That’s a saying I’ve heard from more than a few people. It expresses the sentiment that more power means a bigger engine. Intuitively that would seem obvious, but it’s not quite that cut and dried. What you do with a given displacement will have a lot of effect on the power you get.
This point was brought home to me recently as I attended an antique bike show. I was looking at at 1916 Triumph motorcycle. It had a 500cc engine. In a modern motorcycle like the Kawasaki Vulcan, or Ninja, one would expect 50 to 60 horsepower. The old Triumph engine for 1916 used it’s 500cc to produce a whopping 3.5 horsepower. Just as well I suppose, as this old bike was fitted with a wicker sidecar. Can you imagine strapping a wicker side car to a Ninja?
While that is an extreme example, you can still see the effect of engineering decisions on performance for a given displacement in contemporary engines. For a short time, I recently owned two bikes with 1500cc displacement. I was amazed at the difference in the two machines. A friend of mine commented that, “There’s 1500 and then there’s 1500.” The big V-twin bike topped out at about 85 horsepower, while the flat-6 produces roughly 100. That’s even more surprising when you consider the V-twin was a newer design, fuel- injected power plant while the flat-6 was older with a carburetor.
Both of these bikes are big touring cruisers where comfort and reliability is more important than raw power. A co-worker recently bought a Honda CBR 1000. Checking the specs on that bike I was surprised to discover that the 998cc power plant of this little bike produced 148 horsepower.
There’s been a huge gain in efficiency from better mechanical engineering since 1916. But the improvements in the last 10 to 15 years have been smaller. We’re probably getting close to the best performance we’re going to get from simple mechanical advances. The greatest impact on engine performance in the last couple of decades have come from electronics. Engine control units, or ECUs, now integrate several mechanical systems to control fuel delivery and ignition producing not only more power over a greater RPM range, but better gas mileage as well.
Dual cams and variable cams have improved valve performance, but these are still mechanical systems with some limitations. You can time when a valve begins its cycle with variable cams. But so far the lobe, or time the valve stays open, is constant. Some engines are experimenting with electronic valve control that offers the opportunity to alter the time the valve is open, but also can alter the area through out the cycle for maximum combustion.
So, what do we do when we reach the limit of performance using electronics? Well, there’s always chemical engineering of the fuel.
－ Guy Wheatley