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The Correct AFR for your Lotus

6K views 11 replies 7 participants last post by  rwbakk 
#1 ·
#2 ·
Especially with your access to the ECU, would it be possible to use a wideband O2 sensor with analog out to force the ECU into closed-loop at WOT and target a richer setpoint? Wideband gauges can have a programmable output, so it seems like one of those "how hard can it be" kinda things.
 
#3 ·
@turbophil - Great article!

After reading this and some past comments, I’m concerned my OEM setup is running perpetually lean. I have a 2011 Elise SC and most of my operations are at 6,000’ - 10,000’ MSL with occasional excursions to 4,000’ and 12,000’ MSL. It’s not very precise, but cruising at 4,000 RPM, I get 30 MPG at 9,000’ and 35 MPG at 12,000’. That seems pretty lean to me.

I know the MP42 with no intercooler is a dead end for more power, but is the AFR something BOE could address on my vehicle?

Glen
 
#4 ·
^ That's normal and is not necessarily lean. Air density decreases aloft, so the fuel injectors are scaled back to maintain ideal AFRs (this is not lean since there's less air being compressed). You'll see similar MPG results driving in extreme cold versus extreme heat at the same elevation.

The only problem is you're making less power from reduced compression at higher altitudes. Just be glad you have forced induction to help out!
 
#8 ·
^ That's normal and is not necessarily lean.
I’m a pilot and quite familiar with leaning out the mixture as altitude increases. But Phil’s document indicates the (OEM?) system tends to produce leaner than desired mixtures as altitude increases. It’s probably not a problem for most drivers, but I’m routinely two miles above sea level and occasionally as high as 14,250’ MSL (bonus points for figuring out where - I’m 20 miles away).

Glen
 
#5 ·
For general tune and running verification, you’re interested in a single operating condition, which is steady state WOT. You’re not interested in AFR at idle, as the ECU is forcing the idle to be at stoich through its closed loop operation using the primary NB02. It is common for a WB02 to not have great idle AFR data and will often show a lean indication at idle. This is due to a variety of reasons. Do not be alarmed if you observe a lean idle on your WB02 as it likely erroneous. You can verify your idle AFR with a scan tool and monitor the factory NB02 and the corresponding fuel trims. The idle AFRs are not a common problem and generally not a tuning issue due to the closed loop operation.
At idle, my AEM Wideband creeps up and off the charts pretty quickly and goes to --, also the exhaust gets really loud (like I have a 67 Hemi GTO or a small top fuel dragster) and when I touch the gas the AFR pulls right in to normal ranges (goes green) and it sounds smooth as butter. Does this sound normal?
 
#6 ·
It sounds like your sensor is too close to the exhaust tip which allows outside air to corrupt the signal when the exhaust pulses are spaced out. Sometimes you have to just run open loop at idle for this reason, especially if you have no cat between the sensor and outside air. This is why I always cringe when I see shops put a sensor in the exhaust tip to do dyno tuning. Odds are that reading is garbage at low-ish rpm. Or you could just have a calibration issue.
 
#7 ·
Running PPE headers (SC) with their decat. Sensors are in their bung holes. I may try it with the sport cat instead of the decat and see if it changes anything.
 
#12 ·
'I have a 05 Elise with a BOE REV300 and operate a 4700' in elevation. If I install a smaller blower pulley to get my boost up to the same psi as a sea level car will my AFR read the same as a sea level car? Currently it runs just under 13:1 under boost all the way up to redline, witch scares me.
 
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