I have updated the following with some more specific information, mainly due to some questions raised here on the M20M power output and on what MAP is required for takeoff.

 

TL;DR

1. The TIO-540-AF1A/B is making more power than most people realize; the AF1B is NOT a detuned version of the TIO540, it is turbo-boosted up-tuned version (per Lycoming).
2. The MAX MAP of 38” is the high-limit on the density controller, normal Full Throttle takeoff MAP should be 35-36” if the density controller is adjusted properly
3. On take-off you’ll likely be making well over 110% power from this engine
4. 2725 RPM and 35” MAP on a standard day at SL is where that engine is making 100% rated power of 270HP according to Lycoming

 

I’ve updated the power charts below, (lines start with a | where I do that and I have included some additional supporting Lycoming data)

DVA

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Hi!

 

I’ve seen a lot of threads that have good information on power settings when it applies to the TIO-540-AF1B, but they are buried and disjointed making it hard for someone new to glean an understanding. So I thought I’d try to put something together to help. Fire away.

 

For those who are struggling to understand % of power on turbocharged vs normally aspirated engines, allow me to take a stab at an explanation: (Experts will say I am oversimplifying here, but I think they will agree in principle):

 

A normally aspirated (NA) engine, like the IO 540 will be build to make a certain amount of horsepower (torque, actually) at a certain RPM, Mixture, Air Temp, Altitude and MP. NA engines are rather predictable when it comes to computing the power fall-off from 100% as the airplane gains altitude. With a turbocharged engine it is not as easy, but it can be done.

 

Let’s start with some basics: You can only get 100% power from a NA engine under one general set of conditions: That being at max design RPM, at a certain air temp (usually Std day), at sea level and at the highest practical atmospheric barometer; the latter condition is important to understand.

 

For the sake of this discussion's simplicity - experts please indulge me - your NA engine's highest possible MP, at sea level, will equal the barometer reading at that location. So if you are on the beach, in Florida, sea level 0 feet, and the barometer is 30.00 (nice day) - your NA engine, at full throttle will only produce 30" MP on the ground. If the next day is rainy and nasty and the barometer reads 28.50, then your NA engine will only make 28.5" MP on the ground. 

 

At wide open throttle, MP in a NA engine is very close to the real barometric pressure no matter where the engine is, and barometric pressure decreases by about 1" for every 1000' of altitude. So if your NA engine made 30" of MP on take off at sea level with wide open throttle, and if everything else remains constant, at 1000' you would have 29" of MP,  at 2000' you would have 28" of MP,  at 5000' 25" and at 10,000' 20". And as you climbed and the MP decreased, the engine would be making less and less power by virtue of the fact that the air is less dense as you go higher and higher.

 

You can think of a MP gauge as a 'air density' gauge. To add a little more precision here, it's the oxygen in the "air" that the engine wants so it can support combustion, and as you go higher in the atmosphere, while the percentage of O2 in the "air" remains the same (apx 21%), the density (or partial pressure) of that O2 gas gets less and less.

 

Take-away: MP directly affects the engine's ability to make and maintain power - more MP equals more power.

 

So do other outside conditions affect the NA engine's ability to make power? Yes, but some conditions are not to the same degree as MP.  For instance, the temperature and humidity of the air that is sucked into the NA engine will affect power; colder dryer air will help make a little more power, where warmer moist air hurts power. Commonly, about 1% / 10 degrees +/- is a good ratio. For the sake of this discussion, intake air temp for the NA engine is not that meaningful or significant when compared with MP, mixture, and RPM. However, you will see why intake air temperature becomes more influential with a turbo in a moment.

 

Take away: Cooler intake temperature and lower humidity help an engine make more power. Warmer and/or moist air hurts power.

 

Speaking of RPM: Cars, motorcycles, boats, etc all have much higher RPM where max HP is made; my car makes maximum HP at nearly 7000 RPM.  Why do aircraft engines have such low maximum RPM, commonly below 2900? 

 

It's all about the propeller. For a lot of good reasons a prop just can't spin faster than about 3000 RPM without the tips going transsonic and the efficiency of the blade (really a spinning wing) falling off rapidly. Given that limitation, direct-drive aircraft engines have to be made to produce adequate torque at a relatively low RPM. This is not as easy as you'd imagine. Higher RPM does a lot to contribute to overall torque when you design an engine. Side note: torque is the product of a running engine. Horsepower is simply a calculation derived from measured torque.

 

Take away: If everything else is equal, If you lower RPM you lower torque and thus you lower horsepower.

 

Then there is mixture. Four-stroke internal combustion engines designed around the Otto cycle create combustion (and thus power) identically. Car. boat, airplane, lawn mower, it doesn't matter. There is nothing special about an airplane engine except it is simple and unsophisticated compare to a modern car engine but they all work the same way.

 

While this is a whole nother' topic, mixture basics are simple: If you want to make the most power from an simple internal combustion engine, everything else considered equal, you want to have a mixture that will produce an exhaust gas temperature that is 80-100dF rich of peak at any particular power setting. This is where your Briggs and Stratton lawnmower will make the most power and run its CHT coolest for that power. If you want your lawnmower to cut more grass because the fuel lasts longer, then you want to run your lawnmower at exactly Peak EGT or better yet, slightly Lean of Peak EGT. These mixture settings will reduce power somewhat, but will save fuel and lower the CHT for that setting. You never want to run your lawn mower near full power at less than 80dF ROP - say 30 or 50dF ROP as some manuals suggest - because you are then operating the lawn mower in a combustion regime that will make excessive heat, with the highest internal pressures, at LESS power. Your lawnmower will hate you! So will your airplane. (Follow the POH for mixture settings, unless you know what you're doing. This discussion is theoretical)

 

Take away: Mixture affects power to a great degree - proper mixture setting is very important. Author's note: You should really read as much as you can about the Red Knob and how it affects your engine, and what effects it has on your engine. Proper mixture control, in a properly tuned engine with proper engine instrumentation will properly serve you. I cannot overemphasize this.

 

Lets summarize the NA engine now in flight, this assumes a good solid working engine with no hidden problems.

 

It's a standard day at sea level, 59dF and pressure 29.92 and the atmospheric lapse rate is also standard.  We take off with full throttle (close to 30" MP), mixture rich, prop full forward (if applicable) 2575 max RPM and fuel flow, fuel pressure and all temperatures within limits. We can assume we have 100% power as we lift off and we are climbing at Vy at, say 750FPM. 

 

By 3000' we notice our MP is down to just below 27" and our VS is now 500FPM, Why the loss of VS? We naturally lost power due to the air density going down as we climbed. What can we do about? Nothing really, every engine control is already full forward.

 

A quick check of the POH graph at this point may show that 2575 RPM at 27" MP is 80% power in this example airplane.  By 5000' ASL our MP is under 25" and our VS is 350FPM and the graph may show 70% power - still nothing we can do to make more power. In fact we will continue to lose power, VS and *indicated* airspeed the higher we go, until such a point as the airplane will no longer climb due to lack of sufficient power; this is the engine's 'critical altitude’ and a term you also hear used in a turbocharged engine's POH. The critical altitude (sometimes this is also the Service Ceiling) is simply the point at which the engine can no longer make enough power for the aircraft to climb.

 

Let's add a turbocharger to the above airplane. (I used the Bravo's 2575 RPM in the example above on purpose)

 

Recall above that the NA engine on take off made 30” of MP at 2575 RPM with all controls full forward, and the manufacturer called that 100% rated power. If we took that identical engine and flight conditions, and simply bolted on a turbocharger could we use that turbo to make MORE than 100% power? Yes we could! 

 

By simply using the turbo to boost the MP past 30" - anything past 30" - we will make more power. Another question: If we used the turbo to just keep 30" as we ascended (so not lose MP as a NA engine does) could we essentially keep 100% power as we go to 3000', then 5000' and beyond? Indeed! That’s how a Turbo-Normalized engine works. It doesn't make more MP than the engine has naturally at sea level, it just makes up for the natural lose of MP as you ascend.

 

But that is not what the we have on the M20M - we have a turbocharged engine that indeed makes more than 100% power at times (over a identical sized and configured NA engine)

 

The TIO-540 AF1x engine uses its turbo to over-boost the intake air pressure well above atmospheric pressure, up to as much as 8" or so. Its MP maximum is 38" and normal POH cruise is 30-32".  At any altitude that is way over the natural atmospheric pressure, making this engine a very high performance, hot, power plant. As you will will see, running this engine at high cruise power of 34”/2400 125dF ROP (like what the green visor suggests) is running this engine at approximately 93% of rated power. Lycoming and Mooney have certified this engine to run at percentage of power levels higher than you'd expect and that's what makes the M20M go so fast. But should you routinely run this engine at that high power level?  You can, it’s certified to do it. I don’t, however.

 

You have to look in both the POH and in the Lycoming Engine Operating Manual to put together a % of Power Chart for this engine. I have done some of that work below by averaging out the power charts, but first I think a little turbo talk is in order.

 

|The Turbocharger will make-up for the loss of air density as the plane ascends, and allow the engine to make more power, higher up. This gives us service ceilings of FL250 in the M20M and faster forward speeds ... that’s the up-side. The downside is that a turbocharged high performance engine also makes more heat - a lot of it - which causes faster wear and stress on engine and exhaust parts. Not to mention that the turbo and its parts adds to the cost and complexity of the engine. Running a turbocharged engine incorrectly can also cause premature failure and wear of the engine.

 

|IMHO, to get the most time and efficiency out of the AF1B, for cruise at altitude, you should routinely run this engine at a power point where it is not over-boosting but rather normalizing. That would be in the 28-30” of MAP as a max at cruise. From the chart below you’ll see that is 75-80% of rated power. I run mine at 28-30“ and 20-30dF LOP often, and it goes fast and uses a lot less fuel. (I have GAMI injectors and use Tempest fine wire plugs to help achieve smooth running LOP) If I am running at a higher power level with this engine, which I do occasionally, I will always run 100dF ROP TIT, and never at Peak TIT.

 

I find this to work well, you might have a better plan:

 

| 35-38” / 2575 / Full rich for takeoff                                                  =   ~100%-114% of rated power (270-308 HP)

  34” / 2400 / Full rich @ 120 for initial climb                                     =   ~93% power

  32" / 2400 / 100dF ROP @ 130 enroute climb                                =   ~87% power

  29" / 2400 /  30dF LOP for cruise (airspeed varies with altitude)    =   ~75% power

 

| Here is the interpolated chart, supported by the graphs below derived from the Lycoming power plant manual. Please note that the power curves are not linear for some settings, higher up you’ll make more power.

 

| ACTUAL HP corrected for RPM/MP/ALT/TEMP (Std Day (+/- 1% per 10dF from Std)) TIT ROP -125dF Max Power mixture  

 

| 114%        308        2575           38”    0-FL220 (This is the max authorized MAP and will rarely if ever be encountered when properly adjusted)

| 106%        285        2575           36”    0-FL220 (this is the normal “max power” takeoff upper limit where most density controllers will be set)

| 100%        270        2575           35”    0-FL220 (this is the normal “max power” takeoff lower limit where most density controllers will be set)

 

93%          250           2400        34”    0-FL220

87%          235           2400        32"    0-FL230

80-85%    215-225    2400        30"    0-8K 8K-FL250

75-80%    200-220    2400        28"    0-8K 8K-FL250

65-70%    180-195    2400        26"    0-8K 8K-FL250

60-65%    160-180    2400        24"    0-8K 8K-FL250

55-60%    145-165    2400        22"    0-8K 8K-FL250

 

85%          230          2200        34”     0-FL250

80%          215          2200        32"     0-FL250

75%          203          2200        30"     0-FL250

65%          175          2200        28"     0-FL250

60%          165          2200        26"     0-FL250

55%          150          2200        24"     0-FL250

50%          135          2200        22"     0-FL250

 

| (Note, I don't normally fly 2200 RPM. I don’t have a good reason why) I changed my mind, I tried it and I like it, I can get 2200 RPM and 31” MAP LOP and she runs nice and quiet.

 

| Finally,  recall from the discussion of the NA engine that we illustrated intake air temperature was not a huge contributor to power gain or loss. On a turbo such as the AF1B if can be. As the turbo spins up from the exhaust gases passing by impeller, the intake side of the turbo is sucking in outside air and compressing it, which raises the density of the air. This compression effect also heats the air - a lot.

 

| On my Bravo I have a JPI 830 with all the options and I see Compressor Discharge Temps approaching 220dF on some hot days high up. The intercooler after the turbo will take a full 80-100dF out of that hot air and present an Intake Air Temperature of just over 100dF to the engine.  If you look at the fact that every 10dF of hotter air takes away 1% power, that intercooler is helping by at least 10% - that’s a huge power help. 

 

| Here are some graphs to look at. The first is from the Lycoming TIO-540 operating manual. It shows us that that the full rated power of the AF1B is 270 HP at 2475 RPM and 35” MAP. This calculation is corroborated many times over in the manual.

 

 

| Of note, all of these figures are based on the TIO-540 engine, but notice that many variants of the engine make more of less power. Some of the engines are modified with lower compression ratio or different fuel flows along with higher or lower maximum MAP to create the difference in power. Note the Note: The Mooney POH suggests to us that full rated power comes on at 2475 and 38”, but Lycoming counters that with numerous examples, such as this:

 

 

| And finally, here is the graph used to adjust the density controller for full throttle operation.  Note the fact that the Compressor Discharge Temperature is critical in making this adjustment. From what I have learned, many A&Ps who are not thoroughly familiar with how a density controller works, will simply “eye-ball” this adjustment which is the likely reason why some M20M (and others) pilots have such large variations on the full throttle MAP.

 

| If you are consistently making at least 35” to around 36” of MAP with a fuel flow of greater than 27.5 GPH at a little less than 2475 at the start of the ground roll - you are in great shape.

 

I hope this helps, I am not an expert but I listen to and learn from them. I'll try to answer any questions as best I can.

 

Enjoy your Turbo Monster 20 M!

Dave