Forced Air? TURBO? Options

[bob91403]

Could someone tell me why a scoop can't increase air pressure, as you go faster, and create the same kind of HP "boost" you get from turbo or superchargers? Excuse me for being ignorant. But, isn't it just a matter of increasing air pressure to the intake, and pumping in the right amount of gas for mixture? What about a pressurized tank of air, or both air and propane tanks? Couldn't you use a small compressor for boost, or as a source to refill a "boost" tank? I really don't understand this obcession with tubocharging. Nitrous systems have been around for a long time, and are much simpler. I was told, by my top rated Porsche mechanic, that, "Yeah, higher compression heads, turbocharging, supercharging, nitrous oxide, will all increase horsepower. But, anything you do to increase the horsepower will seriously lower the longevity of the engine." He doesn't even recomend european compression heads for a rebuild. I do understand, tubocharging a smaller engine gives you increased horsepower, plus the weight advantage of not having to put in a 3.2L six. But, if you have to rebuild it every 50K, instead of 150K miles, doesn't conversion cost less? Curious to hear your replies.

[lapuwali]

A properly placed and ducted scoop would raise the inlet pressure some, but not by a lot, and very high speeds are required to gain much. Kawasaki fitted such a system to a very fast bike 10 years or so ago, and the system was only effective above 75mph or so, and even then it was pretty marginal (less than 1psi of boost).

1 atmosphere = 14.7psi = 1.02 bar = 102 kPa, which are all of the commonly used measures of air pressure. A typical turbocharger will provide between 7 and 15psi of boost, or 0.5 to 1 atmosphere, more or less. Ideally, 1 atm of boost will double the power. In practice, it usually ends up adding about 75-90%. And a turbo will do this at 10mph, not just above 100mph.

Nitrous is useful for short spurts, but it's basically just a drag racing thing, good for a few 1/4 mile runs or the odd spotlight drag.

Compressing the air to provide even 6psi of boost takes a lot of power. A Rootes-type supercharger of roughly 70% efficiency takes about 4hp to do this. That's 3kW, or enough to power 2-3 averaged size houses with all of the appliances turned on. The "small" compressor can't be very small and still provide a useful amount of boost.

The primary benefit of a turbocharger is it's using exhaust heat that's otherwise wasted (roughly 70% of the energy in the gasoline you pour into the tank is lost out the exhaust stream, only 15% of it makes it to the drive wheels) to drive a turbine to provide the pressure, so you're substantially increasing the efficiency of the engine by capturing "lost" energy to do useful work.