Dyno vs Roadtest A/F Ratio

[martyb]

I have that with my newly installed wideband, at WOT, I am showing about 11.8 A/F with my 94 Roe car. On the dyno sheet from PBJ, its more like 12.0 to maybe 12.2. PBJ tune was done on a 70 degree day, and I was on the road yesterday with the temps in the 50's. I would assume the car would lean out a bit with the cooler temps, so I was surprised to see 11.8 on the guage. Is there that much difference from dyno to the street?

marty

 

[ViperTony]

On the dyno, what did they use to measure your AFR? Did they use your wideband sensor, their wideband sensor or did they sniff via the tailpipe?

 

[martyb]

tailpipe sniffer

 

[BOTTLEFED]

The difference between where your WB sensor is and a sensor at the tailpipe could be the answer.

Do you have a VEC2 or VEC3?
the VEC3 has an IAT correction so the air temp would be accounted for, otherwise, it should be leaner at low temps if you have to correction

also the load on the dyno is going to be different, so AFRs can vary with different loads

 

[Jack B]

I have that with my newly installed wideband, at WOT, I am showing about 11.8 A/F with my 94 Roe car. On the dyno sheet from PBJ, its more like 12.0 to maybe 12.2. PBJ tune was done on a 70 degree day, and I was on the road yesterday with the temps in the 50's. I would assume the car would lean out a bit with the cooler temps, so I was surprised to see 11.8 on the guage. Is there that much difference from dyno to the street?

marty

Here are some of the issues:

1. The viper PCM is map based therefore, it doesn't matter whether you are on a dyno or road testing. The exceptions would be very high speed and/or heat sinking.

2. A tail pipe snifer is always going to show leaner. In addition, widebands used by dyno shops are notoriously inaccurate. The dynojet wideband can be almost a full point off.

3. As was stated, if you are not using a VEC3, the viper PCM has a bad habit of adding fuel when the temps get higher, theory says it should be the opposite. That can also throw you off a full point.