Motorcycle Blow-by: A Shower of Sparks Abridged Version by Bunn Rex

Author:Bunn, Rex [Bunn, Rex]
Language: eng
Format: epub
Publisher: A. R. Bunn
Published: 2013-11-13T16:00:00+00:00

Ch

apter 4- The Power Question

Top speed is 140 more or less, depending on how the thing [Egli Vincent], is tuned... but the vibration almost fused my wrist bones and boiling oil from the breather pipes turned my right foot completely black - Hunter S. Thompson, 1973

Does optimising motorcycle crankcase ventilation increase engine power? The subject continues to be discussed across web forums for many motorcycle marques. When I began this research project, there was wide scepticism of any power gain from optimising crankcase ventilation. This reflected the lack of pure and applied research into the subject; though much of the first century of motorcycling, as argued in this development history. This is slowly changing. However, the matter could and/or should have been resolved as far back as 1996 when two authors set out different but complementary proofs for the existence of power gain, or more accurately, reduced power loss. Yet it was not.

During the 2000’s, I was from time to time contacted by riders reporting apparent evidence of power gains after fitting one of my kits. These are mainly used by classic and vintage riders and with bikes of 20-30hp, there’s often little interest in power. Hence, my research emphasis has always been on conserving these irreplaceable crankcases, of which there are only 300,000 pairs left in the world (Bunn 2007 ). Protecting them from deterioration by acid blow-by, ethanol and moisture is one primary goal. The other has been reducing oil leaks, with which British bikes often present. Many oil leaks are associated with poor ventilation and with improved breathing riders can once again park without embarrassment.

Thus, I initially ignored such reports of power gain. Yet they kept coming… claims of increased idle, a louder exhaust, decreased engine braking, nippier takeoff, even easier starting! Then I came across something that tweaked my interest…a snippet of an old sticky debate on the net. This involved the US physicist and motorcyclist Charles Falco with his 1996 views on piston drag, pumping losses and wasted work. With the author’s permission, I reproduce this verbatim, as it is one of the first cogent arguments I can find in print.

"...an even "more ideal" situation -- albeit an unrealistic one to achieve--would be if it could somehow be arranged that atmos. pressure were in the cases during the upstroke, and...a... vacuum were created under the piston only when it reached TDC. Otherwise the piston, which has a surface area of ~9 in2 in the case of a 500°C c, would have to travel the entire length of its stroke against a force of ~130 pounds (9x14.7=132). Although, I hasten to add, since there would be no pumping losses for the air in the crankcase--since there wouldn't be any air in the crankcase--it would get all of this back on the downstroke. However, if our new "more ideal" breather could somehow, without pumping losses, introduce atmos. pressure air into the crankcase for the upstroke, and suck it all out again for the downstroke, there would be another ~30 h.

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