Pressure Drop and Efficiency
Two key metrics used to gauge the performance of intercoolers are effectiveness and pressure drop. Effectiveness measures how much heat is being removed from the intake charge, and the flow capability of the intercooler determines pressure drop. The nature of intercooling means that there will be less pressure at the intercooler outlet than at the inlet. Realistically, it’s not practical to design an intercooler that has no pressure drop because there needs to be at least some pressure drop in order to scrub heat from the intake charge. Nevertheless, it’s not just the intercooler that must be considered, but also the tubing. You can’t just flow the intercooler alone. You have to take into account the intercooler tubing diameter and layout. Generally, it’s best to limit the number and severity of the bends. In other words, intercooling works as a complete system, not just the intercooler itself. Generally, at 8 psi of boost our supercharger kits are designed to keep the inlet air temperature within 20 to 30 degrees of ambient air temperature. Not only is this good for power, but for engine longevity as well. The key goal is repeatable performance, not something that requires a lot of cooldown time, and our superchargers and intercoolers quickly recover enough to maintain performance in back-to-back runs. A lot of testing is done on the dyno, and sticking a fan in front of the bumper can’t replicate the dynamics of driving on the street. Likewise, sometimes there are interesting aerodynamic effects over the front of the car, which is another reason why you have to go to the track to see how well an intercooler really works in the real world. Through these efforts, we have been able to design air-to-air intercoolers that are over 70 percent efficient.
Three of the primary components in supercharger compressor design are the inducer, exducer, and volute. The inducer is the inlet where air enters the supercharger. Increasing the inducer diameter affects airflow potential, which is why sanctioning bodies place limits on inducer size. The exducer is the outside diameter of impeller wheel, and changing its dimensions affects pressure at a given rpm. Exducer design is very important because the number of blades, shape of blades, tip height, and stagger all affect flow. The exducer diameter together combined with shaft rpm gives you tip speed, which gives you pressure. The volute is the compressor housing, which merely acts as a collector for the compressed air. An undersized volute will negatively impact performance, so the goal is to design it so it’s not a constraint. Generally, you just want the volute to be neutral in design. Interestingly, you can’t just scale a small blower up to make it work on a bigger motor. You have to completely redesign a supercharger for the pressure and flow range of each application. CHP