Here is another case if where not properly tuning or understanding how PCM functions costs how engine functions which effects performance

In this real world case a C5 Corvette using 18 PSI of boost was having engine knock problems even though the owner installed and ported water/methanol so that there was a nozzle for each cylinder that flows as much as 3 gallons an hour per cylinder.
They are using 70 plus lb/hr injectors and still had real lean conditions by only 4,000 RPMs.

Owner had PCM tuned by others and problem still existed.

Owner sent us a scanner recording and our results showing fuel trim values ( top graph) was as lean as 13% even with those large injectors.

Owner sent us the PCM flash file we then analyzed to find the injector offset table values incorrect for the injectors used.

We corrected that and the fuel trims went from real lean to at least 10% rich which again shows that values for offset that are wrong will cost more fuel being used as PCM was trying to drive the lean trims values back down and by properly making the adjustments for those injectors that we then not only gor rid of the lean conditions but was still able to reduce injector flow by 10%.

Bottom graph is after base tune, after owner does a drive relearn we will do a final tweak to lean up even more at the low engine loads.