Dexta:
***I have a C172 with the O320-E2D, I have a Single point CHT & EGT fitted, when in cruise I will lean until I get an RPM drop which corresponds to peak, or just past peak EGT. I only do this once I'm running less than 75% power, determined by RPM and density altitude (approx. 2500 @ 2500', 2600 @ 5000' and 2700 @ 9000'). I do not know if I'm running LOP on all cylinders, but it is usually on peak or just lean of peak - 40 deg or so, on cylinder 3 (the one with the EGT probe), but what I do notice is my CHT's which drop from 380-400 degrees to below 350. I am getting about 118 KTAS at 8000' for 30 litres per hour and the engine is running smooth. I have been told that this procedure should not hurt the engine as it is under 75% power. I would love to do the APS course (and will next time it is within 300nm of home) but I'm hesitant to install an EMS in the aircraft (as I'm hoping to upgrade the aircraft in the near future). I could of course run 100 deg ROP but the CHT's get upto 400 and as I understand, the lower the CHTs the better. Of course if the above is all wrong then I would be very interested is learning what I can do practically to run my engine at its best with what I have.***
If you are LOP on the cylinder with the EGT probe and the engine is running smoothly it means that your method is spot on and you DO KNOW that all cylinders are LOP. If the engine is smooth, the F:A ratios are reasonably well balanced, you have a good ignition system and no induction leaks. Everything is fine. If all cylinders are not LOP, your six little engines running in tight formation sharing a common crankshaft will be running rough since they will be making different HPs.
One suggestion:
While you are definitely not hurting anything with your method, you may be leaner than optimal and giving up some performance. You might try setting the mixture on the EGT probe to about 20dF LOP at the altitudes you are flying. See how you like that?
You have proven that you do NOT need to have an EMS to set mixture efficiently or properly if you run LOP. BECAUSE it is smooth, you know that the engine is healthy. If, OTOH, you run ROP and have unbalanced F:A ratios, poor ignition health or induction leaks, you will NOT know it without an engine monitor because it will still be running smoothly! ROP operator need an EMS more than LOP operators!!