I've got a friend that asked my to post this question. He is flying a new Piper (I don't recall the model) that has an avidyne panel and TC engine. He says that the Avidyne does not have individual cylinder EGTs but the POH says to use the TIT to lean to either 20 degrees ROP or LOP for cruise.

I told him, that I thought that was the worse place to fly the plane that he should be 70 degrees LOP. Of course the plane will not fly that far LOP.

Any thoughts on this practice?

What's a "Piper"?

A nice looking plane that you can't buy parts for.

It's who you pay....

Those numbers would scare me. Can't a separate JPI or other monitor be installed just to keep an eye on things?

Turbo or not, the amount LOP that you set the mixture would depend on the power being produced. It really should be that simple. The higher the power, the farther LOP he would need to be. If using 20 LOP I would assume he's only getting around 65%. I don't know how much MP recovery the Piper can achieve but that would be the key to the answer.

Engine Type would be helpful, however since it's a TC Piper it's likely a Lycoming, possibly a TIO-540 and therefore Best Economy would be OK at Peak or slightly LOP, and Best Power should be 100~125+ ROP. All this assumes your keeping track of CHT's and the stay below limits.

If it is a TIO-540 Lycoming, I would cruise at Best Economy or Peak up to 65% power while keeping CHT's below 400. If I really needed to go fast I would go to Best Power (75%+), richen the mixture to 125+ ROP then add MP to desired FF & power setting

When operating a true TC engine (MP's in excess of 32" up to 46") you need to be critically aware as to what power setting you can reasonably operate at Best Economy (Peak) up to without exceeding your CHT limits. Generally speaking that's about 65% power and about 29~32" MP & 15~17 GPH in a TIO-540. Greater than 65% you have to go to Best Power which is going to be around 34" MP and 22 GPH.

65% vs. 75%, That's a 30% increase in fuel consumption to get an additional 10% HP which is going to yield a less than 10% increase in airspeed.