Warlord makes a valid point that measurements have to be consistent.
The setup at Hilltop is that the temperature and air flow rates are the same for the control run and assessment run. The same operator does the runs on the same Dyno about 40 minutes apart. A bike's two runs are usually consecutive, so there are no changes to the Dyno setup. For transparency, you are invited to be in the Dyno cell standing beside Geoff the whole time. There are also CCTV cameras in the Dyno cell so you and others can observe from somewhere more comfortable, if you wish. The only variable is barometric pressure which, unless there is a significant front passing through between runs, would have a negligible effect (I think the barometric pressure is shown for each run). As the Dyno beds is, it may show slightly improved figures, but most people seemed to only be interested in the before and after for their own bike, as it's the only one they ride and not against other bikes measured months before or after.
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