Rolling road tuning

Having access to a chassis dyno is vital, if you want to know if you are improving the performance of your engine.
A 205 gti road rally car recently came to us for some checks following a head gasget replacement. A skim of the head was required and the owner wanted to make sure this had not upset the previous settings.
A base run to check fuel ratios, detonation, and power output showed a healthy spred of power but with some detonation. Slightly rich mixtures across the rev range and no thermostat resulting in low operating temps.
Adjusting the fuel pressure reg showed increased torque readings with slightly less fuel pressure. However these 'gains' were reduced by retarding the ignition timing to prevent detonation.
Repeating power runs with a set operating temp closer to 86 degrees C than the previous 58 degrees C showed a slight loss due to heat soak.
Advancing the cam timing showed a little improvement but not enough to keep testing.
Rolling road time is expensive and knowing when to stop is important, otherwise repeated tests cost more than they bring in improvements. But if you want to wring the last few bhp or lb/ft out of your engine you have to test all the possible settings to see if they can make the difference.
The importance of repeatable results is paramount, otherwise you are wasting your time.
We use a dyno that gives often depressingly accurate results time after time, with the ability to overlay previous runs to where or if you have made that improvement.