While the corrections by the dyno software do a good job to compensate for ambient air temperature and humidity, it can never totally accounts for all the factors. For instance, if a car is baselined on a cool morning, but gets tested again in the hotter afternoon, the dyno will compensate for air density. However, it has no idea what sort of intake manifold temperature the car's ECU was registering. And in many cases, the ECU will retard ignition timing, thus reducing power, if the temps are too high. This is further compounded because the air coming out of the intake manifold has less oxygen to burn (something the dyno again can't know), hurting power even more.
To accurately test our car...
To accurately test our car we needed MKC Performances' MD9001 Snap Tool to monitor parameters like intake air temp and ignition timing
Years ago, we performed an extensive exhaust test on an E36 M3. All exhausts tested in the morning showed gains over our baseline, but not those tested after lunch, until we established a new "afternoon" baseline.
We also saw this after installing the UUC underdrive pulleys. When tested at the end of the day they yielded no gains against our crisp morning baseline. But thanks to the workshop's SnapOn MD9001 scanning tool, we interrogated the ECU and discovered the ignition timing was retarded 5 compared to the morning baseline. When we returned the following morning, the ignition timing was restored and so was the horsepower.
These factors come into play very often, but many dyno operators don't take the time to properly test an upgrade. They assume it provided no gains, when it's not always true.
So when we did our tests, in addition to using the shop's SnapOn scanner (and testing in similar weather) we also installed an SPA Technique dual gauge for oil and water temps. We got it from BimmerWorld and it's claimed to be accurate to the nearest 0.1F. While we could already read coolant temps from the scan tool, the ECU doesn't register (or respond to) increased oil temps. And since higher oil temps can hurt power, it's important to monitor them on the dyno. In addition, if the oil is over about 280F it can prevent proper engine lubrication, so it's smart to monitor it in any high performance car.
Hopefully, these pointers will give you an idea of what to look for when your testing parts on the dyno. Sometimes, parts just don't work but you'll only know that's the case if you haven't overlooked any of these common dyno testing problems.
The Peake Research R5/FCX-3...
The Peake Research R5/FCX-3 fault-code scan and reset tool (left) with the SRS scan and reset tool (right) and code book
Read Cels
Unless you plan to throw money at the dealer every time you get a Check Engine Light (CEL), the Peake Research R5/FCX-3 code reader and reset tool is a must-have for any BMW enthusiast.
Each time your Check Engine Light comes on, which can be caused by several dozen things, you'll be able to quickly diagnose what triggered it (with the supplied code book for all BMW models) and reset it.
The company also sells the SRS fault code reader for the airbag system. On high mileage BMWs it's not uncommon to have airbag failures, or even see the airbag light for no apparent reason. This tool will allow you to see what's really going on.