Everything Counts at 15,000 rpm


Our interest in the internal-combustion engine mainly focuses on the flame and fury in the space above its pistons. But recent events have shown that significant power loss can occur below the piston. The proof of this can be seen in the latest generation of 15,000rpm 600cc sportbikes, which have nearly as much careful porting below their pistons as above.

Some engine builders history

This story begins long ago, when overhead valves were first fully enclosed and lubricated by circulating engine oil. A single piston, rising and falling, compresses and expands the air and oil vapor below it in the crankcase. There is no loss in this simple process for the energy required to compress the gas is recovered as it expands on the piston's upstroke.

Adding rocker- or cam-boxes to the head complicated this. Oil pumped to the valvetrain must somehow return to the crankcase - normally left to gravity via a camchain gallery, pushrod tubes or a dedicated passage. Now when the piston descended, the rising pressure in the crankcase rushed up these new passages and into whatever volume existed in the head. Because the passages were small, the first problem noted was failure of the oil to drain back at certain engine speeds, Careful dyno work would also have revealed a power loss, for it takes power to force a "fluid" - crankcase air, in this case-back and forth through small passages.

Crankcase Pressure

Reducing crankcase pressure was one solution. British engines pumped themselves down through times breathers, and recent engine builders use reed valves for the same purpose. The aim is to reduce the gas pressure in the crankcase, thereby reducing pumping loss and any problems with oil drainback from the head. During the Flathead era. Harley racing engines occasionally "wet-sumped" - oil would evade the crankcase scavenge pump and be whipped around by the large flywheels inside their close-fitting crankcase, generating enormous friction. A former H-D racing manager, said you could see the bikes slow down when this happened on the banking at Daytona. Later Harley race engines cut the pressure change in the crankcase by adding volume a large, under-engine sump and a vent into the primary chaincase.

Formula One Racing

More recently, in Formula One auto racing, engines have been configured as a stack of five separate V-twins, with the crankcase of each pair of cylinders sealed off from its neighbors. As these are drysump engines each of the five crank chambers must have its own scavenge oil pump. The problem of gasflow between crankcase and camboxes is solved by separating these as well-each cambox has a scavenge oil pump at front and rear. It is common for scavenge-oil-pump capacity to be quite large, such that they really operate as vacuum pumps to pull crankcase air/vapor density down, allowing use of power-saving low-tension oil rings.

Direct Air-flow from cylinder to cylinder

Twenty years ago, Yamaha discovered that 4 to 6 horsepower could be saved in one of its large, air-cooled, four-stroke fours by taking an opposite approach-making it easier for crankcase air pumped by pistons to move back and forth between cylinders. Structurally, it is desirable to make main bearing webs solid, but this forces air pushed by the descending #1 piston to travel all the way down, through the rotating crank, then make a U-turn to come back up through the crank again to fill the space created by the rising #2 piston. What Yamaha did was to cut large holes through the main-bearing webs above the crankshaft. This allowed air to travel more directly from cylinder to cylinder, no longer having to make two trips through the spinning crank, reducing the pressure required to force it back and forth. That released a useful amount of extra power.

The modern 600cc sportbikes

Today's 600cc sportbike engines redline as high as 15,500rpm. At this speed, anything in the way of cylinder-to-cylinder airflow becomes a restriction. The market forbids expensive measures such as separated crankcases and multiple oil scavenge pumps, so a more direct technique is used: The cylinders are cut away on the sides, almost up to where the oil rings stop at BDC, to create the largest possible cylinder-to-cylinder air ports. Although no one is putting numbers to the potential gains, it seems believable that between the best and worst designs, 10 percent of peak power could disappear. No high-speed engine should be designed without evaluation of this form of power loss.
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