All this tuning looks great...
All this tuning looks great on paper (or more correctly, a computer screen) but what does it do in the real world? To find out, we strapped our Cobra to the Superflow "lie detector" chassis dyno at Motion Performance in Winnipeg, Canada.
Spark advance is where there's power to be made, but with the greatest risk. As such, we're not just going to jump in and add 10 degrees more spark advance, then grit our teeth and grimace during WOT, hoping it holds together. In the borderline detonation spark advance table, we added a couple degrees of spark advance just about across the board for part throttle, then carefully tailored the higher Load (WOT) spark to make more power but still be safe.
If you recall from Part 2 of this series, the ECU will calculate spark advance from several independent sources, then select the lowest value calculated. One of those sources is the MBT spark advance table. So, once we were done in the borderline spark table, we double checked the MBT spark table and found it was in fact now calling for less spark advance at 6,500 rpm than our borderline table, so we bumped up the MBT values to match.
With the Cobra still strapped on the dyno, cooling off after the baseline runs, we simply flashed the new tune into the ECU using our Predator flash tool, and we were ready for more action. Using a Raptor to data log the ECU, we ensured the coolant temperature was consistent with the baseline runs, then let 'er rip with the custom tune. On the still otherwise-stock Cobra, the custom tune put us up over 17 rwhp, but with close to a 30-lb-ft wheel torque improvement (that you can really feel). A hot-lapped backup run got us within 1 hp, so we knew the results were real.
When a cold-air intake system...
When a cold-air intake system is added, the MAF sensor accuracy more often than not goes out the window. As a result, our first step in the tune for the now modified Cobra, was to get the MAF sensor function properly programmed. Using an old SCT Raptor, we data logged MAF voltage and STFTs at various rpm steps while in closed loop mode then applied the necessary corrections to the lower end of the MAF sensor transfer function. To tune the higher end of the curve, we made dyno runs and compared the commanded A/F ratio (logged) to the dyno WBO2 numbers (and did our ethanol-blend correction math). It's important to mention that while we sorted out the MAF sensor function on the dyno, we ran conservative spark timing and would abort any dyno runs immediately if we saw the A/F ratio go leaner than 12.0:1.
Modified Cobra
Modifications to the Cobra included a cold-air intake system, which changes the way air flows into the MAF meter, thus requiring some custom tuning to get the MAF calibration accurate again. The added boost from the smaller blower pulley also required changes in spark timing, A/F ratio, and load ranges. The after-cat exhaust didn't really require any specific tuning changes on its own, so it was just along for the ride.
To get the low end of the MAF transfer function accurate, we simply did a "step" test (warm engine in closed loop mode), where engine rpm in neutral would be held for several seconds at say, 1,000-, 2,000-, 3,000-, and 4,000-rpm steps, while we data logged the MAF voltage and STFTs. The result would be discrete data points showing us how far off the MAF curve was for each step. For example, at around 2,000 MAF, voltage corresponded to 1.26 volts, and our average STFTs were around 0.90. Thus, our MAF curve flow numbers needed to be increased by 1/.9 or 1.11 at 1.26 volts to correct the error. Using the other data points from the step test, we made several adjustments to the MAF curve up to 2.15 volts, and smoothed all points in between.
To get the higher end of the MAF curve fixed, we'd need to run the engine under load to get the airflow up higher (i.e., dyno runs). Again, from the data logs, for straight gasoline you could simply compare the A/F ratio commanded at any given rpm to the actual A/F ratio achieved (as reported by the dyno WBO2) and make corrections as necessary (but for us there was slightly more math involved, due to the 10 percent ethanol blend of fuel). Of course, to be safe, we'd first set the commanded A/F ratio safely rich (and commanded spark advance safely conservative), then watch the dyno WBO2 display like a hawk during the dyno runs, to be sure we weren't still going dangerously lean. If things looked like they were going lean during a dyno run, we'd immediately abort the run, then richen the MAF curve as necessary before having another go at it.