Editor’s note: This is the first part in a two-part series on the differences between a race bike and a street bike. Scott Holton starts things off with his notions about what makes a good race motor. Tomorrow’s post, written by Anthony Todd defines reliability and drivability as the most important factors that make up a good street bike.
By Scott Holton
On a recent J&P Cycles forum, I was asked how a particular engine combination would be for around-town riding. I looked the specs over, and told the guy that the engine setup he described wouldn’t be all that suitable for strictly short trips around town. That engine — while possessing very good components — was too much like a race engine to be practical around town.
That got me to thinking about what it is exactly that defines a race engine. Here’s what I came up with: I told this fellow that the cylinder compression ratio/cam combination in his bike would create too much cylinder pressure for today’s low-quality fuel availability. The higher the engine’s cylinder pressure is, the more power it makes. A well-mannered street engine will have cranking compression in the 170 to 180 psi range with a compression tester. If the cylinder pressures creep much higher, it will be more difficult for the engine to turn over, which creates an abundance of heat. This wreaks havoc on the starting system, and if not run long enough, the charging system won’t have time to fully charge the battery between starts. This will strand you quickly if you aren’t careful. It will seem like the charging system is failing, which of course, isn’t the case at all.
A good rule of thumb for definition of a race motor is one that exceeds the octane rating available for standard commercially available gasoline. Let’s define this octane. The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than a spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine. Lower-octane gas (like “regular” 87-octane gasoline) can handle the least amount of compression before igniting. The compression ratio of your engine determines the octane rating of the gas you should use in your bike. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a high-performance engine has a higher compression ratio and requires higher-octane fuel. The advantage of a high-compression ratio is that it gives your engine a higher horsepower rating for a given engine weight — that’s what makes the engine “high performance.” The disadvantage is that the gasoline for your engine costs more.
So my definition of a “race motor” is any engine that requires higher than normal octane in its fuel delivery system. Hope this sheds a little light on the inner workings of an engine. Until next time, ride safe.
To get a straight-away description of street bikes, read Anthony Todd’s column tomorrow morning in this space.