Understanding the Tech of a Motorcycle Engine

Learn the ratios that help your motorcycle run

Understanding the Tech of a Motorcycle Engine
The mixing of elements inside the engine's combustion chamber is what ultimately makes your motorcycle go. What ratios of the various mixtures is correct?Photography by Dean Groover

When a cloud of gasoline and air is compressed and ignited in an engine's combustion chamber, the tremendous heat released causes the trapped gases to expand with great intensity. The enormous pressure works against the piston, driving it downward, rotating the crankshaft and ultimately making the wheels of your motorcycle go round and round.

It sounds simple, and for the most part, is. There’s only one catch: The proportions of air and fuel have to be right on the money. Get them wrong and you’ll be lucky if the engine starts, let alone runs properly.

In theory, something on the order of 14.7 pounds of air to 1 pound of fuel should work nicely, and in fact, when air and gasoline are mixed in those proportions, the mixture is said to have the proper stoichiometric, or chemically correct, ratio. Unfortunately, the operative phrase here is “in theory.” In the real world, the best performance is usually obtained when the mixture is biased toward somewhat more fuel. (The amount being determined by engine temperature, throttle position and rpm.) Allow me to digress, though, and point out that the terms “richer” and “leaner,” as used to describe the air/fuel ratio, refer to the engine’s requirements and not to the stoichiometric ratio. As you’ll see, an engine fed a rich mixture of, say, 10:1, can still be lean in a given circumstance.

When an engine is initially started, for example, it needs a very rich mix. At cranking speeds, intake velocity is minimal, and because the intake tract and combustion chamber are cold, thermal efficiency is low. Since chilly, slow-moving fuel neither atomizes nor vaporizes very well, ignition is difficult, and only a small portion of gasoline evaporates and burns. Under these conditions, a blend of around 4:1 is needed just to light the fire.

As the engine warms up, vaporization improves, and the mixture can be leaned out. Idling, however, even when the bike is warm, also demands a rich mixture. Because intake velocity is leisurely at low rpm, fuel and air don’t mix well and combustion is inefficient. Consequently, idle mixtures should hover in the vicinity of 10:1 or 12:1.

At normal operating temperature, turning moderate rpm, thermal efficiency and intake velocity are both high. The fuel and air blend well and the mixture ignites easily, so we can lean the mixture out even further. Typically, at a steady cruise, an air-to-fuel ratio of about 14.8:1 to 15:1 provides the best compromise between economy and power. Unless what we’re riding has an EFI system operating in closed-loop mode, this is as close as most of us will get to our ideal stoichiometric mix of 14.7:1.

Conversely, during WFO operation a slightly richer mixture of around 12:1 or 13:1 is usually the hot setup, though that’s by no means carved in stone. Due to design and tuning anomalies, some engines may make their best peak power with slightly richer or leaner ratios.

So what happens when the mixture gets too far to one side? When the mix is overly rich, excess fuel cools things off, slowing combustion and reducing power. It also raises emissions because unburned gasoline components exit the tailpipe straight into the atmosphere. Additionally, rich mixtures foul plugs and create carbon deposits in the engine, and in severe cases can dilute the engine oil (especially on the cylinder walls), thus causing premature engine wear. Lastly, and for obvious reasons, a rich mixture stunts fuel mileage.

If the mixture is too lean, there’s not enough fuel to combine and burn with the available oxygen, so power is also reduced. Furthermore, a lean mix burns slowly, which increases the head’s and cylinder’s exposure to heat and makes the engine run overly hot. Run it too lean for too long and there’s a good chance you’ll do some serious damage. Ironically, a lean mixture can also increase emissions because the burn is incomplete. The engine can develop a lean misfire, causing it to skip a combustion cycle and release unburned fuel, just like a rich mix.

In sum, both rich and lean mixtures emit excess pollutants and create running, starting and driveability problems, which is why good tuners and the factories work so hard at getting the mixture spot on. Problem is, they don’t always get it right, and even if they do, changes to anything that affects airflow through the engine—an aftermarket pipe or modifications to the airbox, to name a few—can undo all their good work and necessitate the installation of a jet kit or fuel-management system to restore correct air-to-fuel ratios.

As you’ve probably realized, internal-combustion engines are somewhat intolerant devices. Like obsessive-compulsive technical editors, they tend to want everything just so. Vary air-to-fuel ratios too far in either direction and they become moody, cantankerous and, in some cases, downright vindictive.