Suggestions for diagnosing lowered static rpm?

[humptybump]

Over the past year I've felt my O320-E2A (150hp) hasn't been performing optimally. It has 450 hours since a major overhaul when it was install circa 2007.

Initially it was mostly a feeling. Recently I needed to test my current Catto propeller against my old Aymar Demuth (here after as AD).

In 2012, I did RPM tests of static and WOT@8000' DA:

AD: 2300 and 2725
Catto: 2025 and 2750

Now in 2016 I've conducted multiple RPM tests of static and WOT@8000' DA on different days:

AD: 2260 and 2720
Catto: 1985 and 2740

Also in 2016 I did a couple of static tests immediately after landing and would see a static RPM close to 2012 number but wishing 2-3 seconds it would drop back to 2016 numbers.

I'm looking for thing to examine and test. The A&P I work with is investigatng this issue with me.

Thus far I have checked:

• throttle linkage for binding and full travel - no performance changes
• air filter was cleaned and then replace - no performance changes
• MAG checks are consistent over the past 4 years
• fuel burn on XC flights appears to be unchanged (but could be up a tenth or so)

[Kent Ashton]

Did you park into the same headwind? Was the humidity the same? These will all affect your static.

In-flight testing is a better check.

[WA85]

Possibly check the engine tach for accuracy. When I dynamically balance props, I also check the tach accuracy to actual prop RPM, finding 50% or more read low.

[lr172]

Your WOT @ 8000 #'s are 10 RPM different. That is essentially the same, given all the variables that could cause small fluctuations in RPM, including the tach, at any given moment. .1GPH either way in mixture would probably be worth 10 RPM or more when near peak. I don't know enough about the static tests to know what can cause these fluctuations, beyond power. On my IO-320 with Catto, I can create a spread of well over 200 RPM at cruise with just mixture (throttle position constant). All comparative cruise performance needs MAP and RPM (their combination represent power output).

I am not discounting your potential performance degradation. If your senses are in tune, you can usually pick up on little things like this. Just be sure you are accounting for heat, humidity and weight, which will all create small power reductions or perceived power reductions. For example, if the temps were 30-40 degrees cooler with lower humidity on your '12 static test, that's could be your difference in static.

For small drifts in power output with no roughness, I would be looking at timing, mixture distribution (incl intake leaks) and compression.

When doing comparative tests, it is important to note some of the variables, such as OAT and EGTs, that impact performance.

Larry

[RONSIM]

W85 is correct. As they age, they do read on the low side ----- I have measured really old tachs at 100+ RPM low.

R.

[humptybump]

OK, thus far ...

• WA85: I will ask the A&P to bring his independent tachometer.
• WA85: I have a very real sense that take off distance is a bit longer and my climb out is initially shallower.
• Kent: this year I have done several tests on different days and at different time and in different locations and all showed the 40 RPM decline. (I'm not sure how to test static RPM in the air)
• ir172: It does seem odd that the static is off by 40 RPM but the WOT is basically the same.
• ir172: I will ask the A&P to check timing, intake leaks, and compression.

Keep the suggestions and questions coming !

[tim2542]

In the ballpark still I think.
Given the variables, off the top of my head.
Air temp
Pressure altitude
Relative humidity
Wind
Fuel temp
Spark plug/mag wear & timing settings
Tachometer resolution and rounding

Probably an incomplete list. If your have good leak down numbers, clean filters and an otherwise good running engine I'd say run it.
Tim Andres

[lr172]

Originally Posted by humptybump

ir172: It does seem odd that the static is off by 40 RPM but the WOT is basically the same.

Keep the suggestions and questions coming !

Just want to be sure you understand WOT is not a measure of power, even in an FP installation. It's RPM & MAP, at a constant DA. In my FP installation, I can be WOT at 8000, delivering anywhere between 60% and 80% power, all of which will net different RPM's and TAS with just mixture. The constant is that the RPM/MAP combo, and corresponding % of power, will be unique for every mixture setting.

Your static measurements are a better comparison. However, temp, humidity and mixture also play a factor. If you test one day at 29.70" of baro and 30.40" the next day, that is almost a full inch of MAP and without looking at the charts, is probably good for at least 40 RPM (a somewhat extreme example to make a point. Heat and humidity further reduce power output.

Larry

[Neal@F14]

Originally Posted by tim2542

Spark plug/mag wear & timing settings

Mag timing is one of the first things I'd check. It can change as the points wear.

[humptybump]

Larry, thanks.

When I recorded my data for "WOT" I did adjust for temp, baro, and humidity by calculating and flying DA. I also recorded TAS and cross checked with the NTPS calculations.

The one thing I did not preserve in my 2012 data was baro pressure. However, with my 2016 data I have at least three test days ranging from 30.30 to 29.91.

I also did check mixture for its effect on static and WOT to find the peek RPM for each.

I'll remember to record better environmental data going forward.