Thorp Air Command - T18.net

Guys,

I was out last night turning gas into noise and thought I would play a bit with respect to mixture settings. My Thorp has an IO-320-D1A engine with a C/S prop. I have been typically cruising at 2200 Rpm and 24.2 " to get an indicated speed of about 158 mph. The CHT's are usually about 345 degrees F with the EGT at 1320 degrees F at peak. The fuel burn is about 7.8 GPH. Last night I set the MP to 24.2" and the same 2200 RPM. Then leaned to 6.8GPH, which was about the lowest I could go and still have the engine running smoothly. The EGT went down to 1270 degrees F and the CHT was 330 degrees F. It did slow down a bit to 154 mph. The decreased fuel burn equates to about 12.8% with only a 2.5% speed decrease. It looks like there may be something to this "lean of peak " way of running our engines. I will play a bit more to ensure it is repeatable.

If you guys can think of anything expensive I may be doing to my engine in doing this, please let me know.

Best regards,

Jim Mantyla

Here's a link I found. I'm sure you've seen it. I have no first hand experience in LOP operations. On page 4, left column, just after the bold type, it says: the technique of operating LOP and power recovery were discontinued due to the resulting increases in service issues.

http://www.lycoming.com/support/trouble ... SP700A.pdf

I have always felt that a "rich" mixture was better for the engines lubrication vs a lean mixture that could burn valves,etc.
Trying to save $5.00 gallon of gas does not make sense when an engine cost 25K. Just old head strong me!

There has got to be something to this LOP, there are guys that are using this leaning method and doin what appears to be fine. It would be my luck to do this and smoke a valve or 2 or worse and end up doing a ring and valve job........If I am in error I will error to the side of caution when my billfold is involved. But for those Gentlemen that can do the LOP deal and have it work well for them and their equipment saving a few bucks on fuel with todays fuel prices I guess is a good thing, after all, filling up means making a down payment on an oil company.................Dan

I certainly don't want to burn a valve but with the EGT lower than it was running close to peak,( about 65% power) it would seem to me that there would be less chance of this happening.The 1290 degrees F I see at LOP is about what I see when running full rich. I wonder if I should take a look inside the cylinders with the scope as a before look and then after running a few hours again to see if there is a noticable difference?

Best regards,

Jim

I think the key part of Ryan Allen's post and the SP700A link is "the technique of operation LOP and power recovery were discontinued...". Jim does not mention any power recovery in his technique therefore he should not be at risk for detination. 2200 rpm at 24.2" MP is ~65% power, my understanding is detonation can not occur below 75% power so he is operating with a margin of safety.

_________________
Matt Smith
Des Moines, IA
55RC

The article also pretty much referring specifically to turbo or supercharged engines. Also, the 2000 date of the article put it a few years before the average joe had easy access to full engine instrumentation.
There was a great article in AOPA or EAA about 6 months ago talking specifically about inner cylinder pressures and LOP operations. It works fine as long as you understand and follow the guidelines...
If I'm at 10000- 15000 feet with a normally aspirated engine and lean it was hard as I can, I lose a little speed, but my engine is very happy and I don't foul plugs either...
...or so it seems to me.