How lean is too lean at 4k cruising?
 

Asking cause my brain thinks running 16.0 at highway speeds is too lean, should I make it a bit richer? Down to 15.0 maybe?

Id run as lean on the highway as I could get away with.

Run as lean as you can without getting too high egt. You need a lot of timing to run that lean in cruise too.

EGT before or after the turbo? May be something to do down the line.

EGT is measured about an inch from the exhaust port. Its actually something like 1.5x the width of the port.

You can get thermocouples for like $10 on ebay. I'm running mine into a TinyIOx to log with Megasquirt.

I run right at 18:1 for cruise. Lycoming "aircraft" uses a process of leaning the engine to find peak EGT @75% power. Then +100 deg richer EGT for power or -50 deg leaner for best economy. That works out to 18:1 economy and 14.5 power. I have run as lean as 22:1 in the car. PW goes up on mine beyond 17.8 so I settled in at 18 for cruise. I also run 50 deg timing up there. This nets me better than 30mpg at 80/85mph with 4.3 gears. Slowing to 65 ups to 34-35mpg. I do bring it down to 15:1 pretty quickly crossing 70kpa. Running a stock 2001 with MS.
The concern you will have is cylinder balance. The lean to peak applies to the leanest cylinder.
BTW - A high Fuel ratio better than 20:1 does not generate enough heat in EGT to do harm.

Most will say 16:1.

How do you tune timing at cruise. Just raise the timing until the EGTs are safe?

There is no way my 1.6 would push under 70kpa running 18.0, it either won't hit those lean tuned cells, or it will and then quickly jump right back out to higher rich cells. I've tried +/- 10degrees or so around cruise, can't really tell much of a difference. I'm around 40 degrees or so in cruise now.

15.6-15.8 seems to be best for me. There's no way I could run lean as 22.0, pulsewidths would be too small and it would kick and buck like nuts coming in and out of power for hills and stuff. Around 50kpa cruise at 15.8 = 2.9ms

yeah, even with seq. injection and seq. spark, i couldn't cruise leaner than 16:1. Anything leaner and it would feel down on power and effect drivability. Maybe if I had better injectors, but 16:1 is the most efficient cruise area for mpgs and that was all I was concerned with.

Save the environment. Run 14.7 like every OEM does in every section of the map except for power enrichment.

'cause NOx?

Yup. Your Cat converter is designed to operate at 14.7. I do calibrations work for a living which involves cert lab emissions. After all the reading and research I've done, I'll always preach that saving 5% on fuel efficiency is not worth the 10x or greater emissions increase.


https://www.miataturbo.net/attachmen...ine=1450113188

what if i care more about 5% gas savings vs. whatever insignificant amount of extra NOx my car makes?

would running lean, then not be "worth it"?


https://www.miataturbo.net/attachmen...ine=1450113372

Then run whatever AFR pleases you. Everyone is entitled till their own opinion.

I'm just stating mine for those who aren't aware how AFR affects post cat emissions. There's a reason why every OEM calibration runs 14.7 pretty much everywhere except WOT.

What if no cat.

i passed emissions, so whatevez.

shiiitttt. I need to pass emissions in march.

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. :party:

Sorry guys. a couple posts past while I wrote my novel. Maybe it is a sign to never write a novel. lol

according to brain's chart (cyotani - your graph cuts off data) my NOx is back down to same as 14.7. CO is lower and HC also looks about the same level. Going leaner to 20:1 does crank up my HC aggressively. Given I am also using 33% less fuel at 18:1 it does sound like the start of a win/win. hmm