Now, you may ask, well why not bolt a 14-71 on a 454. Well, for two reasons. For starters, you'd be at a very low drive ratio. The negative result of that, can be simply inefficiency, especially at low rpm. The rotors turning too slow, can result in carryback. The other reason, is its simply taking alot of power to turn that large blower, when its not warranted. 20lbs of boost? Ok, now we might be getting close, but for the typical marine 5-8psi, it's just not needed.
Why cant my small blower work just fine?
Well, it can. To a point. What happens when you spin the blower too fast, is it creates heat. Lots of heat. Not only does heating the intake air charge, help invite detonation to the party, it kills power output. Lets say you have a small 174/177 blower just screaming back there. You may have intake manifold temps easily exceeding 200+ degrees. By simply going to a larger blower, say, 8-71, 420, 10-71, you may drop the temperature down 50, 75, or even more degrees. How much of a gain is that worth, well, think about how you're N/A car would run on a blistering 120* degree day, vs how it would run when its 60* at the track. There was an oso member, who converted his 575sci mercury engines that had the 256 superchargers, to 8-71's. With the same amount of boost in the manifold, he picked up around 5mph in his 40' outlaw. I, also personally switched from 250 blowers, to 420 blowers, and gained about the same in my 38 Fountain, again, at the same boost level.
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skav said: Click to expand...
Screw Superchargers are actually an air compressor which is more adiabatic efficient than a Roots Supercharger. A Roots supercharger, which most originated from a 2 stroke diesel engine, typically a Detroit ie; 8-71 (8 cylinder, 71 cubic inches per cylinder). Centrifugal superchargers are kind of a cross breed between turbos and Screw or Roots Superchargers. All of these are mechanically driven.Turbo Chargers work primarily on the Radiant Heat/Expansion of exhaust gases not so much exhaust pressure or back pressure. In a Diesel engine they work perfectly. However with a gasoline engine they can cause a snowball effect once the engine fuel/air ratio goes lean as it will case more radiant exhaust heat which will spool the turbo to make more boost which will only further lean the engine, and so on.All types of supercharging used to increase the volumetric efficiency of the engine which will raise the power output or Brake Means Effective Pressure.Here are a couple of links that you should find interesting. All of them are pretty basic and short reads but will give you a better understanding of what you are asking about.Hope this helps.
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