That isn't really how engines work...

You can tune for power and efficiency by optimizing the fuel and ignition curves, but the factory maps are pretty close with an err on the safe side - especially with electronics nowadays that constantly revise the mapping for given variables.

Power is gained by leaning out the fuel mixture and advancing timing, but this isn't a "10% across the board" sort of thing. It has to be tuned for every RPM and throttle position under the sun, among other things. It could be 10% less fuel in one spot with 2 degrees of timing advance, 5% less with 3 degree advance in another, etc

I'm sure he was just oversimplifying things for you, but saying he only added 10% of the power is very misleading. It should be described as "optimizing the fuel and ignition mapping allowed for a 10% increase in power."

But even then, he would have no way of knowing how much power was gained unless he did before and after dyno runs...

Frankly I doubt there was anywhere near a 35hp gain. Turbocharged vehicles are the only ones that can get that sort of gain from tuning, and that is because they can increase the amount of air going through the engine by adding boost.

__________________
12 X5 M - 06 X5 4.8iS - 03 X5 4.4i - 03 M5 - 02 X5 4.6is - 99 M3 Dinan S3 - 98 M3 - 92 850i-6 - 92 850i - 91 325ix - 89 M3 2.5 - and a few parts cars