PowerPoint, the Balderdash Flag and Normal Riding Questions | Ask the Pro


By: Jamie Elvidge, Tim Carrithers, Photography by Gusset Clothing Company, Joe Neric

Got a question for answers? Send it to mcmail@sourceinterlink.com

Power Point
In the New vs. Old Hayabusa article (Motorcyclist, December '07), you spoke about how the '08 'Busa makes its power in less time than its predecessor. This is something I have always wondered about. Take, for instance, a Buell XB12R and Honda's latest CBR600RR. They both make similar horsepower (though the Buell makes much more torque), so why are they so different in actual engine performance? Is it because the Honda makes its power in less time? Almost every dyno chart published in magazines shows rpm and power measurements; should there also be one for time? How much difference is there between bikes with similar power and the time it takes them to make that power?
Tyler MacKenzie
White Bear Lake, MN

The short answer is plenty. It all depends on what sort of power you want and where you're riding. Let's dismiss missing variable number one for now: the CBR600RR is 8 bhp stronger, 49 lbs. lighter and will drop the Buell like a bad habit. An engine is basically an air pump, and the XB12R's 601cc cylinders move combustibles without working as hard as the CBR's 148cc quartet. The twin hits its 97-bhp peak at a leisurely 6750 rpm, while the Honda's 106 horses arrive at 13,800 rpm-nearly double the Buell's engine speed. Buell's 1203cc Thunderstorm V-twin can't spin that fast, but it can crank out 77.5 lb.-ft. of torque at that same 6750 rpm, whereas the Honda's 44.4 lb.-ft. show up at 10,800 rpm. Confused? Relax. We posed your question to Bret Williamson, technical support engineer at Superflow Technologies, makers of the majestic Motorcyclist dyno.

"It helps to understand what horsepower means and how it is derived, from Mr. Watt's equation: Horsepower is the ability to do work, and in Mr. Watt's case, lift 550 pounds 1 foot in 1 second. Torque is an angular force. Without a time factor, torque alone does nothing.

"Mr. Watt's equation states, 'torque x rpm a constant (5252) = horsepower.' Torque applied with a given amount of engine revolutions over time will do some amount of work: horsepower. Said work done is the same, whether the torque figure is low and the rpm high, or the torque high and the rpm low. As long as they generate the same horsepower figure mathematically, the same amount of work gets done. Ultimately, horsepower does the work, not torque.

"As for the '08 Busa making its power in less time, this means the engine produces more power at lower rpm, due to more torque in that part of the rev band. The flaw with the dyno charts is a failure to look beyond the peak power. Overlaying the '07 and '08 Hayabusa power curves on the same graph tells the true story."

Throwing The Balderdash Flag
I've got to throw the balderdash flag. After reading the "Me & My Bike" bit on page 44 of your December '07 issue, a 648- horsepower Hayabusa is ridiculous. Most motorheads know that Speed = Money. Our Canadian brother must be loaded.

Now, I'm no engineer. In fact, I'm not even a brakeman. However, some motorheads know that bhp = BMEP x D x N 792,000, where BMEP is brake mean effective pressure (a parameter equating to the force propelling the piston down on the combustion stroke), D is engine displacement in cubic inches, N is engine rpm and 792,000 is a constant. If we work this backward, we find (based on a 1300cc engine at about 12,000 rpm-because we have no other engine information on this particular mill) BMEP is 535 psi. By comparison, a Formula 1 auto-racing engine produces a BMEP of 219 psi. My Speed Triple produces a paltry 10.7 psi. What kind of fuel is he using to prevent detonation at the extreme compression he is running? Purple 140-octane aviation gas has not been available for decades. I expect a little more justification of claims like this from your publication. If you believe these claims, I've got some land in Florida and a bridge in New York I'd like to sell you.
Tim Berridge
Columbus, OH

According to Richard Peppler at RCC Turbos up in Fordwich, Ontario-where the bike in question was born-not everything adds up to a formula. He tells us 550 bhp is the norm for a high-end Hayabusa these days, and that there are at least a dozen bikes in the U.S. making that kind of power. The record stands at 701 horses, set two years ago in Valdosta, Georgia.

"The fuel we use is VP C-16," Peppler explains. He runs about 36 psi with a liquidto- air intercooler that maintains a constant 150 F. He offers a bit of simpler math: "A stock 'Busa makes 150 bhp. We lower the compression to 8.5:1 and the bike makes 135-140 bhp. Typical turbocharger applications add approximately 14-17 bhp per psi of boost pressure. So at 10 psi we are adding a minimum of 140 bhp for a total of 280 bhp. At 36 psi we are adding 504 bhp, plus the stock 140, and you have 644 bhp. This is on the conservative side; the particular turbo setup on the bike in the magazine article could be pushed as far as 40 psi."

So pushed, our featured bike produced 648.45 bhp-good enough to win the 2007 Brute Horsepower Shootout at Daytona Bike Week.

Relax, You're Normal
I am in the middle of doing the 15,000-mile service on my 2005 Kawasaki Z750S. All my intake valves are within spec, but I have seven exhaust valves that are under the recommended .22mm. Do I have a problem or is this normal with a modern liquid-cooled, 16-valve inline four?
John Tyree
St. Clairsville, OH

You're not telling us how far under spec those exhaust valves are, but if clearances were down to zero, you'd be one seriously unhappy camper. Assuming some sub-spec clearance, Kawasaki says the scenario you describe is normal after 15,000 miles. Exhaust valves operate under hotter, harsher conditions than those fuel-cooled intakes. A bit of wear on the seats and valves is inevitable at that point, so no worries. That's why you're supposed to adjust the valves in the first place.

By Jamie Elvidge, Tim Carrithers
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