Thorp Air Command - T18.net

What do others with the 290 lean to in cruise and climb? I just fixed my EGT and have been leaning to 1350, I know I can just do the BMP but wondered what others do.

The actual egt's don't matter as they change daily. What matters is how many degrees you want to run it lean or rich of peak. There are a lot of opinions for running either side of peak egt. Pick one you like. Me, I like to go fast so I run rich of peak for best power. John Deakins has some good articles on the web for lean of peak. Not for me after trying it and burning a hole in my exhaust, though. Your mileage may vary.

I have tried lean of peak with my O-290G with O-320 sump and MA4 carb. It worked best with part throttle and carb-heat 'on' between 4,000 and 7,000 ft msl. Mainly I was trying to compare and adjust the 4 widely different EGT reading between the cylinders. I ended up finding an economy cruise power settling. But my 4 cylinder EGTs are pretty wide, and peak over a wide band of leaning out, so I usually just lean until the engine feels like it's running well. I quickly lean to an EGT set-point for the #3 cylinder, let the airspeed stabilize for the flight, and then lean from there. I recently installed an EGT instrument that reads-out all 4 cylinders on a screen, in place of a meter with a 4 position switch. That should help.
As posted above, I limit the exhaust temp too. I generally keep it at or under 1400F to reduce stress on the exhaust system.

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Scott Emery
EAA Chapter 326
T-18 N18TE

We didn't have all these issues before we had engine analyzers.

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Bob Highley
N711SH
SN 835
KLAL

LOL at Bob - I know someone who now swears by lean of peak....I don't trust my exhaust temp probes. I lean to cough and enrichen three quick turns - a really smart airline pilot and GA A&P told me recently that the only burnt valves he had ever seen were ones that were run lean. I've replaced every cylinder on my 0-320 before 300 hours (mostly because they were refurbished installed on a "major" overhaul) and wish to keep the new top end happy since they got broken in well. One quart per 24-25 hours and still not black at 50 hours. Gas is cheaper than cylinders....for me at least

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Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

Just fly the aircraft ! We spend way too much time with our heads down trying to figure out that new black box..."What the hell is it doing !"

RB

I don't have the equipment nor do I want to run LOP in the Thorp - I only have 1 EGT and 1 CHT and was just curious. My last Thorp was provided with a nice manual on the ops and in it stated leaning to 1350 at all times if I recall. That was a 360 so just wondering if the 290 is any different in that regard. Because it's one small digital gauge my head isn't down long enough to matter

Bob & Fraze and Rich are right of course!
Chad;
I don't think you can go wrong at 1350F. My wife Marilyn does the same flying an RV-4 with an O-320, and she has a single EGT - CHT gauge like you. I think she has about 1000 hours in it.
You didn't mention the type of O-290 you have. There were at least 3 different exhaust valve types used from O-290G thru O-290-D2.
The valve stem diameter was increased each time, with the final 1/2" type including a sodium filled valve head to transfer heat up the stem to be carried away by the valve guide.
I have an O-290G with 7/16" solid valve stems. Early O-290 aviation engines and all Ground Power Units have mechanical tappets that must be periodically adjusted. I think all O-235 engines are mechanical tappet also. These have cast-in ditches in the case that collect oil thrown off the crankshaft. this oil flows thru drillings into the pushrod tube sockets where it then flows down into the valve cover area, over the valve stems and out the drain to the sump. Hydraulic tappet engines do not have this feature, and the heads have less oil flowing over them. This extra oil flow could be a contributing factor to the low CHT on my T-18?
I run or maintain an O-290G, O-320E with 160 HP conversion, and an IVO-360A converted from a helicopter. They all run about the same, but the O-290G alone has the possibility of solid tappets and solid exhaust valves. There was much debate about these, and the TBO is less.
I would consider the maximum permissible CHT to be about 380, not the published 500 degrees...but I don't know anyone who considers running their CHT close to 500 in any Lycoming.

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Scott Emery
EAA Chapter 326
T-18 N18TE

Thanks Scott. Pistons are 0-290 D-2-B, Intake Valves 0-290-D2, Exhaust Valves 0-290-G, All other parts are 0-290-D

I have spent a lot of time researching leaning techniques. There is nothing wrong leaning to "Lean of Peak" (LOP). It's been proven time and again. However, you have to know how much power you have before you lean. How many out there calculate your horsepower at specific power settings at different altitudes? If you have been looking at lean of peak operations at all, you should know that you don't do it with more than 75% power in a Lycoming and 65% power in a Continental. With ether engine you can't hurt it with the red knob at 65% power or less. Most of these airplanes can't go lean of peak with carbureted engines. They just won't do it. The injected engines will. Many carbureted engine may not reach peak EGT without running rough. The technique with a fixed pitch prop is to lean to the edge of roughness and smooth out with some carb heat. That doesn't always work either. I would lean to peak EGT in my injected Lyc powered Beech set with 75% power or less. That results in about a 4% power drop and the same CHTs as using best power mixture. I got good speed and decent economy and the cylinders loved it! I had 4 new Lycoming angle valve cylinders that I had flow matched. They were 2 AMUs each, plus an additional .25 AMUs to flow match. I was very careful with my cylinders. My personal CHT limit is 400 degs. I agree that 500 degs will not melt your cylinders, but 501 degs just might. I've had a engine overhaul facility tell me that the aluminum in the cylinder heads starts to "do funny things" around 450 degs. I believe them. If you want your cylinders to exceed TBO, you really need an engine analyzer and learn how to use it. The "red zone" or "red box" as GAMI describes, is 20-80 degs ROP at cruise power settings. This is the area to avoid and if you don't have an engine analyzer, its hard to tell if you are there or not. The problem with the "red zone" is not only with the cylinders also with the rods. At 20-80 degs ROP the pressure build up at ignition is very fast, with the rod at 25-20 degs BTDC it is not in an optimal position to handle that pressure (this is why the CHTs are the highest in this zone). So you get a lot of resistance against the piston that transmits it to the connecting rod. If the mags are not timed exactly together the flame front propagation across the piston will be uneven and piston rock will occur. Prolonged operation in this zone can reduce your cylinder life and twist/bend a connecting rod. A well maintained 0-320 will burn about 9 gph at best power mixture setting @ 75% power settings. Going to peak EGT will usually result in about a gallon/hr decrease in fuel burn. So, a 100 hr/yr owner will realize fuel savings of ~100 gals/yr without a significant drop in cruise speed. It saves some money and helps keep the lead from building up in your plugs. As always, T/O and climb with full rich below 5,000. For the fixed prop normally aspirated airplanes, lean to max RPM at 5,000 (best power mixture) and above during climb, then chose your leaning technique according to your engine power, how your airplane is equipped and the mission profile.

Marty Vanover
Tucson, AZ

SHACK!!

Highly recommended book, available on Amazon:

Spanky

What Lycoming has to say … No mention of absolute EGTs. I don't think Lycoming is all that fired up about LOP operations.

Spanky

There is no EGT absolute or EGT limit. EGT is a reference temp only. Peak EGT will vary from engine to engine. I have read some older Lycoming publications that recommended leaning to 50 degs ROP. I don't know if they have changed their position on that, it is probably the worse place to lean your engine to. You can get some serious fuel savings by LOP operations, if you have an injected engine. But, after peak EGT, power also drops with fuel consumption. Significantly drops. You can get the same speed/fuel flow just by reducing power. You do loose some efficiency with partial throttle operation. I live in the Southwest and we generally climb pretty high to keep ourselves cool. That allows full throttle operation that reduces pumping losses and while making less than 75% power (generally, you can only produce 75% power at 7,500 - 8,000 ft with 2500 rpm). At that altitude or higher I would lean at cruise until the 2nd or 3rd cylinder peaked. Of course, there are variables and the main thing is to keep your CHTs lower than 400 degs. That can be accomplished with more fuel in the fuel/air mixture or more air in the fuel/air mixture. My Beech would be the fastest with best power mixture producing ~12.5 gph. With the same power setting, leaning to peak EGT I'd see 9.5 gph and loose 5 kts of airspeed. I thought it was a good trade-off. Mike Bush's book on engines is a must read for airplane owners and pilots in general.

Dang!! I forgot to load the file …

Spanky