Delta betwwen Outside Air Pressure and MAP with Wide Open Throttle and max RPM?

[Fry]

1 hour ago, Hank said:

Check the data plate on your engine, see if it shows the timing to be 20° BTC or 25°. The former has lower temperatures and slightly less power, leading to what you have described. Seems there was an SB a few years ago to change the timing, and not everyone was happy with the results. 

Thanks Hank! Where do I find that data plate?

And if your assumption turns out to be true: I understand the SB is not mandatory, so I should be able to tell the maintenance shop to set the timing back, correct?

(a quick Google lookup points me to SB 380A, but as far as I understand (still learning :-), this applies to older models than this IO-360-A3B6D on my 1995 M20J MSE. Also, looking up in the operating manual for the IO-360, I understand my engine with serial no. L-1015-51E should have mag timing 25° BTC, correct?)

[PT20J]

Changing the timing also requires changing the impulse coupling lag angle so that the impulse coupled mag fires near TDC for starting. A3B6Ds have Bendix dual mags (that’s what the D signifies) and can only have 25 deg timing because that’s the only lag angle available with that mag.

Skip

[Fry]

1 minute ago, PT20J said:

Changing the timing also requires changing the impulse coupling lag angle so that the impulse coupled mag fires near TDC for starting. A3B6Ds have Bendix dual mags (that’s what the D signifies) and can only have 25 deg timing because that’s the only lag angle available with that mag.

Skip

Thanks Skip. Does that imply that my mag timing cannot be off, no need to check?

[PT20J]

2 minutes ago, Fry said:

Thanks Skip. Does that imply that my mag timing cannot be off, no need to check?

No, the timing could still be set wrong, it’s just that the A3B6D does have any options — it must be set to 25 deg. The timing is normally checked at every annual inspection. A single mag with retarded timing will show up as an excessive rpm drop during the pre-takeoff mag check. But the dual mags are mechanically connected, so they are timed together. There is internal timing in the mag (E-gap) that can be off and affect timing also. That would best be a job for a mag shop. But, it’s unusual for mag timing to drift enough to make a major difference in power. I would definitely check that the mag hold down hardware is correct and secure. There have been issues on the dual mag with it coming loose. 

Skip

 

 

[Fry]

35 minutes ago, PT20J said:

No, the timing could still be set wrong, it’s just that the A3B6D does have any options — it must be set to 25 deg. The timing is normally checked at every annual inspection. A single mag with retarded timing will show up as an excessive rpm drop during the pre-takeoff mag check. But the dual mags are mechanically connected, so they are timed together. There is internal timing in the mag (E-gap) that can be off and affect timing also. That would best be a job for a mag shop. But, it’s unusual for mag timing to drift enough to make a major difference in power. I would definitely check that the mag hold down hardware is correct and secure. There have been issues on the dual mag with it coming loose. 

Skip

Ok thanks, I will ask the shop to have a look at that. My thoughts were, since manifold pressure is fine and fuel flow is fine, the obvious next thing that may influence engine power is the ignition timing... any other thoughts welcome. And BTW, it's not a major difference, I'm just not seeing the 2700 RPM on takeoff and I'm not seeing 160 KTAS at 75% power in cruise.

[PT20J]

Have you checked the tach accuracy?

[tmo]

Also, if all the above fail, perhaps a weak spark due to improperly gapped / dirty spark plugs and / or a sub-par ignition harness?

[Fry]

32 minutes ago, tmo said:

Also, if all the above fail, perhaps a weak spark due to improperly gapped / dirty spark plugs and / or a sub-par ignition harness?

Certainly no. I am using Tempest FineWire Spark Plugs. The cold engine starts on the first turrn.

[Fry]

47 minutes ago, PT20J said:

Have you checked the tach accuracy?

No, but it is a JPI EDM primary, I guess that is digital and does not drift. Also, my main observation is a slight lack of cruise performance.

Next annual I will have the shop check the mag timing and the gear doors.

[carusoam]

Other methods of x-checking HP output...

1) Speed in level flight... 3 ways and crunch the numbers... compare to POH...

2) climb rate... direct comparison to the POH... including all details... temps and altitudes...

3) My favorite... T/O distance... WAAS + CloudAhoy app... compare to POH...
 

CloudAhoy can capture a ton of flight data...

Your WnB needs to be pretty well recorded to get repeatable and comparable data...
 

Your engine monitor can show if one of your mags is timed differently than the other...

To know what their timing is would require a standardized location of the EGT sensors... typically found on an IO550... still a calibrated chart would be needed...  EGT vs engine timing...
 

Easier to spend some time with your mechanic and put the timing instrumentation on your engine...  it is a step done at every annual... for proof...logs are a good source... but, if you have a polished spinner... look for light scratches in a ring on the spinner... this is where the timing equipment has left a mark... 

PP thoughts only, not a mechanic...

Best regards,

-a-

[takair]

I sometimes see a little under 2700, depending on oil temp. And the type of oil.....maybe 2680 instead of 2700....but speeds are still good and I attribute it to tach accuracy variation and governor technology and condition.  Use of a second tach might be helpful here.  There is a good app called “Engine RPM” that is easy to use and fairly accurate. You do have to pay for it.   
 I don’t think mag timing will cause significant reduction in cruise speed.  My E has (had....SureFly now) 20 deg timing and speeds were still over 150TAS.  As I mentioned previously, rigging is a major factor.  Gears and gear doors sagging are worth multiple knots.  From other observations, if all else fails, the biggest reduction in power seems to come from cam wear.  This is hard to diagnose without looking at the cam, or if your find metal in the oil.

Not sure if you mentioned this, did this problem occur suddenly?  If it was slow occurrence or you are new to the plane, I would start with rigging....

[Fry]

1 minute ago, takair said:

I sometimes see a little under 2700, depending on oil temp. And the type of oil.....maybe 2680 instead of 2700....but speeds are still good and I attribute it to tach accuracy variation and governor technology and condition.  Use of a second tach might be helpful here.  There is a good app called “Engine RPM” that is easy to use and fairly accurate. You do have to pay for it.   
 I don’t think mag timing will cause significant reduction in cruise speed.  My E has (had....SureFly now) 20 deg timing and speeds were still over 150TAS.  As I mentioned previously, rigging is a major factor.  Gears and gear doors sagging are worth multiple knots.  From other observations, if all else fails, the biggest reduction in power seems to come from cam wear.  This is hard to diagnose without looking at the cam, or if your find metal in the oil.

Not sure if you mentioned this, did this problem occur suddenly?  If it was slow occurrence or you are new to the plane, I would start with rigging....

I'm still new to the plane. Guess I add rigging to the list. What kind of rigging error would lead to slower cruise speed?

[Hank]

3 hours ago, Fry said:

I'm still new to the plane. Guess I add rigging to the list. What kind of rigging error would lead to slower cruise speed?

Anything off will make the plane fly with a yaw to go straight ahead. That adds drag.

[takair]

3 hours ago, Fry said:

I'm still new to the plane. Guess I add rigging to the list. What kind of rigging error would lead to slower cruise speed?

Flaps (Too low or too high), elevator, stabilizer, rudder, ailerons (are they streamlined in flight?, rod end wear), gear hanging down, geardoors hanging down, cowl flaps, engine baffles, These are all very real And significant contributors that are routinely neglected and can add up to multiple knots.  Some of it requires rigging fixtures, but gross errors are detectable and are typically the major contributors.  

[Hank]

10 minutes ago, takair said:

Flaps (Too low or too high), elevator, stabilizer, rudder, ailerons (are they streamlined in flight?, rod end wear), gear hanging down, geardoors hanging down, cowl flaps, engine baffles, These are all very real And significant contributors that are routinely neglected and can add up to multiple knots.  Some of it requires rigging fixtures, but gross errors are detectable and are typically the major contributors.  

One IA wanted to replace my control rod ends [all of them]. When I asked, he did not have Mooney travel boards. I declined his offer, despite his protests of "taking careful measurements and putting the new ones on right where the old ones are." Then he did it anyway . . . . I went to my friendly, semi-local MSC for rigging, and was asked if my takeoff was exciting? My elevator travel is supposed to be 20º ± 3º up and down, but they only gave 15º Up while the Down deflection went off scale on the board [it is marked in ½º degree increments up to 26º both directions! The same person adjusted my nose gear doors, because he didn't like how they closed [I found out when I picked up the plane]. there was a new clunking noise and a giant jerking of the yoke when raising and lowering the gear--the MSC said that I was lucky the nose gear came down, it was rubbing and binding . . . . . No, I have not gone back to that shop . . . .

You gotta be careful, and whoever does any work or rigging needs to do it correctly, have Mooney travel boards and know how to use them.

[jlunseth]

Well, how far LOP are you flying? When LOP, fuel flow dictates power, not manifold pressure, and it is certainly possible to dial the engine far enough LOP to slow the aircraft down. That would not explain failure to make 2700 on takeoff unless, of course, you are leaning for takeoff also, which some NA operators do. You can, however, see a slight drop in RPM if the oil is still cold. I generally only cycle the prop once during pre-flight in the summer, but during the winter up here, at least twice and maybe three times to get warm oil into the governor.

A fairly common and often overlooked issue is the gear. If the gear does not completely retract into the wheel wells then you will have quite a bit of drag and a slow aircraft, and there is no way for the pilot to detect that except by low airspeed. Early on, when I first got my aircraft, I had that discussion with my mechanic several times. The plane was slow. I asked him to check the gear and he would always say they routinely did, it was fine, but I could tell they were not doing what I asked. Finally I convinced them to really check the gear retraction. One wheel was hanging partly down. They got the gear tucked up and voila, 5-7 more knots on the ASI.