT which leads to more efficient fuel burn and lower CHT. Lower compressor speed also results in lower TIT, EGT and exhaust back pressure.

1,600°F and 111,000 RPM!. If you think about the kind of heat and RPM that poor little turbo is expected to operate under, you would install an automatic wastegate system in a heartbeat. Customers often ask "Should I install an intercooler? I say, "Just install the automatic wastegate." If you want an intercooled engine in your 231, then get an engine that was designed with an intercooler as an integral part: the -MB engine conversion or a 252 Mooney.

Setting Proper Fuel Flows and TIT Values for Cruise:

TCM says that you can run 1700°F TIT for a maximum of 60 seconds to determine peak and 1650°F continuously. Remember, these numbers mean never exceed. Once you level off at your cruising altitude, the TIT will vary from day-to-day at the same altitude depending on air density. Never set your TIT to a specific number.

We have found that a good way to set the proper fuel flow at higher power settings (70% and above) is to watch the manifold pressure gauge. The manifold pressure gauge is more accurate than the TIT. The reason for this is as you get close to peak TIT the TIT needle moves very little and responds slowly. Most engines have a steep initial leaning curve, but as you get close to peak this curve flattens out and the actual peak point is very easy to miss. By watching the manifold pressure gauge as you slowly lean the engine you will see the manifold pressure jump up one-half to one inch right at peak TIT. This is because the engine is now burning every drop of fuel with all cylinders operating at their maximum.

When that manifold pressure peaks, note the temperature on the TIT gauge. That's the accurate peak TIT value for your flight conditions. Then, my recommendation is to reduce it by 100°F (max continuous 1600°F) by enriching the mixture. I advise using caution running economy cruise (25°F rich of peak) at any power setting higher than 70% BHP. Variations in fuel flow among the cylinders may have one or more cylinders too lean and too hot. A per-cylinder engine monitor would spot this. Try to remember that fuel is a coolant in an air-cooled engine and fuel is the cheapest thing you will ever put in your airplane. By running 100°F rich of peak TIT, it is my opinion that you are adding life to your turbocharger and cylinders. The extra fuel burned at 100°F rich-of-peak will vary from altitude to altitude but will be well worth it in longer engine life and lower maintenance expense. The bottom line here is, in cruise, make 1600°F TIT a limit.

Summarizing Aftermarket Wastegates and Intercoolers:

An automatic wastegate and aftermarket intercooler installation do not turn a 231 into a 252. The 252 has a completely different exhaust/turbo system, intake system, and fuel system. The -MB engine was built and designed to operate with an intercooler and an automatic wastegate, all in conjunction with the fuel injection system. Another key difference between the 231 and 252 is the intake system. Be careful with aftermarket intercoolers and power chart reductions, remember, without applying power chart reductions after you install an intercooler, you will be over boosting your engine. I prefer the automatic wastegate installation for controlling engine temperatures in the 23l.

How I Set Takeoff Power the 231:

One characteristic of many turbocharged aircraft (including the 231) is slow turbo spool-up time. If you set a manifold pressure value at the start of the takeoff roll, by the time you are airborne, the manifold pressure reading will be a couple of inches higher with a set throttle position. As the aircraft's speed increases, ram air effect kicks in and adds to the manifold pressure. This trend continues, more or less, up to reaching cruise speed.

To counteract the effect of ram air, I will typically run the manifold pressure up to 36" and hold the brakes for three or four seconds while the turbo speed stabilizes. After the turbo has spooled up, I release the brakes and by rotation the manifold pressure is close to 40". Don’t do this on an unimproved or dirty runway. Rocks will tear up your prop.

A Word About Induction System Icing:

231 models were known for developing ice in the induction air inlet. This caused the engine to lose power at altitude, sometimes resulting in a total loss of power until the airplane descended into warmer air and the ice melted. The factory recognized this and issued a service bulletin recommending a different alternate air box be installed on the firewall. This solved the problem.

Avoid Shock Cooling Your Engine in Descent:

One thing that all turbocharged aircraft have in common is the possibility of shock cooling the engine. When my customers ask me what shock cooling is exactly, I use this example: You are driving on the freeway in your car when your radiator runs out of water, you pull off the freeway into a gas station. You get the water hose out and you fill the radiator right away. Bad move – you may have just blown a head gasket or worse, cracked a head on the engine. That is shock cooling in a water-cooled engine. To shock cool an air-cooled engine, run at high power settings at altitude and then just push the nose over and remove all power. Remember, power equals heat. Bingo, you just shock cooled your air-cooled engine. The red-hot exhaust system is especially susceptible.

Speed brakes are a valuable asset on a turbocharged airplane. If you have the luxury of speed brakes, you can keep the power up and descend at the same time, avoiding the possibility of shock cooling. If you don't have speed brakes, then you should try to come down from altitude using a reduction of 1" of manifold pressure per 1,000' in altitude lost, at least until lower altitudes result in warmer temperatures. Using that gradual power reduction lets the engine cool down over the course of the descent. Plan ahead!

Another item to avoid is running too rich a mixture setting (heaven forbid, full rich!) at altitude. The excess fuel not only makes the engine run rough, it will also shock cool the engine and foul the plugs. Also, remember that opening the cowl flaps at altitude can shock cool your engine. Remember, anything you can do as the pilot to keep the engine warm during descent will go a long way to keeping your engine healthy and happy.

Once on the ground, let the engine idle for four minutes before shutdown. Sometimes a long taxi plus two minutes is enough time for the turbo to cool down. You must realize that at altitude your exhaust system and your turbo exhaust housing are hot enough to glow cherry red. If you have ever flown in a turbocharged twin at night and looked out the window at the engine nacelle, you can see the glowing exhaust system under the cowling. By allowing a few extra minutes at idle for the exhaust and the turbo to cool down before shutting down the engine, they will last longer.

Cowl Flap Rigging:

One thing I constantly see on the 231 is that someone has rigged the cowl flaps so they won't close completely in the "full closed position". The reason pilots are asking their mechanics to do this is to increase cooling air flow through the engine without having to place the cowl flaps in the draggy "trail" position. I think a better solution to obtain increased airflow through the cowling with the cowl flaps closed is to trim a portion of the outside of both cowl flaps. Cowl flaps are made to be trimmed for each installation to obtain proper engine cooling. Don't re-rig them incorrectly to solve a cooling problem, just trim them. Here in Wyoming, it is not uncommon to see 100°F summer days and -30°F in the winter. Having cowl flaps that close completely, trail properly and open fully is very important for proper engine cooling.

An Indication of Induction or Exhaust System Leaks:

A question often asked is "Why does my manifold pressure increase when I close the cowl flaps?" This is an indication of an intake or exhaust leak. If this starts to happen, get your exhaust system pressure-checked right away. A small crack in the exhaust system of a turbocharged aircraft could result in carbon monoxide in the cabin and/or an in-flight fire.

Tailpipe Security:

Another item to pay close attention to is the V-Band clamp that holds the tailpipe onto the turbocharger. Make sure that the tailpipe has very little or no movement. You can check this by tugging on the tailpipe with one hand and watching the turbo V-Band area for any type of slippage. I have seen the damage done from a tailpipe that fell off a 231 and it was severe. I always safety wire these clamps together after torquing them. If the bolt securing the V-Band were to break, the safety wire will hold the clamp together. I know this, as a Turbo Arrow I owned had a V-Band bolt break. The safety wire possibly kept that flight from developing an in-flight fire.

As always, if you have a question about this article or a previous ShopTalk, you may contact me via e-mail: shoptalk@knr-inc.com or by the old‑fashioned telephone at our aircraft repair facility: 307-789-6866. I will be glad to help with any questions about an owner’s 231. Until the next ShopTalk, enjoy flying your Mooney.