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Attn: NSX'ers with A/F gauges

13 September 2000
Encino, CA

I'm hoping to hear about the experiences that other NSX owners have had with the readings from their air-fuel-ratio meters. There's probably not too many people out there with them so if you have one I hope you'll post

I've had a digital A/F meter installed that reads off one of the two factory (primary) O2 sensors. After recently installing a twin turbo kit and working to dial in the fuel, I've hit an issue with the A/F readings.

In a nutshell, I can increase the fuel during wide open throttle (WOT) and not see any change in the A/F reading - it seems to have hit a ceiling. Back before the TT kit, when everything was stock, I saw average readings of .85 to .86 at WOT, and as high as .88 to .89 when I went WOT in 6th gear at 75mph (lugging it a bit). Now with the TT kit and the adjustable fuel system, most of my WOT readings are still in that .86 to .89 range, regardless of increasing fuel.

I *know* the extra fuel is having an impact, because the exhaust gas temperatures (EGT's) dropped substantially as fuel was increased. We went from hitting 800 degrees Celcius at the top of 3rd, to hitting around 740 degrees, by adding extra fuel. Yet there was no difference with the A/F voltage readings.

On other turbo applications, usually the range is somewhere between .92 and .96, and being under .92 is typically considered too lean for a turbo car. That's part of what has me concerned. Maybe NSX O2 sensors just scale a little lower?

Anyways, my questions to those of you out there with A/F gauges and modified NSX's.. What readings have you gotten at WOT? Do you get beyond .90 volts or do you hit the same ceiling? Did you attempt to tune to a specific reading?

At this point I plan to tune mostly via EGT's in 3rd and 4th, since higher gears are where you build up load and blow engines, and typically if you tune the car in for 3rd and 4th, it'll run just fine (perhaps even more safe/fat) in 1st & 2nd. It would be nice to be able to use the A/F as a meaningful secondary tool however. And it sure becomes tough trying to tune the car in 3rd and 4th considering how fast it gets up to 120. I'd like to keep my license.


[This message has been edited by SpeedDemon (edited 19 May 2001).]
I had some digital A/F gauges installed for
the NOS recently. I must have different gauges than you because mine cycle until I go
WOT. I can show you tomorrow at the drive.
They basically cycle back and forth meaninglessly until I hit WOT (for NOS). Then, they actually work and hover in the Rich/Stoich range. It doesn't provide me with actual numbers as it just lights up a range. I'll show you what I'm talking about tomorrow. In my case, that's good enough as I just need to make sure they don't show lean while I'm releasing the nitrous.
Actually, I thought they weren't working because they were cycling back and forth. Larry said that it's normal so I guess that's how they work... just a bit strange to me. I'm glad I mounted them hidden in the glove box, otherwise the car would look like KIT with those 2 gauges going like christmas trees all day.
The unfortunate reality is that most A/F gauges aren't very accurate. They are generally O.K. for getting the tuning in the right neighborhood, but once you are close, you should put the car on a dyno and fine tune it with their equipment.

Marc, I don't know why yours is giving you the problems that it is. Perhaps you should give the manufacturer a call and make sure it operates in the same voltage range as the NSX 02 sensors.
Originally posted by David:
Marc, I don't know why yours is giving you the problems that it is. Perhaps you should give the manufacturer a call and make sure it operates in the same voltage range as the NSX 02 sensors.

I have actually seen it go into the mid 90's when tapping quickly on the throttle & such. Just not when I've gone WOT, which simply might mean I've got to dump some really excessive fuel to get there. The gauge I have by the way is basically a digital voltage meter, it's simply reporting the voltage coming from the factory O2 sensor.

Ilya - it is normal behavior for the A/F gauge to fluctuate back and forth like you described. When the car is in closed loop (medium, little, or no throttle) the ECU cycles it like that. When you mash the gas (WOT) is when you'll get a consistant reading without major fluctuations. Then the ECU isn't playing with the fuel, it simply dumps the amount it's been programmed to.