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- Quarter Scale Merlin

SUPERCHARGER: WORTH IT?

There are two reasons why high-performance aircraft engines like the Merlin have superchargers. One, of course, is to increase power, particularly at high altitude. The other is to improve mixture distribution, which is always a problem for inline engines fed by a single carburetor. Even a slight pressure increase seems to help even out mixture distribution among the cylinders, but much of the improvement is simply due to the supercharger's agitation of the fuel/air mixture. These factors should work equally well on the Quarter-Scale Merlin, but the downside of adding the supercharger is complication and extra weight. The wheelcase/supercharger section is almost as complicated as the rest of the engine, and some of the castings are fairly challenging to work with.

The supercharger designed for the Quarter-Scale Merlin is based on the early Merlin's single-stage version. After consulting with supercharger experts, it was decided to gear the impeller to run at approximately 10 times crankshaft speed, or 36,000 rpm at the target 3,600 rpm engine speed . Calculations indicate that this will provide a small but significant pressure boost, sufficient for the target horsepower/rpm goal. Of course, boost pressure would be considerably higher if the impeller were geared to the maximum possible rpm for its size. Based on the general rule for centrifugal superchargers, that the impeller tip speed should not exceed roughly the speed of sound, this would be on the order of 160,000 rpm. Since the supercharger design is completely untested, even gearing it at 10:1 seems risky enough.