engine performance mystery

erich weaver · #1 11-27-2011, 07:19 PM

Greetings

I need some help diagnosing an engine issue. Background info: RV-7A, IO-360B1B (vertical induction) Aerosport power engine built from ECI parts, Whirlwind 200RV constant speed prop, AFP fuel injection, dual p-mags, GRT EFIS, and EIS.

Lately I have noticed an in-flight change in my engine performance that seems to spontaneously show itself after anywhere from 1/2 to 1 hour of flight time during cruising operations. Today I went up and tried to quantify things a little bit. After normal preflight, startup and run up, departed airport and climbed out with WOT, full rich, and just short of max RPM, somehwere around 2650. After 9 minutes of flight and at 8500 feet, I level off, cut my RPM to 2300, and do the big mixture pull to LOP. Everything normal at this point, and I am fat dumb and happy, because my no tail wind ground speed is 185-187 mph with about 7.2 gph fuel flow, and the EFIS says that means over 25 mpg. Cant beat that with a stick. CHTs 1-4 stabilize at 325, 345, 373, 370 while OAT is 57 degrees F. Biit of a spread from high to low there, and I plan at adding an air dams in front of #1 and #2, but just havent done it yet, and anyway, these numbers are what constitute 'normal' for me over the last three years, taking into account OAT variations.

OK, so far so good. But after about 30 minutes of hands-off autopilot flying today, and no changes to anything by me other than very minor tweaks to the mixture to keep it at 7.2 GPH, my numbers and performance suddenly change. CHTs drop by 25 to 30 degrees across the board, EGTs drop 30-45 degrees, my ground speed drops (dont have a hard value for that, but something like 15 mph) and the engine no longer runs smooth at 7.2 gph; I have to richen up to somewhere around 8.3-8.5 gph. I dont like the power loss, and certainly not the increased fuel flow to get things running right again, but other than that, the engine seems to keep running ok - I dont get the feeling its gonna quit on me.

These symptoms are now happening on a regular basis, although the flight time at which it happens isnt exactly the same from flight to flight. It occurred to me that perhaps I was losing the use of one p-mag? Gas was running low, so I didnt get a chance to fully check this out today, but during my less than full throttle, probably ROP descent, I did cut first the the left p-mag, then the right p-mag, and saw all the EGTs go up each time, so they both were working at that point, and they both functioned during pre-takeoff run up, so that theory seems unlikely.

What else could be happening here? Maybe some kind of induction leak that only shows up after the engine reaches normal operating temps? I dont get it. Any theories or trouble shooting advice?

thanks in advance

erich

rhill · #2 11-27-2011, 07:34 PM

P mag on both sides? Timing changes moving around?You might want to consider a mag on one side.

N941WR · #3 11-27-2011, 07:35 PM

It could be that the P-mags are doing what they are supposed to do. Have you taken a very close look at the vacuum lines feeding both P-mags and are they tied together?

If you have a leak that opens up with heat, then the timing will return to close to 25 degrees. (You didn?t say if you were running the ?A? or ?B? curve.)

The other thing to check is to try turning on the boost pump to see if your speed picks up.

Mark Burns · #4 11-27-2011, 07:40 PM

Eric,
You didn't mention what the manifold pressure does during the change.
Also, I assume the RPM remains constant?

Thanks,
Mark

N941WR · #5 11-27-2011, 07:49 PM

Check your PM's.

erich weaver · #6 11-27-2011, 08:06 PM

I wasn't able to spend as much time as I wanted in the post-change regime to fully check everything, but my notes for the few minutes I did have show no change in MP; it stayed at 22.2. I did not adjust RPM, and it's a constant speed prop, so that stayed constant and my last reading was 2310.

Bill, I don't understand your comment about the pmags "doing what they are supposed to do". Why would the timing spontaneously change? I definitely have the MP line teed to go to both pmags, but I will double check for possible leak points. I am using the more conservative timing curve; I think that's the A curve, right?

With respect to changing out a pmag for a standard mag, gonna pass on that for now. If this is a timing issue, I want to understand what is happening and get to the bottom of it. I would try swapping out both pmags for replacement pmags as a trouble shooting technique if that becomes necessary.

Thanks, keep the replies coming.

Erich

NDrv8r · #7 11-27-2011, 08:21 PM

I agree with Bill R.
I have one bendix mag at 25 deg on the left and one p mag on the right running the b curve. The mag is running the bottom plugs, but there is quite a difference in sound and performance between the mag and pmag.
since both are going at the same time and you do not see any individual cylinder difference, I would look real hard at the mp plumbing to the pmags.

does the mp line to the pmags come off the same place as your efis mp sensor?

N941WR · #8 11-27-2011, 08:35 PM

Quote:

Originally Posted by erich weaver

...Bill, I don't understand your comment about the pmags "doing what they are supposed to do". Why would the timing spontaneously change? I definitely have the MP line teed to go to both pmags, but I will double check for possible leak points. I am using the more conservative timing curve; I think that's the A curve, right?...

The P-mags are designed to revert to fixed timing magneto mode, if there is a vacuum leak. When that happens, the timing will go to around 26* BTC. So, if you are cruising along at 65% power, LOP, the P-mags will be between 30 and 33* BTC and IF you have a vacuum leak they will revert to 26* BTC. (Those numbers change with the 'B' curve and the 'B' curve is completely configurable by the user.)

Mark Burns · #9 11-27-2011, 08:54 PM

Quote:

Originally Posted by erich weaver

I wasn't able to spend as much time as I wanted in the post-change regime to fully check everything, but my notes for the few minutes I did have show no change in MP; it stayed at 22.2. I did not adjust RPM, and it's a constant speed prop, so that stayed constant and my last reading was 2310.

Thanks, keep the replies coming.

Erich

Eric,
I don't know aything about Pmags but based on the data supplied I agree with the others that it's the timing that is most likely causing this change in power.

What we know:
Fuel flow is constant (GPM)
Air flow into cylinders is constant (MP)
Engine looses power (heat) All cylinders.
Prop flattens to maitain RPM which maintains MP.
Airspeed drops.

Pretty much either the timing changed or the fuel composition changed.
I vote for timing unless you switched tanks a few minutes prior

Mark

G3i Ignition · #10 11-30-2011, 06:30 PM

Timing reducing down from 33/36 BTC to 25-27 BTC would not cause the EGT?s to drop maintaining the same amount of fuel consumption. Question, when fuel flow is increased up to 8.3 gph to obtain a smooth running condition does the CHT?s and EGT?s respond also, back up to your original readings?

Thomas S.