Rob's Impala SS

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Initial Runs: 09/02/2004

Today's G-Tech testing had a pair of firsts: the first front wheel drive vehicle, and the first supercharged vehicle. Rob's 2004 Impala SS has a supercharged 3.8 liter V-6, where the previous Impala SS had a 5.7 liter naturally aspirated V-8 with RWD. Regardless of the powertrain differences, both cars are equally fast, and it was very interesting to test the new Impala.

In addition, Rob loaned me his OBD2 (on-board diagnostics 2) data logging device. This device connects to the OBD2 port and logs data directly from the car's ECU and other sensors. We have a nice fat log file of data, but it's raw and very hard to correlate. I will look at the data carefully and put out an update comparing the G-Tech's RPM and speed accuracy versus the logged OBD data a bit later. The logs are available in the GTP file archive for these runs.

Acceleration:

We performed three instrumented acceleration runs, but no power curve runs since Rob's automatic transmission cannot be locked into a particular gear. Although the Impala has pretty wide tires (P235/55WR17s), it was immediately apparent that a supercharged engine and a FWD car with an open (non-limited slip) differential means lots of smoky wheelspin and fat black lines left on the road! Woo-hoo! The first and last runs started with a bit too much wheelspin and produced a 15.6 and 15.7 second quarter mile. However, on the middle run Rob launched very cleanly with no wheelspin and netted a 15.1 second quarter mile at 93 mph!

Rob's first launch was a flat launch without brake torqueing, while his second and third were brake-torqued. Both launches that had some wheelspin resulted in 0-60mph times of 7.7 seconds, while the clean launch gave a nice 7.1 second time. These times are very much in line with published figures for the Impala SS of 6.8 - 7.1 seconds 0-60, and quarter mile times right at about 15 seconds flat. We'll look at the launch results and techniques in more detail below.

Power:

The Impala SS is rated at 240bhp @ 5200rpm and 280lb-ft of torque @ 3600rpm, and due to the supercharger the power and torque delivery is immediate and remarkable flat/non-peaky. The maximum measured power was 171bhp @ 5100rpm, and 201lb-ft of torque @ 3600rpm. Using a calculated 15bhp aero drag loss and a 20% AT driveline loss, the adjusted figures are 223 bhp and 260lb-ft of torque. These figures are right on the published figures, especially since I'm not (yet) adjusting for the ambient temperature and pressure. Since today was a nice warm day it would lower the output figures artificially.

Technique:

The main problem we ran into was that the powerful and torquey supercharged V-6 overwhelmed the front tires. Briefly, the challenge launching a FWD car for an acceleration run is that as the power is transmitted to the road, the car's inertia resists the acceleration. Since every suspension has some level of travel built in, this means that the weight of the car travels or transfers more to the rear of the car as the nose pitches up and the rear squats down. For a rear-drive car this is ideal since it places more weight on the rear where the power is delivered, but it is the reverse for a FWD car.

So, Rob tried a variety of ways to launch the Impala. He launched flat on the first run, and since it is an automatic he brake-torqued briefly on the second and third to try a different rpm launch. On the first and last launch he went a bit too deep in the throttle and got a lot of wheelspin, where on the second launched he hooked up very well.

Based on these results, especially given the launch speed graph, the fastest way to deliver the power should be to brake-torque the engine to get the RPM level up earlier. However, this means that Rob has to release the brake and feather the gas at the same time (at different rates), and is much harder. Done smoothly it should deliver the fastest time, but it also requires the most precision. An easier and nearly as effective launch would be to smoothly roll onto the throttle from a stop without using the brakes, and try to minimize wheelspin at all costs. With an open diff any wheelspin immediately drops the acceleration significantly.

More Info:

As discussed above, the primary requirement for a fast run is a smooth, no-wheelspin launch. The automatic transmission in the Impala made the shifts perfectly, revving the engine up to the power peak but shifting as quickly afterwards as it could while still dropping the RPMs (after the shift) perfectly on the torque peak. The shifts were also pretty quick at about 0.6 seconds, but there is a TCU reflash that Rob is considering to speed up the shifts. Fun!

Adjustments to be made to the run data include: