Stressful week

Went to dyno tune an E9 GSR earlier this week.

First, a baseline pull before dyno tuning... came up to 400 HP. Fine. So I spend the next hour or so trying to get the fueling just right, making sure there's no detonation etc. After this the car makes another power run. 350 HP! I immediately load the map before the dyno tune to see what's wrong. Logically it should also produce close to 400 HP, or the same power it came in with.

Nope, it still showed 350 HP! All temperatures were normal between runs.

At this point in time it was getting late and the owner decided to bring it back and refuel. He pulls into the gas station and we hear something similar to brake caliper pistons getting knocked back into the seats. i.e. if the calipers seized they might be preventing the car from accelerating, hence explaining the loss in power. So we head back to the workshop and take it apart, but we find nothing. The pads are in good shape too. :(

The next day, the car went onto the dyno again, and another baseline power run. Now it is 340 HP! Loaded back the other dyno-tuned map, it is still 340 HP. At this point in time the owner made a decision to just trust what I've done so far. OK... so I mess around with the map a little more, taking out a little bit of fuel and advancing the spark in small increments. Nothing too radical. After about an hour I make a couple of runs. The dyno shows 370 HP. The owner decides it's enough and make a final power run. Here is where it gets weird. Final numbers are 420 HP @ 7000, peak torque of 440 Nm @ 6000. We take the car out on the street and it does seem to have a much better pull, especially on 4th and 5th gears.

In summary, we went from 400 to 350 to 340 to 420 HP. Very strange. All tuning and power runs were done in 4th. The ambient corrections were either 2-3%. I can understand the 350 to 340 HP drop since conditions may vary enough such that a 3% loss in between runs, while significant, is possible. But the 50 HP drop is still unexplained, same goes for the 340 to 420 HP gain. Could it really have been the brakes? It also doesn't explain why I saw 370 on the dyno yet the print out showed 420 HP.

From the figures, the following would seem to make more sense. 400 HP via some conversion factor becomes 350 HP. Net gain of 20 HP gives 370 HP. 20 HP gain applied to original 400 HP yields 420 HP. Comments?