Re: How to calculate the increase in HP of an engine with forced-air induction?

Greetings:

In turbo-supercharged engines, the air intake turbines must turn at very high
RPMs to boost horse power and they consume a large amount of the available
engine power todeliver compressed air to the engine air intake. Thus a ram air
system would be desirable for small engines such as those used on motor cycles.

I have found the following web site that has information on the testing of
various ram-air- equipped airboxes on motorcycles that were conducted by
Sport Rider Magazine and reported in the October 1999 and December 1999 issues.

http://www.factorypro.com/magazine_articles/mag_sportrider,Ra mAir,part2.html

For some reason I can not access the web site directly from the above URL
and I have to search for “ Sport Rider” and “ram air” on Altavista.com to
connect to it.

Sport Rider made pressure measurements, versus engine RPM, for a number of
popular racing bikesat speed on a track and on a dynamometer test stand while
simulating ram air. The difficulties that they encountered in the tests are
described in detail in the article on the web site.

Air is a compressible fluid and increased compression and air flow into the
motorcycle engine is what a ram-air induction system is attempting to do.
However, it turns out that air behaves more like an incompressible fluid at
speeds less that Mach 0.3. Mach 1 is the speed of sound at the local
temperature and air pressure. At standard temperatures and pressures (20 C,
760 mm Hg) on the earth’s surface, the speed of sound is 1236 km/hr (768
miles per hour). Thus an air velocity greater than 370 km/hr (230 mph) would
be required to begin to observe air compression in an air intake duct. To
accomplish this in a ram air system requires that the air intake duct be tapered
down to a smaller diameter to increase the velocity of the air flow to be
greater than Mach 0.3 into the engine. This is similar to the intake of a jet
engine and the design of these type ducts is highly dependent on the
inlet pressure of the engine (- 11 milibars relative to ambient in the article)
and the ram air compression (+11 millibars relative to ambient in the article)
at 12 to 14 thousand RPM for a YAMAHA YZF-R6 bike where they measured a
5 horsepower increase using the ram-air system. Also, compressing air
increases its temperature, and this changes the Mach number as the air passes
through the various stages of the intake system making the design problem
even more complex.

Best regards, Your Mad Scientist
Adrian Popa