Yes, NOx is way up there at lean conditions.
Lets see if I can touch on a few without writing a book. It would not sell well anyway. lol
aidandj- Tuning timing at cruise is pretty simple if you can find a steady state condition. Adjust it and watch PW. Obviously lower PW is better. Timing will affect AFR so keep up with AFR Though timing should always lead the process. Being your car and your motor, go with what you are comfortable with. Note that if you keep leaning the motor, at some point it will peak and go colder as you lean more. Leaning to peak under high power will melt things. I have run 1400 at the turbo for cruise. Timing was too slow then and was at 14.5. Had I added timing and pulled back more fuel EGTs would have dropped.
I have two cars running 18:1 with no problems. the 01 does have stock injectors, but the 95 has 440s. Now I may have some difficultly with 20:1 after putting in my 610ccs for the turbo. But expecting to maintain 18:1. I was showing a friend the MS and set the AFR target to 21.5 once and forgot to set it back. Noticed the next hwy drive the O2 meter was maxed at 20. Laptop was confirmed 21.5. You could not tell by how the car drove. VVT and Tonguerun front lip does help a lot on mileage for me. Car only got 20-21mph stock at 80. So pretty happy with it.
Couple things here on timing. First forums tend to be very conservative on timing. Understandable. However, using the standard for the NA crowd. Stock system runs best with adding 6-8 deg of timing. Well add that to the stock WOT timing of 34 on a 95 and we are in the 40 range under full throttle. Atmosphere pressure (95-100kpa) of course. 37 to 38 would be conservative. Are you guys in that range? Giving 38 100KPA. 45-50 timing at cruise becomes real. I cruise about 50-55KPA at 80. Goes to 65kpa real quick with hills.
I also run my O2 closer to the engine than most. The delay of further down tied to a slower responding O2 is a headache for swinging tune. Leaner as you all know is more critical to have right.
deezum - I expect timing is the reason you are falling out of cruise numbers. Though I do not have experience on the 1.6. fwiw I started where you are with my 01. Was conservative due to 10:1 CR. But once I brought timing up it all fell into place. 01 only likes 34 100KPA where the 95 with 9.5CR has no trouble with 40-42. Take my info with a grain of sand. I also do not run below 25 timing on boost. 12-14psi #3/T04E. I recall comments of more than 17 will destruct... At something like 40k miles on it I figure it is living. Tuning is key as always.
Brain- Yeah hp does drop and at some point 19+ is a losing deal. It will feel softer at 18, but not bad. Beyond 18 and power goes in the tank. Once I cross 75 kpa I add fuel aggressively down to my 13.5. 440cc RX7s in the 95 So not great injectors but they will do it. Do have seq fuel. Needing to do seq ign on both cars. Was able to do 18:1 on the MS1 but a pain to maintain tune with the seasons.
Experience with a bird (O320 160hp Lycoming) having 4 EGTS with 4 O2 sensors has really changed my standards for cruise settings. Lycoming is very well documented on power/tune settings (charts etc). For those not aware it is all manual tune. Fuel adjustment is needed as you change power settings, or altitude, or fly thru a front.
With 3.6 gears going in soon I hope to see in the 40s mpg for 65mph.