Understanding Blowby in Motorcycle Engines

Every now and then I receive an e-mail from someone wondering why he's suddenly finding large puddles of engine oil in his air filter. While some oil misting in the air filter is normal and nothing to worry about, these guys, who invariably own large-displacement, high-mileage V-twins, are talking about a regular Exxon-Valdez, enough to make a real mess and require topping off the oil supply on a regular basis. Because this is an area of general interest, let's discuss what's happening.

By nature, all four-stroke engines require ventilated crankcases. The reason is twofold. First, when an engine is started or run at low temperatures, condensation takes place and fuel vapors migrate into the crankcase. These contaminants, essentially water and raw gasoline, mix with the oil to form engine-damaging sludge. Fortunately, once the engine reaches operating temperature the water turns to steam and the fuel remnants percolate out of the oil. As long as the crankcase is vented, either by the atmosphere or by using a positive-pressure system, these nasty byproducts are free to drift out of the engine and go their merry way without causing any harm.

The second reason the crankcase needs a vent is because no matter how hard we try, there is no practical way to eliminate air from entering it. Anytime the engine isn't running, atmospheric pressure pushes air into the engine through open valves or any other convenient entry point. When the engine is running, some combustion gases normally make their way past the rings to pressurize the crankcase as well. If the crankcase wasn't vented, pressure buildup would soon cause every seal in the engine to rupture as the trapped air sought a way out.

Originally, crankcase vents were nothing more than tubes connected to valves timed to open when crankcase pressure was too high and close when crankcase pressure was too low. This allowed the bad stuff to be forced out and prevented dirt from being drawn back in. A variation on this scheme vented the crankcase through a series of baffles, accomplishing the same thing with less complication. Although they were efficient, these breather-tube ventilators were little more than open pipes spewing engine contaminants directly into the atmosphere. As you can imagine, the EPA took a dim view of the situation and demanded that manufacturers, first car and later motorcycle, do something about it.

Although system details vary, one solution was to plumb the breather into the air cleaner. The fumes coming from the crankcase are then recirculated back into the engine and burnt.

Here's how the oil ends up in the air cleaner. Crankcases can only contain a given volume of air and its attendant pressure. Once that pressure is exceeded, you've got problems.

What causes excess crankcase pressure? Excellent question, grasshopper. For starters, anything that can reduce the crankcase volume, such as overfilling it with oil, creates problems, as will a restricted or damaged breather system. In V-twins, crankcase pressure sees rapid fluctuations, decreasing as the pistons rise in unison, then increasing sharply as they descend, compressing the gases in the crancase. But the most common culprit is usually something called blowby.

Blowby occurs when the piston rings fail to do their job properly. They may be worn or they may never have seated properly in the first place. This failure to seath the rings tends to be somewhat common is large-displacement V-twins. That is partially because they make good power down at low rpm and small throttle openings. However, it takes large throttle openings and high rpm to get the rings seated. If you went through those first few hundred miles without ever holding the throttle open or revving to the power peak—or only did it once or twice—your rings probably never seated.

The bottom line is that the piston rings can no longer effectively seal the combustion chamber. Combustion gases are leaking past them, pressurizing the crankcase. This creates all kinds of problems; seals leak, gaskets fail and, because the pistons are forced to compress the excess air on the downstroke, horsepower is lost. But these things don't happen right away.

Because the hot oil in the crankcase is easily displaced by the increased pressure and because it has a perfect path out of the crankcase via the breather, the first sign of impending trouble is nearly always oil migrating into the air filter.

Once the oil is pushed into the air cleaner, much of it condenses and drops out of the airstream, making a mess in the process. If this was the only problem it'd be bad enough. Unfortunately, some of the oil is pulled back into the engine along with the vapor and burnt, where it does more damage by glazing the cylinder walls and creating carbon deposits in the combustion chamber. At this stage the engine is on a downward spiral. It's only a matter of time before you'll be doing a top-end job.

I'd be remiss if I didn't mention that some engine designs are more likely to experience top-end sealing problems and/or breathing issues than others. In particular, large-bore twins, V or otherwise, tend to push more oil than an equivalent-displacement multi. This is simply because dinner-plate-size pistons moving through sewer-pipe-dimension bores are more difficult to seal and move greater volumes of air. As a side note, there are also some bikes with poorly designed breather systems, but that's a separate issue to be discussed another time.

Please don't get the mistaken impression that all large twins are just ring-eating monsters waiting to self-destruct; they aren't. Chances are the majority of us will never have a problem with blowby. But like they say, recognizing a problem is always the first step in curing it.

For more articles on how to maintain and modify your motorcycle, see the Tech section of MotorcycleCruiser.com.