In the early days, this unintentional overlapping was at first considered detrimental. (Both intake and exhaust valves are slightly open at T.D.C. Lengthening of valve timing in the gasoline engine brought on accidental overlapping of the intake and exhaust valve events. Unless these burnt gases are completely expelled from the combustion chambers, they will remain behind to displace and contaminate the incoming fresh fuel/air charge. An important advantage gained from lengthened valve timing is that the greatly expanded gases are eliminated more efficiently and virtually by their own pressure. The stretching of the intake valve timing allowed the engine to breathe deeper and take in greater amounts of air and fuel, thus creating a more powerful explosion in the combustion chamber. BRAZEN EXPERIMENTS WITH LONGER DURATION CAMSīy experimentation, the more progressive cam engineers of the 1910's and 1920's discovered that the midrange and high-speed power could be greatly improved by lengthened valve timing. These early engines had Oo overlap or no overlap whatsoever. The burnt gases, due to their high pressure, virtually expel themselves, and the piston drives the last of the gases out the exhaust valve closes at T.D.C. The spark plug fires and ignites the fuel/air mixture which drives the piston down to B.D.C.

The piston rises, with both valves closed to compress the fuel/air mixture. The old valve timing then was: Intake valve opens at T.D.C., and as the piston lowers, it draws in the fuel/air mixture the intake valve then closes at (B.D.C.) hence, the intake stroke.

Even in their wildest inspirations they would never have believed that a quarter or half century later, with better structures, these same engines would be revved five times as fast and produce many times more power. They were merely concerned with getting the engines to run at slow speeds. The engineers of the late 1800's were only concerned with harnessing power of the gasoline and air explosions in an internal combustion engine to propel an automobile, hopefully, a little faster than a horse. In the old days, the first four cycle engine had very short valve timing, but rightfully so because these were slow speed engines. Its purpose is to operate the intake and exhaust valves in the correct timing with the piston as it sequences thru the four strokes. The camshaft is connected via the timing chain and sprockets at a 1:2 ratio to the crankshaft and therefore revolves once for every two turns of the crank. Two revolutions complete one sequence of the four strokes. 4 cycles x 180° = 720° or two revolutions of the crankshaft. Each stroke represents one-half of a revolution of the crankshaft or 180 crank degrees. You probably already know the four basic strokes of the four cycle engine: INTAKE, COMPRESSION, POWER, and EXHAUST. Indeed, cam degreeing is simple, but first let's make sure you have a good understanding of the cam's function in the four cycle engine.

Degreeing harley evo camshaft