I'll provide some thoughts on the mods and results below. As usual, each dyno sheet is showing runs from the same day with equivalent coolant, oil, and intake temperatures. This will provide true before and after results.
Step 1 - Headers, Off-Road Pipes, and Plugs:
The dyno sheet below shows some VERY interesting results from my Kooks header installation... no power gains! This was a bit disapointing at first, but simply proves that the factory manifolds and catalytic converters provide sufficient flow for ~600rwhp. This behavior is not uncommon on supercharged vehicles; we see it with Mustang Cobras and GT500s too. Blown motors do not experience the same benefits from improved exhaust scavenging as naturally aspirated vehicles because the supercharger is pushing the air through the motor. The exhaust simply needs to provide a hole big enough to let the air out.
Step 2 - Tuning for the Headers:
Though the headers did not realize an increase in power, they did provide a reduction of back pressure as seen in the slightly lower boost levels. This provided an opportunity to adjust the tune to gain some power as seen in the chart below. After the tune there was an increase of 15whp and 15wtq above 4,000rpm. These are certianly nice gains, but the cost of this power was quite high: $1,200 + install time + tune session. At this point in the build, my recommendation to ZL1 and CTS-V customers who have done my previous "Stage 1" mods would be to not invest in long tube headers unless further mods are planned...
Step 3 - Injectors:
My experience with many different injectors led me to Injector Dynamics I850s for the ZL1. These are not the cheapest injectors available, but the quality and precision of the injectors and injector data is second to none. I often recommend these injectors to customers, because injector data is something a tuner cannot make-up or backwards engineer. There are multiple vendors who provide good quality injectors with data, but many who do not. The dyno graph below shows two pulls which are nearly identical... this is because the only change to the car was the installation of the ID850s and the only change to the tune was the injector data provided by Injector Dynamics for HPTuners. You can see the variance between the pulls is within the range of "run-to-run variance." Most low cost injectors would be unable to match this level of precision! This is why many tuners will not tune a vehicle without proper injector data.
Step 4 - 10" Lower Pulley + Tuning
My final modification for "Stage 2" of the ZL1 build was to install a 10" lower pulley. I previously installed the Innovators West Harmonic Balancer with Metco 8.6" lower pulley which provides ~2lbs extra boost over stock. The 10" lower pulley provides ~5lbs extra boost over stock. This large pulley does require an idler relocation bracket, but the installation is very straight forward and reuses the factory idlers. The tune was updated to compensate for the additional boost.
Note: the before and after pulls were not done on the same day although the operating temps were monitored to provide as much consistency as possible. Mid way through the installation my wife called with a "slight emergency." Long story short we ended up at the hospital for the birth of our first child. Some distractions are quite worthwhile :)
As you can see in the dyno sheet below, the gains from this pulley were very large: +48whp, +86wtq, and +4lbs boost. The torque increase was much higher than expected and the horsepower increase is on target. Taking a look at the dyno sheet you can see the boost spike above 5,000rpm. This boost spike and the ramp-down in the torque curve is indicative of a restriction in the system, which can be cured with a blower swap and/or upgraded cam ;)
All in all I would consider the "Stage 2" modifications a great success. The header swap did not offer much increase with the "Stage 1" modifications, but I feel the reduction in back pressure was a stepping stone for the big gains seen with the 10" lower pulley. In other words, I would not want the pulley without the long tubes and the long tubes are not a worthwhile investment without a large pulley.
With the installation of a lower pulley, idler bracket, belt, cold air intake, injectors, long tube headers, off road pipes, boost-a-pump, heat exchanger, and of course Performance Dyno tuning the ZL1 is up 150whp and 183wtq over stock!!