LOP - What kind of Fuel Flows do you see?

[Steve]

So, as I have noted in the past, I could never run my old engine LOP (and never did the GAMI process). I rationalized that avgas was the cheapest component of my flying costs. About 5 months ago I replaced my engine with a Lycoming factory-rebuilt engine. Aside from the fact that it has roller lifters and is zero-timed, the other big difference I have noted is that I am able to run it LOP.

Today, I flew it at 6500' and 5500', both at Best Economy (BE) settings (8.2 GPH) and at 25-50 LOP. 2200 RPM. Adjusting for LOP is somewhat difficult in my airplane for a couple of reasons:

1) The non-vernier mixture control is difficult to make small changes with, and
2) The VMS Microsystems engine monitor only samples CHT and (more importantly) EGT about once a second, so you have to be very slow in your adjustments around Peak EGT to get an accurate peak value. I don't like to have the engine running at Peak any longer than necessary...

Other than a small difference in MP at the two test altitudes, engine parameters were similar. EGT was 1450, CHT was 356, and FF was 6.3 GPH. I was a few KTs slower LOP. At BE EGT was 1400, CHT was 368, and FF was 8.2 GPH.

Saving nearly 2 GPH would be a significant operational factor in my trips to the FL panhandle, allowing non-stop flights vs my routine one stop for fuel. I also sampled some vibration data under the two conditions. The engine felt just a slight bit rougher LOP, but you likely wouldn't notice it unless you were looking for it. The recorded vibration data differed slightly in the X- and Z-axes only. X- was 0.162 g(rms) LOP and 0.138 g(rms) BE, Z- was 0.098 g(rms) LOP and 0.082 g (rms) BE. Both testing conditions had a Y- component of 0.096 g(rms). I have not (yet) dynamically balanced the new prop.

So, a bit surprised at the FF I'm able to get LOP. Is this near what others are able to get? Obviously, I only tested at a couple of (close) altitudes at one RPM setting. In the future I plan to "expand the envelope" of LOP operations.

Steve

[Rich]

I didn't keep really hard data, but the one trip I used this technique to speed up by slowing down was a leg KPAE -> KBYI a number of years ago, en route to KFNL. This mad the trip one fuel stop instead of two.

The actual route covered on this leg, due to ATC vectoring, was about 525 nm. The IFR/VMC flight included a climb from takeoff at 500' MSL to 11,000'. Being it was summer, DA at cruise was likely about 12,500. The hard data I have is that this leg was 4 hours in the air and the plane took exactly 30 gallons at KBYI. So 7.5 GPH average and average TAS 131. I was only able to get about 20 deg LOP but CHTs plummeted to something like 320. I now know this low for CHTs isn't a great idea. As I recall I was using 2200 RPM.

Steve, I didn't know the -M1A was available with roller cam lifters. Did you use another variant?

[Steve]

Rich:

The Lycoming factory rebuilt M1As (and most of their other rebuilds) all come with roller cams. Apparently, the case has to be machined for this modification. Roller cam engines will have an "E" as the last letter of the SN, as mine does.

As for the FF on mine, I actually had a hard time believing it was accurate, since most of the numbers I had seen were similar to yours, around 7.5 GPH. I was limited in my ability to confirm the FF number by fuel usage on the short flight, but I started with 20.8 gallons and finished with 10.3 (filled the tanks). So I used 10.5 gallons for the 1.5 hours (average of 7 GPH for the whole flight including taxi, takeoff, climb, and 2 short runs of 8.2 GPH, plus the descent, landing and short taxi to the fuel pump). My fuel gauges are very accurate.

[blsewardjr]

My last cylinder to peak (#4) usually peaks around 7.8 gph so when I run LOP I'm usually at 7.5-7.6 gph. Bernie

[MarkA]

I’m sure there will be lots of opinions on how to best set up the IO360 for lean of peak operation at > 5000 ft but here is what I do:

• Climb wide open throttle, full rich, 2700 RPM, booster pump on, takeoff flaps

• At 400 ft AGL, reduce to 2400 RPM, turn off booster pump, raise flaps and continue the climb

• At the target altitude, continue wide open throttle and set the desired cruise RPM – I typically use 2400 or 2200 RPM

• Lookup the resulting manifold pressure and RPM on the engine performance table from the POH (attached)

• Lean to slightly less (~.2 gph) than best economy for lean of peak or slightly more than best power for rich of peak operation

• This keeps the engine outside the “red fin” area as defined in the Mike Bush’s articles

• Confirm the expected fuel flow, RPM, manifold pressure, altitude, and OAT all make sense

• Monitor the cylinder head temperatures to make sure they don’t exceed 380 in cruise

• If the CHTs are getting close to that threshold, either lean more aggressively (if LOP) or enrichen (if ROP) to reduce the CHTs

In my plane, on a recent long cross-country flight lean of peak at 9,500 ft (density altitude), 2300 RPM, I was getting ~136 knots true airspeed at ~7.7 gph.

.

[perossichi]

My experiences are as follows

8-10k: 8.2 GPH, 138 TAS ( this is about 50 degrees LOP)

12-14k: 7.2 GPH, 133 TAS ( this is about 70 degrees LOP)

At 2300 RPM, WOT.

Of course at the higher altitudes, you don’t get as high MP.

Max CHT is 370 at 8-10k and somewhat lower at 12-14k.

I’d love a vernier mixture control and a turbocharger.

[Lou]

Since installing Tempest fine wire plugs last year, I fly LOP nearly always. Cylinder 1 is the last to peak for my aircraft and as soon as it has and is running a few degrees LOP I stop. If I go further it gets just a teensy rough and I prefer not to do that. No GAMI.

Sample data from today:
6500’, 10C OAT, 29.78”
24.0” Manifold
2500 RPM
9.0 GPH
142-144 kts

If I dial it back to 2200 I see low 7’s fuel flows and mid 130’s. But if I can cruise at 140+ at 9 gph, life’s great.

[danno2000]

Other than a small difference in MP at the two test altitudes, engine parameters were similar. EGT was 1450, CHT was 356, and FF was 6.3 GPH. I was a few KTs slower LOP. At BE EGT was 1400, CHT was 368, and FF was 8.2 GPH.

Running LOP in my steam-gauge 2003 is a must for CHT management, but I can't get more than 0.5 to 0.8 gph lower than best economy before I get more engine roughness than I'm comfortable with. So high-7s or low-8s is much more common for me at my typical 2200-2400 RPM range.

best,
dan

[Charles]

Dan, I experienced the same CHT issue and needed to run LOP or reduce power on hot days on my 2002 DA40 until I had the Arizona baffle installed, which resolved everything. Although my engine runs fine LOP, I always run at peak EGT now. Engine has 1940 SNEW and is still running strong.

[Derek]

Since installing Tempest fine wire plugs last year, I fly LOP nearly always. Cylinder 1 is the last to peak for my aircraft and as soon as it has and is running a few degrees LOP I stop. If I go further it gets just a teensy rough and I prefer not to do that. No GAMI.

Sample data from today:
6500’, 10C OAT, 29.78”
24.0” Manifold
2500 RPM
9.0 GPH
142-144 kts

If I dial it back to 2200 I see low 7’s fuel flows and mid 130’s. But if I can cruise at 140+ at 9 gph, life’s great.

I was somewhat surprised to see this as well on a new to me 2002 DA40. It runs 7gall/hr around 128knots, or I can use 25% more fuel to go 7% faster!! Sweet spot is definitely cruising at 7gall/hr or so. I guess I’m slower than most because I have the two blade metal hartzel and no power flow. Engine runs perfect though.