The sales pitch has been long and heavy: "incredible volumetric efficiency"; "large volumes of high-pressure air...significant boost in high-rpm horsepower"; "force-fed, pressurized air results in...major increases in horsepower and torque." Ram air is one of the most highly touted advances made for sportbikes in recent years, yet its benefits are rarely objectively stated, or scientifically evaluated. First appearing on Kawasaki's ZX-11 model in 1992, ram-air induction has now become de rigueur; virtually all sportbikes these days come equipped with gaping air intakes on their fairings, along with the proclamations stated above.
Theory
The basic concept behind ram air is that a moving bike will stuff air into its forward-facing intake runners, resulting in a free supercharging effect. As speed increases more air is forced in, resulting in more power and, in turn, more speed. The faster you go, the faster you'll go. The first impression upon viewing the gaping maws of some ram-air-equipped bikes is that an enormous amount of pressure must be built up at high speeds, but this is not so. As air is jammed into the opening, the high-pressure area that is created builds outward, effectively stopping more air from entering. Picture a balloon being inflated with an air tank. If the balloon is sealed to the nozzle, it will expand easily. Hold it slightly away from the nozzle, however, and the balloon will only inflate so far.
The pressure build-up can be defined using the Pitot-static tube theory:
P = .5 x r x v2
Pressure (P) is force divided by an area. In the English system of measurement the units of pressure are (lb - force)/in2 which translates to psi. Density (r) is mass divided by volume. The units of density in the English system are (lb - mass)/in3. Velocity (v) is air speed, with units ft/sec. Plotting pressure vs. speed gives a graph that has theoretical pressure rising with the square of speed, and this is why ram air has much more effect at greater speeds. For a speed of 150 mph, the resulting maximum theoretical pressure would be about 27mb (approximately .4 psi). Millibar (mb) is a metric unit for pressure. We used millibar instead of psi to give more workable numbers.
Of course, this air has to find its way to the carb mouths, and there is some loss due to the inefficiency of intake runners and the airbox itself. Without going into a lot of theory, suffice it to say that efficiency depends on the relative areas of the fairing intake, airbox intake and the airbox itself; even with a good system the loss can be as much as 70 percent. There is also the niggling problem of the engine gulping air from the system. This will lower the efficiency further, especially at higher speeds.
The test: Part One
So how much horsepower is ram-air induction really worth? Astute SR readers will recall Jon Doran's ram air/dyno test (Ram air: What's it worth?") in the August '95 issue. While the article was well done, we decided to take the concept a step further. We took one of Pi Research's advanced, System 3 data-acquisition systems and hooked up one of its air-pressure sensors into the airbox of eight different modern sportbikes. We also mounted a wheel-speed sensor which allowed us to precisely measure and compare roadspeed with airbox pressure. Absconding with all the motorcycles to our top-secret, high-desert test site, we then proceeded to wring the piss out of each machine and gather data from each of the top-speed runs.
The results will-as Doran stated in his test four years ago-surprise you. All ram-air induction systems are obviously not created equal. In the following graphs, the upper line denotes wheel speed and the lower line represents airbox pressure.
YAMAHA YZF-R6: First up was Yamaha's 15,500 rpm YZF-R6. Note that the pressure drops below zero-that is, below ambient pressure-accelerating through second gear, and it finally builds once the bike gets past 85 mph. The spikes (present in all of the graphs) represent pressure buildup during shifts, since the throttle plates are closed momentarily. Pressure peaked at 17mb. | 
SUZUKI GSX 1300R HAYABUSA: Here was our first revelation: Going 190 mph doesn't necessarily mean airbox pressure will be sky-high. Note that the pressure stays below ambient until the Hayabusa reaches 145 mph! It finally peaks at 16mb, but the pressure fluctuates heavily getting there, due to the big 1298cc motor gulping down huge quantities of air. The Hayabusa's cylinder head flows a tremendous amount of air, demonstrated by the motor's incredible 160-horsepower output. | 
HONDA CBR1100XX: Revelation number two: If the bike isn't making 160 horsepower and going 190 mph, it doesn't necessarily mean the airbox pressure will be low. Note that the CBR's pressure builds above ambient once the bike gets past 90 mph, and increases steadily and consistently up to a peak of 28mb. Honda didn't make a big deal about the new XX's ram-air system, but it's obvious this isn't just an add-on feature-they did their homework. |
KAWASAKI ZX-7R: Kawasaki was the pioneer of ram-air induction on sportbikes, and this graph shows why. Compare this graph (and the ZX-9R's) with all the others. Note that there are virtually no pressure spikes during shifts. And airbox pressure builds past ambient at approximately 60 mph, not 90 or 145 mph-just a smooth crescendo up to a peak of approximately 23mb. Kawasaki builds all sorts of aircraft, and that experience is obviously trickling down to the motorcycle division. | 
SUZUKI TL1000R: Whoa. What happened here? The fluctuations in airbox pressure in the TL1000R are definitely huge. This is probably due to the V-twin's huge pistons gulping down mass quantities of air with each intake stroke vs. a four-cylinder's rapid little (comparatively) sips. The graph is so rough it's nearly impossible to determine the peak airbox pressure. But by averaging the last section, we'd estimate it to be approximately 12mb. | 
KAWASAKI ZX-9R: Another impressive showing by a ram-air-equipped Kawasaki. Even though the intake ducts are smaller than the ZX-7R's, the Nine's airbox pressure builds quickly (albeit a tad rougher than the 7R), peaking at 28mb. It's also interesting to note that the Nine's slimmer ram-air ducts run over the frame spars, while the 7R's ducts run through the frame spars; yet the Nine's airbox pressure builds much quicker and higher. It makes you wonder what kind of pressure the new ZX-12 is going to make. |
SUZUKI GSX-R750: Although the Suzuki's ram-air induction system looks similar to the Kawasaki ZX-7R's (twin nacelles on the front, leading into ducts running through the frame spars), their pressures are vastly different. The GSX-R doesn't build positive airbox pressure until the bike exceeds 100 mph, and its peak of 20mb is somewhat erratic compared with the 7R. Of course, it doesn't matter that much when you notice the Suzuki's superior top speed and overall acceleration displayed on the graph. | 
KAWASAKI ZRX1100: As a test control, we fitted the Pi System to a non-ram-air-equipped motorcycle. If you think figures like 17mb seem insignificant, take a look at how much vacuum is present in a regular airbox and you'll realize even that amount of positive pressure can make a huge difference. With a pressure of -27mb, it's obvious that power gains can be realized by converting that vacuum into positive pressure. | |
Note: In the next issue of Sport Rider, we will complete the second part of this test by pressurizing the airboxes on each of the bikes to the amount measured by the Pi System-while running on the dyno-to find out just how much power ram air is really worth. See Ram Air Test: Part Deux.
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This story originally appeared in the October 1999 issue of Sport Rider.