Optimizing Your Harley's Engine Efficiency

In previous Motor Series installments, we looked at component dimensions and relationships influential for achieving engine synergy, the relationship of torque and horsepower, and airflow factors. In this segment, we'll deal with optimizing engine efficiency. We all know that if you want to go fast, the easiest way to do it today is to drop in a large displacement "crate" motor. With pump gas and a street-legal exhaust system, most of today's typical crate motors are good for about one horsepower per cubic inch, which results in a mild but relatively powerful street engine from just the large displacement alone. Surely these engines offer more power than a well-optimized 80-inch Evo engine from a few years ago that was putting out 90 ponies or so, assuming the owner tuned on it hard enough.

However, if you've had your mildly built street engine for a while and want to take it to the next level without installing bigger cams, higher-flowing heads, or a larger carb or throttle body-which may reduce low-end torque and sometimes make an engine less streetable-what can you do? One thing you can do, regardless of whether you have a big or small engine, is maximize engine efficiency through optimizing the compression ratio, combustion process, and cylinder sealing. In fact, optimizing efficiency improves power on any engine, no matter how big or small it is or how mild or racy its state of build and tune.

The Pump

Although increased displacement, airflow, and rpm are keys to making more power, more power will not be realized unless combustion chamber pressure is optimized and harnessed for maximized pressure on the piston. We all know that the internal combustion engine is nothing more than an air pump. However, what we must not forget is that it is a pump designed to generate heat and harness cylinder pressure. Making heat and controlling cylinder pressure is the secret to making power. For optimized cylinder pressure to occur, heat must be contained and converted into usable pressure on the crankshaft while minimizing heat loss to the cooling system. Of course, a balance must be achieved between maximizing heat and minimizing its harmful thermal and mechanical effects on components. When designing and building an engine, remember that it is critical to maximize cylinder pressure on the power stroke while minimizing pressure on the exhaust stroke. This starts with optimizing the compression ratio to the engine combination.

Corrected compression takes into account the point at which the intake valve closes after BDC on the compression stroke. The later the valve closes, the lower the corrected compression ratio. For an optimized engine, the corrected compression ratio should be matched to the vehicle's application and fuel octane. In general, the later the intake valve closes, the higher the mechanical compression ratio must be to maintain a given corrected compression ratio. To determine corrected compression, the mechanical compression ratio and the cylinder displacement remaining from the time the intake valve closes must be known.