Dennis Asbury:
Hi Mike,
I was working on my LOP operation today, and wanted to report in and tell you my findings. They seem very similar to yours, and I just want to know if it all sounds right. I saw the lowest CHTs that I have ever seen!
I have a Super III with the Lycoming IO360. Cruising at 4500 ASL, 2400 RPM, I leaned it out and saw EGTs ~ 1420 – 1424 and CHTs around 360. I kept leaning and just as you stated, the EGT dropped to about 1380 & and the CHT to 330! I had to add a bit of power, and it was maybe a little rough – is that possible, or just my imagination?
I noted that my cruise speed (actual) was also a bit faster. I usually see about 108 Kts, and today I had 114 Kts. Anything that I should be concerned about, or just perfect this process?
Thanks as always for your help and advice.
Technical Editor:
Sounds right to me. The following commentary assumes a CSP prop. If your Super is fixed-pitch, you have to increase RPM to add power. In that case, you can just ignore any comments about MP.
When you are under about 5,000 feet, you have some manifold pressure to spare. At 5,000’, full throttle gives about 24” max MP. If you typically fly under 5.000”, get stabilized in cruise at your chosen altitude and RPM. Make sure that the MP is at or below 24”. Then do the fairly brisk leaning, to the lean side of peak. As you say, you can feel the power ‘sag’. Once you are well lean, meaning 25-50 degrees or more, you will feel a slight ‘buzz’ from the engine. Then move the throttle to full throttle, and lean down again. There is a built-in enrichment circuit when you hit full throttle, so you have to re-lean. Going to full throttle/higher MP helps restore some of the power you lose to leaning, but the CHTs still stay down (because of the leaning).
The slight engine vibration is not a ‘miss’. It is a slight imbalance caused by the cylinders producing slightly varying amounts of power. It doesn’t hurt anything; and it often fades away as everything gets stabilized. If it is bothersome and persists, you can richen very slightly until it clears up; just don’t go on the rich side of peak EGT. Running at peak EGT still won’t hurt anything; CHTs will still be lower than they are on the rich side of peak EGT.
If the power lost to leaning is too great due to higher altitude, and speed declines as a result, you can also choose to increase RPM to regain lost power. The fuel flow will increase with RPM, but the percentage of leaning remains the same. You will still have the low temps.
If you ever do any flying in the Rocky Mountains, you will find yourself at 12,000’ or higher, getting 17” MP if you are lucky, running 2,700 RPM, and 100 rich of peak, just to get 40% power or so. Temps will be way down due to low power production, despite the richer mixture. You can’t afford to run LOP that high, because you can’t afford the power loss (in an unsupercharged engine); but fuel flows are still down in the 7 GPH range, due to the reduced oxygen content.
The key aspect is CHT. There are only four ways to reduce CHT. You can reduce power; you can increase airflow across the cylinder; you can increase fuel flow through the cylinder, to cool down the burn temperature; or you can reduce fuel flow through the cylinder, which effectively cools down the burn temperature due to the excess air relative to the fuel flow. In most cases, you don’t want to reduce power. A very efficient way to improve cooling is to use a flatter climb/higher climb speed, and to cruise faster. Running LOP adds more efficiency to the mix. The right combination will give you the lowest CHT, the lowest fuel burn, the highest efficient cruise speed, and the longest engine life (lower CHT and cleaner combustion chamber and valves).
Once you have this down pat, you will find yourself burning between 1 and 2 GPH less fuel, on average, for a given cruise speed; and CHT will likely range from 290 to 330. I try to keep CHT under 360-370, even in the climb, by whatever it takes. In hot weather, it may take a pretty rich mixture, a very flat climb, and even a power reduction. But once stabilized in cruise, with the higher IAS and associated airflow, the lowest temps can be achieved by running LOP.
Another fun experiment, when you are just flying to some nearby burger haven (or BAC Fly-in!), is to try flying at best glide speed and lean of peak. In my Sierra, I can put the RPM at 2,100 and MP at 20”, lean to 100 degrees LOP, then reduce MP (throttle) until I stabilize at 105 MPH IAS. My fuel flow is then around 3.5 GPH; that’s down in Light Sport Aircraft territory! I have made two-hour round trip flights to fly-ins, and burned under ten gallons for the entire trip.