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Forced Induction types
Different means of mechanical forced induction:
Roots style "positive displacement" blower.
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These are units that move a given volume
of air per rev. If you have a blower that is geared to move 1.5x engine displacement, then
you will always have a (potential) 50% supercharge, instantly. Problems: generally poor
compressor efficiency (50%), very difficult to intercool, noisy, difficult to mount due to
size/shape, excessive drive requirements w/o by-pass valve, limited realistic capacity.
Using the by-pass to circumvent the compressor during cruse, there is very little power
loss due to the drive (<5 hp) = good mpg's. Centrifugal Supercharger. Basically just turbo compressors attached to some sort of transmission driven by the crankshaft. Nature of the beast is quasi-linear boost relative to RPM (i.e. 3k = 3psi, 6k = 6psi) or some multiple thereof. Units are capable of high pressure ratios per stage (e.g. high boost). Benefit: superior compressor efficiency (70%), easy to intercool, relatively small thus easy to install, immune to backfire, simple to manufacture, boost curve often advantageous for traction reasons. Turbocharger. Centrifugal compressor driven by an exhaust turbine. Technically a speed machine, meaning the compressor flow is based on its speed squared. This speed relationship lends itself well to the use of an exhaust turbine to derive the compressors power. With no direct connection between the engine and the turbine, gearing/drive limitations (and complexities) are eliminated. Back pressure is an issue, mostly just on factory applications though (remember, almost all "factory" stuff sucks right...). A properly sized and tuned turbo system will have a boost/back pressure ratio of 1:1. Indy cars do much better. Benefits: superior efficiency, ease of intercooling, limitless compressor/turbine selection, broad boost characteristics. Problems: exhaust plumbing / heat isolation issues, poor converter light off, stigma. Other thoughts: For many Roots blowers are the most satisfying. They look cool, make neat noises, and provide the power increase instantly, at all times. Just like having a bigger motor... Forced induction motors benefit just as much as NA motors from intake and exhaust tuning (headers, porting, etc.). The boost doesn't just "overcome" obstacles, it still has to flow through them. Picture a boosted motor as just operating in a higher pressure differential regime than NA motors and it makes sense. (fi 0-30 psi absolute, NA 0-14.7 psi absolute) Turbines derive their power from the overall energy of the exhaust, not just the heat nor the pressure nor the velocity. Some also claim they can derive power from the acoustic energy present... don't know about that. (thanx Brock) Any of these devices can make high peak power, but broad useful torque, or power under the curve, is best made by turbos and roots blowers. Between these the turbo will be the more efficient. |