[wparsons]

Originally Posted by nikitopo

I don't think that this difference is because of a dyno error. Keep in mind that if you do multiple runs on a dyno, then the actual air flow from the external fun is about 10 mph (and not 80 mph that you might have on road). This means that the radiator, oil cooler, and just about everything under the hood gets pretty hot. This heat reduces power in a number of ways. Not only is the air less dense, but like most modern cars the ECU pulls out timing. The coolant temp starts to rise, and sometimes that kicks on the cooling fan which sucks almost 3-5hp right there.

That's assuming the power kept going down run after run, which is hardly ever the case. 3-5whp spread on multiple runs is actually pretty consistent.