Background: This NC and was purchased used. It is an auto trans car. Stock performance was as expected: somewhat higher acceleration and stronger midrange than my stock 67k miles '99 NB and very considerably stronger than my girlfriend's 103k miles NA. I soloed and tracked the NC. It was responsive and seemed to have plenty of power. I did not dyno it before making changes.
In September 2011, I installed a Cosworth supercharger kit and AEM water/meth injection kit at 37k miles. There were no important problems during the install, just some minor fit issues.
After the install, the car fired right up, still on the original tune. It idled fine and "blipped" just like it did before the install. I took it out for 12 to 15 miles at slow speeds, without kicking in the boost. There were no apparent problems. The car seemed just as before except intake noise was noticeably "hissy." During this first outing, I went once to around three quarter throttle from a roll in second gear to make sure the supercharger actually made some boost. Whoa!!! Rocket car!!! INSTANT SPARK KNOCK!!! Stop that instantly! Go home. Start the tune process. Very happy and expectant.
I contacted calibrator who provides a tune with the kit and received the first tune via email. Installed the tune (with minor software hassles thanks to Windows Vista).
With the first tune, another new car! But this one wouldn't pull the hat off your head. Very, very conservative. No spark knock, but I was back to stock (or less) performance, even though an Autometer mechanical boost gauge showed it making 8 lbs.
I data logged the car (with another $500 gadget), sent in the log and promptly received a second tune. Loaded the tune. No noticeable change.
I took the car to a dyno. It dynoed 153 HP on the first pull, when cool, 142 on the second. Stock NC's do that. Must be the dyno.
I took the car to a second dyno. It dynoed 142 then 140 HP with the dyno sensing 10 lbs. of boost.
The tracing of ALL DYNO PULLS ON BOTH MACHINES showed high variability within the horsepower and torque curves. In other words, the overall shape of the HP curve was about what you'd expect: rising steadily until a peak near 6.3 k RPM with a gradual drop off beginning thereafter. But, within that nice, smooth HP curve, the individual data points were highly variable around the central trend.
I talked to both calibrators, the guy who actually wrote the tunes and the guy who ran the second dyno. Both suggested the boost might be "blowing out the spark" under load. They suggested I switch to a colder plug with an smaller gap.
I installed fresh NGKs gapped at 30 thousandths. When I took the car back to the second dyno, it pulled 151 HP, then 139 HP on the fresh NGK plugs. Cut off the water injection, same numbers (Water alone apparently does nothing.). The same variability in the HP and torque curves is apparent. The curves MAY be a little smoother.
The original calibrator sent me another tune with 4 degrees of spark added. I've loaded that. Haven't yet made another $75 dyno pull to see the results. But seat of the pants tells me nothing is changed.
The car drives fine. On back-to-back 2-18/19-2012 track days (with 450 lbs of driver and coach aboard), I ran the car hard all day in 20-25 minute sessions without incident: no knock, no overheating. It pulls smoothly in midrange and runs out of stuff around 100 mph -- just like before. $6,500 later, my over all impression is still that midrange is somewhat improved over the bone stock used car I bought and the supercharger sounds cool when you stand on it from 3k RPM up. But it makes about the same HP as an NC with a cold air kit and muffler.
Constructive ideas, anyone?