M20K Performance in higher altitudes - what can I expect?

[NicoN]

These days, I had to climb to FL150 on a +15 day. We have 1980 M20K with the -GB enginge

Reaching FL150, I noticed that my engine had only 34.5" MAP with 2650RPM, pulling roughly 16-17gph full rich, throttle all in.

Discussing this with my co-owners one supected the engine not go give the full rate of power - whysoever.

In  contrary, at 800ft airfield altitude, I always get a power indication of 105% from the EDM with 39.8"MAP/2700 and 24gph.

Also, I read about this engine having a "critical altitude" of 17000ft.

So , I am also not sure: If critical altitude means, that you get still 30" MAP in 17000ft, than we should be almost fine. If this means 40" MAP in 17000, then we have an engine suffering short-breathness

In general: what can I expect from my engine in higher altitudes

Sorry to ask, I did not study the performance tables in detail

[N201MKTurbo]

If your engine still has the fixed wastegate, you may need to screw it in a few more turns.

[Shadrach]

5 hours ago, NicoN said:

These days, I had to climb to FL150 on a +15 day. We have 1980 M20K with the -GB enginge

Reaching FL150, I noticed that my engine had only 34.5" MAP with 2650RPM, pulling roughly 16-17gph full rich, throttle all in.

Discussing this with my co-owners one supected the engine not go give the full rate of power - whysoever.

In  contrary, at 800ft airfield altitude, I always get a power indication of 105% from the EDM with 39.8"MAP/2700 and 24gph.

Also, I read about this engine having a "critical altitude" of 17000ft.

So , I am also not sure: If critical altitude means, that you get still 30" MAP in 17000ft, than we should be almost fine. If this means 40" MAP in 17000, then we have an engine suffering short-breathness

In general: what can I expect from my engine in higher altitudes

Sorry to ask, I did not study the performance tables in detail

Critical Altitude is the highest altitude at which the engine will maintain maximum rated horsepower. The altitude given in the POH is for a standard day. The actual critical altitude will vary with temperature and pressure just like density altitude varies from MSL.   You should expect reduced high altitude performance when conditions are hotter/lower pressure than standard and an increase in high altitude performance when conditions cooler/higher pressure than standard.

Calculating  the density altitude at the altitude at which your engine ceases to make full rated power should indicate if it is performing to correct specifications.
 

FAA definition:

Critical altitude means the maximum altitude at which, in standard atmosphere, it is possible to maintain, at a specified rotational speed, a specified power or a specified  manifold pressure. Unless otherwise stated, the  critical altitude is the maximum altitude at which it is possible to maintain, at the maximum continuous rotational speed, one of the following: 

(1) The maximum continuous power, in the case of engines for which this power rating is the same at sea level and at the rated altitude. 

(2) The maximum continuous rated manifold pressure, in the case of engines, the maximum continuous power of which is governed by a constant  manifold pressure.

[Fly Boomer]

4 hours ago, NicoN said:

Reaching FL150, I noticed that my engine had only 34.5" MAP with 2650RPM, pulling roughly 16-17gph full rich, throttle all in.

What are your temperatures (CHT, EGT, and TIT) with wide-open throttle, 2650 rpm, and full rich mixture on the GB engine at that altitude?

[N231BN]

A couple of things, do you have a Merlyn? If not the critical altitude should be around 14k, depending on setup.

Also, that is not enough FF at those power settings. It should be closer to 19 GPH if you want to keep climbing and keep everything cool.

[M20F]

On 6/2/2022 at 9:18 AM, N231BN said:

A couple of things, do you have a Merlyn? If not the critical altitude should be around 14k, depending on setup.

Also, that is not enough FF at those power settings. It should be closer to 19 GPH if you want to keep climbing and keep everything cool.

Would echo the fuel flow concern. 24-25 is right for take off.  

[kortopates]

You can significantly raise the 231's critical altitude by adding Merlyn pneumatic wastegate. I recall the engines critical altitude at 15,700 density altitude (from memory), well below 17K, and I assume temps where well above standard. With the Merlyn, which transforms the wastegate from a fixed bolt to manual wastegate, the critical altitude is raised to over 20K. You can review the claims on the Merlyn website, but be forewarned its largely marketing hype. 

[PeteMc]

40 minutes ago, kortopates said:

You can review the claims on the Merlyn website, but be forewarned its largely marketing hype. 

I won't dispute that just like any other company, they're promoting the best results they got in their tests.

However, I will say that the Merlyn Wastegate makes a HUGE difference in engine management on the K.  Besides getting more performance up high, it greatly reduces the overboost concern at takeoff.  It doesn't remove it, but it is much easier to add power and not worry about as much of a sudden huge overboost.

[hubcap]

In the POH for my 1982 K model it lists the critical altitude as 15,000' with the TSIO-360-GB...so you should be able to get 40" of MP at 15,000'

My plane has been upgraded to the LB engine but I do not know if that has any effect on the critical altitude.

Edited June 13, 2022 by hubcap

[N231BN]

8 hours ago, hubcap said:

In the POH for my 1982 K model it lists the critical altitude as 15,000' with the TSIO-360-GB...so you should be able to get 40" of MP at 15,000'

My plane has been upgraded to the LB engine but I do not know if that has any effect on the critical altitude.

It doesn't change the critical altitude because the instructions for adjusting the wastegate are the same.

The Merlyn wastegate will increase the critical altitude because it is variable.

[Warren]

Your fuel flow at 800' takeoff is about where it should be.

I believe the critical altitude is 14 500 or 15 000 feet as a stock setup at standard temperatures.  This should be the highest altitude you can still get max manifold presssure.

The fuel servo references manifold pressure.  As you climb, hit critical altitude and manifold pressure starts to decrease, you will see a corresponding decrease in fuel flow.

For reference, I have the Merlyn upper deck pressure controller and my critical altitude is approx. 20 000 on a standard temperature day -- this upgrade is well worth it if you want to fly in the lower flight levels.