Last week I went to a dyno to test my self tune. During the dyno session they logged my AFR and thee boost. My car is an EVO 8 and I have a JDM MAP Sensor (from mrfred) and an AEM UEGO WIdeband. I log the boost and AFR with EvoScan. I made logs of my dyno runs and when reviewing my logs I notice a difference between my logs and the dyno on both measurements.
My JDM MAP read a max boost of 23.0 and the dyno said 21.1 (BTW the peak feature on my STRI boost gauge was reading something close to 23.7, but compared to my JDM MAP logs that gauge is reading high by about .5~1 PSI). On the same run the AFR on my AEM logs was 11.6 and the dyno screen I read a 11.9.

On a separate low boost test run I saw a 18.19 PSI on my JDM MAP and the dyno read 16.58 PSI. Have anyone seen big differences between their readings and dyno? Based on past experience which gauge should be more reliable?

 

Depending on where you tap the boost from on the intake manifold you can get a variety of different boost readings. For instance when I dynoed my DSM I street tuned it to 36psi. On the dyno, where we tapped for the PSI, it was reading 32psi tapering to 29 at redline.

Since the JDM map is in the car all the time and its location isn't changing use it. Boost is just a number. It doesen't really matter what the number is other than when you increase it you can see it and when you decrease it you can see that as well. If you have settled on a 22psi tune on the JDM map then stick with it.

I don't always trust the dyno widebands. They typically stick them in the tailpipe which isn't the most accurate. Also lots of cars go to the dynos on leaded race gas and its up to the shop to replace the sensors. Most shops don't really care enough to keep the wideband sensors on a good rotation schudle. Even so a 0.3 difference really isn't that much. Also how old is your wideband sensor? Its up to you to keep that one in good shape and everything calibrated.

 

also remember that if your altitude/boost adder was slightly off your readings could vary from what the gauge was set to at the dyno

 

They tap in one of the lines that connects the manifold to something close to the FPR I have to check what exactly it is. It is just a vacumm line about 3 inches lone that connects the manifold (end left area to something attached to the left side of the manifol). Unless the JDM is off I think it is in a better location since it is more towards the middle on the manifold.

I installed my AEM UEGO back in March 08 and it has about 1500 miles on it. I do not know if the widebands for the AEM can be calibrated. I did run it for about 1K miles on the stock rich tune (<10 AFR) but always with pump 93 oct gas. Maybe the peak diff was a lill higher than 0.3 but since I do not have the exact log of the dyno plots I can tell. For sure it was less than 0.5 diff.

The guy at the dyno told me he changes the wideband on the cars in about a month. But I think he was just talking, blah blah. He also told me that my car should be in the low 300 WHP with a good tune and it was on the 266 HP. Maybe he was just trying to sell me a tune.

 

Just use the ones on the dyno as reference but don't worry about their accuracy too much. Your wideband and JDM once properly calibrated are going to be accurate enough.