Question about LOP operation and HP %age
 

I know that Lycoming doesn't recommend leaning below 75% and I suspect that may be conservative as well, at least when at lower loads (i.e. lower MAP's as seen with FP prop configs). I have been setting up at cruise in the 80% power area and then go LOP. That brings my % of HP down to 70-75% power once LOP. My question to the experts: Does this approach fall within the <75% power recommendation for safe performance? I have seen the detonation charts that Dan has posted and it seems that under square configurations showed no detonation potential. My typical cruise configurations are similar to 2600 & 23".

Thanks,

Larry

Actually, my understanding is that going LOP is not recommended at or above 75% power.

Above 75%. To paraphrase, Lycoming's "Big 'Ole One Size Fits All Statement" says when below 75%, you can lean as you please.

Lycoming's statement was published in the days before they acknowledged LOP operations. In general, the pro-detonation region centers at about 25 ~ 50 ROP. They didn't want operators to go there at high power. We still don't.

However, it's possible to run LOP at well above 75%, with three caveats. (1) You have to pass through the pro-detonation zone to get to the LOP side, (2) when you do, you need to get really LOP, not just a little bit on the lean side of peak, and (3) that means actually finding peak is tricky.

In general, I agree. However, remember that "safe" has more factors than just RPM and MP, notably compression ratio, CHT, intake air temperature, oil temperature, and ignition timing. Here's the thing....watch for adverse CHT indications, notably a rapid rise.

Mathematically and no other factors considered, 23" MP is 76.87% power.

Why push the envelope in this area, pull it back to 22", you can lean (according to Lycoming) to your hearts content.

A minor point in this particular instance, but any time you throttle the engine you are giving up VE. WOT is one of the keys to pulling good BSFC numbers.

This is in part my desire for doing this. At 8000 and WOT, I am around 80% power and 2800 RPM. By going LOP and staying WOT, I drop to 2700 and 70-75% power with a good fuel flow.

Larry

Ok, so if you set 75% and lean to 50 lop you end up with about 65%.
If you now push the throttle to regain the 75% with a Bendix style FI can you assume you still have the same fuel/air ratio?
BTW, Dynon claims the % power reading is still good on the lean side of peak.

Thanks Dan, this is helpful. Getting to the lean side is relatively easy, just by observing the power loss. I am developing a pretty good understanding of the RPM drop associated different LOP levels (I have a FP prop) at cruise power by observing EMS logs post flight. What do you consider "really LOP?"

Larry

This is an approach that I considered as well to avoid the danger areas Dan discussed. It was my understanding that the mixture control was for absolute fuel flow, not relative. Therefore, just pushing the throttle in your example would force you leaner. However, I haven't installed my Bendix FI yet, so can't say with any authority. I know it works that way on a carb.

Larry

Larry,
Have you done a lean range test to figure out how lean each cylinder is running? Fuel distribution in an 0-320/360 can be REALLY screwy leaving one cylinder (or more) much leaner than the others and thus increasing that one cylinder's detonation potential, even when it appears that you are well ROP or LOP on the Dynon. I am in this situation so I am very careful about leaning unless I am absolutely certain I'm well below 75%. Am I being too conservative? I don't know what I don't know, so I don't know. Just something to check on yours if you haven't already. Hope this helps.