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.
ZPI also makes a billet aluminum crank pulley, which takes some of the rotational inertia
We plotted the operating points for every one-second increment for both pulleys, in addition to data-logging the ignition timing. Although the car made more power on the dyno with higher boost if the ECU pulled out more ignition timing (on account of knock activity), the gain in power would not be worthwhile as it only gets worse as the supercharger heats up. Looking at the two operating lines on the compressor map, you can see that although we're still in the same efficiency island, the pressure ratio is much higher, and we are slightly closer to the surge line (the dotted red line on the left.) In theory, if we could generate more flow, we would be able to move into the 76 per cent efficiency island. With the same intake air and coolant temperatures, the nine-pound pulley only pushed the ignition timing half a degree more, to 13 degrees BTDC, compared to the stock TRD seven-pound pulley. We didn't see any detectable knock activity with the Toyota scan tool, either.
More pulleys
The aluminum crank pulley that will replace this stock unit will take a little more work,
Older Scion tCs have a nasty, laggy throttle feel. The electronic throttle with Camry-like programming makes throttle response sluggish at best. Since few tuners bother retuning the ECU that controls the e-throttle map, the only option is to reduce the inertia of the driveline's rotating assembly. There are three ways to do this: get a lightened flywheel, knife-edge or lighten the crank, or fit a lighter crank pulley. Guess which one we chose? Since we opted not to tear the engine or transmission apart for a limited-slip diff, flywheel and clutch, that leaves changing the crank pulley. ZPI also makes a billet aluminum crank pulley, which hastens throttle response by reducing rotational inertia. The new aluminum unit weighs 0.8 pounds, compared to the stock steel pulley's 4.4 pounds, and gives just enough extra pep for decent throttle blips when shifting and balancing the car through a corner.
By Josh Jacquot
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