Which ones?
Shorrick_Mk2 wrote:
So same MP setting should result in less TAS, oder?
Yes, exactly. Best power (ROP) means more power and more CHT. At LOP you extract more energy from your fuel (i.e. better BSFC). All modern engines operate LOP in partial load situations. There really is no question whether you should cruise LOP — only two reasons exist why not: 1) pilot not qualified 2) equipment not setup for it.
One problem I see here is perhaps the old “LOP argument” one which is that people of course get better MPG when they fly a few kt slower.
In terms of physics, “deep” LOP should not deliver better SFC (better MPG) at a given thrust than peak EGT (to be more precise, about 25F LOP, which is near as dammit peak EGT).
And 100F LOP is very deep LOP.
Assuming the original ignition timing was not wrong for the engine operating condition, the only way anybody can get more SFC by going LOP is via second order effects, and probably the biggest of those is that LOP produces a slower burning mixture, which works well when coupled with a lower RPM, and you get lower mechanical losses at a lower RPM.
Electronic ignition ought to make all this work better, via the option of variable ignition timing which currently we don’t have.
Another problem is that to get conclusive data you need to do very careful flight tests, before/after. It goes without question you need a fuel totaliser and very accurate fuel flow data (the analog gauge most people have is no good for this purpose). The POH data means almost nothing in the context of getting the before/after picture.
At LOP you extract more energy from your fuel (i.e. better BSFC)
I don’t agree – put just that way. The best SFC is about 25F LOP. Rich of that, or lean of that, the SFC is worse.
You’re overlooking the turbo case Peter. In a NA aircraft, you are left with very little MP and there Peak EGT is about as good as slightly LOP. In a turbo plane, you have much more MP available (thus power) but the cooling is very limited. Therefore Peak EGT is not available. The classical solution is to be very ROP but the smart way to do is LOP.
Peter wrote:
I don’t agree – put just that way. The best SFC is about 25F LOP. Rich of that, or lean of that, the SFC is worse.
Oh, you just agreed — 25F LOP is LOP 
See above, Peak EGT is usually not an option for CHT reasons.
Any petrol engine gives best SFC at 25F LOP. Turbo or not turbo. All the turbo does is gives you more MP…
Most pilots, when they say “LOP” then mean a lot more than 25F LOP.
25F is very difficult to set up accurately (even on a second iteration of your GAMIs 
) and the SFC at 25F LOP is probably within 1% of what it is at peak EGT.
Yes but you are missing here that a turbo aircraft flies at 65-75% BHP up where cooling is a problem whereas a NA airplane will produce a meager 35-50% BHP. Therefore you need either LOP or ROP to keep the CHT within limits. An NA airplane can just say “Peak EGT is as good as LOP” and go with it. A turbo airplane can’t do that. If all you have is significantly ROP or significantly LOP, then LOP is the much smarter choice.
Yes that’s true.
The PA46, with its difficult engine airflow was thus one of the first (the first?) to have LOP in the POH.
So… what does this mean for “us”?
Does it mean that this electronic ignition product helps only those who fly deep LOP and have a turbo?
IMHO, this suggests that anybody who flies deep LOP (for whatever reason) either
Why, the more advanced spark timing and the longer spark should benefit everybody.
Peter wrote:
Any petrol engine gives best SFC at 25F LOP. Turbo or not turbo. All the turbo does is gives you more MP…Most pilots, when they say “LOP” then mean a lot more than 25F LOP.
25F is very difficult to set up accurately (even on a second iteration of your GAMIs ) and the SFC at 25F LOP is probably within 1% of what it is at peak EGT.
+ 1 !
Achim : We are talking Best Efficiency and THAT has nothing to do with your power schedule, ie. 65% or some such stupid number they put in the POH ASSUMING the engine is run ROP “per the book” …
Michael, my reasoning was CHT based with Peak EGT not being available for thermal reasons, only ROP or LOP. Peter can do Peak EGT because he flies at high altitudes with 30-50% BHP due being NA. So for him LOP is largely irrelevant. I also fly Peak if I can but on my last Africa trip at ISA+22, I had to go LOP which in my carbureted airplane requires very good denture adhesives…
