We have a love/hate relationship with our Project tC. We all seem to love hating it. From the beginning, I've cursed the tC for its lack of an LSD, excessive torque, sluggish throttle response and susceptibility to heat soak. The sentiment lasted until I took it to a NASA Southern California Chapter track event. Although these drawbacks became ever more glaring with every 30-minute session of track-beating, Project tC's brilliance in coping with the abuse made all its flaws seem relatively minor.
I spent three laps at Buttonwillow Raceway chasing down and catching some showboating yahoo in a Boxster. After an Evo MR, the plain-Jane Boxster was the fastest in our run group and I had finally worked through the traffic to get on its bumper. It was dead even on the straights, but the Boxster would pull away through every high-speed sweeper on account of its better balance.
Project tC, in contrast, had the upper hand in mid- and low-speed corners with its superior braking and instant torque, despite always spinning the inside wheel. The chase was on as we played cat-and-mouse, corner after corner. Every bend's exit where our tC had the upper hand to pass led frustratingly into a no passing zone. For three more laps this continued until I finally got the point-by and started to pull some distance from him. That's when I decided Project tC was near completion.
Although my victory over some Kraut rocket is fresh in my mind, I'm back to hating it while driving on the streets. The rear suspension bottoms out all the time, the supercharger heat-soaks and the notchy shifter and throttle make smooth driving impossible. Only its track manners have validated our work so far. So let's focus on that instead.
More Boost
When you look at the compressor map of the Vortech V5 F-trim supercharger TRD used, it's clear that, at 7psi, the blower isn't in the optimum efficiency range. We don't blame TRD's engineers for choosing this figure since they have to make the kit smog-legal and reliable enough to warranty, but we had a hunch that, with more boost (a higher pressure ratio), the supercharger would operate with more efficiency, especially in the mid-rpm range we use most.
To test this out, we increased the boost with Zero Point Industries' (ZPI) billet aluminum 9psi pulley, which bolts on in a matter of minutes. The smaller-diameter drive pulley spins the supercharger compressor wheel faster per engine rpm, which essentially provides slightly more boost at any engine speed. The tC's hydraulic tensioner makes changing to a shorter belt unnecessary. With just a new pulley, our tC now makes 219 wheel-hp and 189 lb-ft of torque. That's a gain of 17 horsepower.
To verify this, we plotted the operating line of the two pulleys on the compressor map. Supercharger efficiency is defined by the ratio of input temperature to output temperature. So let's say, for example, the air entering the supercharger is 90 degrees F and after it's compressed (where some energy is stored as heat) it comes out at 100 degrees F. Divide the temperature in by temperature out and you get 0.9, or 90 per cent efficiency. Unfortunately, even the best centrifugal superchargers can only reach 80 per cent. Refer to the compressor map for the V5-F supercharger and you'll see that it peaks at 76 per cent. That's basically where we want to be.
Compressor maps are basically graphs that show how efficiently the compressor is operating for a given airflow and boost (calculated as pressure ratio). The horizontal axis is the airflow in terms of mass flow rate, like pounds per minute (lb/min), or a volumetric flow rate such as cubic feet per minute (CFM).
On the vertical axis is the pressure ratio, which is a ratio of the absolute intake pressure (roughly 14.7psi for ambient) divided by the compressed absolute pressure. Manufacturers draw little ovals, called efficiency islands, to show how efficiently the compressor would operate at a given flow rate and pressure ratio.
Most designers size a turbo or supercharger by looking at its peak operating point, i.e. the maximum flow and peak boost, to see if the compressor is operating efficiently. The difficulty with mapping a crank-driven centrifugal compressor (supercharger) is that it builds boost exponentially as the revs climb. Unlike a turbo, which spools up, boosts to a regulated pressure and holds it, a centrifugal blower will keep increasing boost to the redline. And not in a linear fashion, either.
To see how our compressor is doing throughout the rev range under real-world conditions, we have to measure three variables: boost pressure, airflow, and rpm. With newer cars, this turns out to be easy, since the ECU reads all these values and spits them out through the OBDII port. In theory, you can plug in a scan tool and read the values as you make a pull on the street or on the dyno. Theory never works out quite right, though. Government mandates only stipulate that the OBDII port communicate at a woefully slow rate, since it's primarily used for diagnostic purposes. It will refresh maybe once per second, which isn't very useful as Project tC will make a third-gear pull to the redline in about eight seconds. So we resorted to measuring boost with our own data logger and a MAP sensor connected to the fuel pressure regulator vacuum line, since it's really the only accessible vacuum source available. Airflow is a little harder. You can't just splice in and data-log the voltage coming off the MAF meter, because there's no way to accurately convert voltage to airflow. We begged the very helpful engineers at TRD to lend us their proprietary factory-issue scan tool that not only allows us to read the ECU, but also logs and exports the data for us to chart.