ProCharger Superchargers Techical Insight - Insider

Jim Summers and Cary Pangrac of ProCharger explain how to make serious supercharged power

Stephen Kim Mar 1, 2009 0 Comment(s)
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Billet Impellers
ProCharger uses billet impellers exclusively in its supercharger units, and the benefits are more than just superior strength. The company says that most aluminum cast impellers have a tensile strength of 36,000 psi compared to the 83,000 psi rating of the 7075 T-6 alloy used in its impellers. "The billet material results in an impeller design capable of withstanding rpm, airflow, and boost levels that are substantially greater than those experienced with cast designs," Summers explains. "By using billet impellers created in-house, ProCharger is able to test and evaluate multiple alternatives prior to production and is not confined to a basic design that can only be `trimmed' in an attempt to meet the needs of a different application. Billet impellers don't suffer from the density variations generally present in cast impellers, and as such are naturally balanced and can operate at higher speeds without generating additional load. Generally lighter than their cast counterparts, billet impellers also reduce blower transmission loads under acceleration and deceleration."

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Fueling
The reason why the recommended WOT air/fuel ratios for supercharged motors are all over the map is because they work well over a wide range. At 12:1, a blower motor typically makes the best power but there isn't much room for error. Arguably the best compromise is at 11.5:1, where power output is excellent while still maintaining a good margin of safety. At 11:1, you're definitely leaving some power on the table, but it's still not overly rich. On the other hand, anything fatter than 10.5:1 is too much fuel and may cause problems with misfiring and plug fouling. It's worth noting that cylinder-to-cylinder variation could be significant in a carbureted application, and the difference between the leanest and richest cylinders need to be considered. As for the rest of the fuel system, you need one additional pound of fuel for every pound of boost on most carbureted applications. So if a naturally aspirated motor runs at 6-7 psi fuel pressure and you add a blower making 10 psi of boost, then you would need 17 psi of total fuel pressure at maximum boost. It is also important that your fuel pressure regulator or mechanical fuel pump be boost-referenced. This will allow your fuel pressure to rise in conjunction with your boost level throughout the rpm range of the motor. Measurement is best done with high-quality wideband air/fuel gauges, and fortunately these devices are getting more and more affordable.

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