What Is Crankcase Windage?

When a piston goes down into the cylinder, it pushes air into the crankcase. What happens then?

One of the proud claims made by the Japanese motorcycle industry is that much of the horsepower gain achieved since the 1980s has come from the reduction of friction. That’s a good thing because any power recovered from friction can be used for intentional purposes, such as rapid acceleration. As Mr. Honda once put it, “Primarily, essentials of the motorcycle consist in the speed and thrill.” He said that in 1959 in the manual for the Benly 125 twin.

I have before me a CBR600RR crankshaft. This is a flat crankshaft because it has two up and two down; the crankpins are at 180 degrees from each other. That means in each of these “twins” the pistons are alternating. When one piston is going down, it pushes air into the crankcase. When another piston is going up, it draws air out of the crankcase.

If you look at the upper half of the crankcase, you see the four cylinders, which are normally blocked with pistons going up and down in them. When those pistons go up and down, the air that they push down into the crankcase has to go from one cylinder to the other and then back again every 180 degrees. If there is resistance—if it is hard for the air to get from one side to the other—that will resist the motion of the pistons. In fact, it is very easy to lose several horsepower at high rpm pumping air back and forth inside the crankcase.

Two things have been done about that issue in this particular engine. First of all, a series of little holes—27mm in diameter—have been made between the cylinders all the way down the crankcase; there are five of them. That makes it easier for the air to flow from under one piston to under the other one as they alternate at 180 degrees to one another. There are also gaps in the crankcase to let that air back and forth.

But when very high rpm is involved, it is better not to have any of this loss. There are two basic approaches to this that are used in MotoGP and Formula 1: In its V-4s, Honda divides the crankcase into two halves, each with its own scavenge oil pump. They are sealed from one another so the piston motion of one V-twin is not communicated to the crankcase of the other V-twin. When those pistons rise and fall, instead of pumping air along the crankshaft, they are simply compressing it and then expanding it. That is a loss-free process, which greatly reduces crankcase windage loss. Honda calls it “semi dry sump.”

The other approach is used by Yamaha, among others, and this is to provide a crankcase evacuation pump, which pumps the crankcase down to a lower pressure so the density of the air that remains in the crankcase is so low its pumping loss is greatly reduced. Either way, they are recovering free horsepower. The holes and other gaps engineered into this production Honda engine are intended to do the same.