With nothing else changed...
With nothing else changed but our custom tune on the otherwise stock Cobra, we picked up an additional 17 rwhp and 30 lb-ft rwtq.
Once the MAF transfer function was corrected for the CAI, we could go back to the tune in search of more horsepower. First, we would rescale the fuel and spark advance tables to extend the load range out to 200 percent (load increases with the added boost), giving us more resolution and a greater useable range on the tables. The procedure to do this was previously outlined in Part 2 of this series.
The next thing changed was to increase the default load at sea level table. This table is used to infer load from rpm and TP if the MAF sensor is judged to be out of range, which could happen on a dense air day in our case, since we're close to pegging the stock MAF sensor with the added boost. Based on the actual load values data logged during the dyno runs, we bumped up the values in the table by around 25 percent.
With the housecleaning stuff covered, we could then tweak the fueling. For the A/F ratio, we prefer to run the engine a bit richer with the added boost, in order to provide some additional charge cooling effect, and to give us some more margin against detonation. So while we ran the stock Cobra at a commanded 12:1 A/F ratio for high loads, now we'd lower the commanded A/F ratio to 11.8:1 at the higher loads and higher rpm cells of the base, and stabilized OL fuel tables (this would show up on the dyno as A/F ratios in the low 11s for our 10 percent ethanol fuel).
For spark advance, we needed to completely rework both the borderline and MBT spark tables (due to the rescaling), ensuring reduced commanded spark at the higher loads. At the top of the tables, we limited spark advance to a maximum of 20 degrees, then added a bit more advance at the lower loads. On the dyno, we snuck up on the spark advance, shaping the power curve with the spark timing table, without adding enough spark advance to make us nervous with pump gas. Had we decided to use race fuel, we could have commanded a bunch more spark advance and likely made significantly more power, but this was to be a pump-gas/street tune.
Another thing we looked at carefully were the borderline spark modifiers, based on ACT and ECT. We wanted to make sure that if the inlet air temperature (after the supercharger) or coolant temperatures ever got too hot, the EEC would dial out the appropriate amount of spark timing to guard against detonation.