Optimizing Your Harley's Engine Efficiency

Knowing the combustion chamber volume is important for accurately determining the mechanical compression ratio. The only sure way to know chamber volume is to cc it. The spark plug that will be used in the engine is installed, and the head is placed at a slight angle. The chamber is then covered with a 1/4-inch Plexiglas plate with a 1/4-inch hole drilled at its highest position. The plate is sealed to the chamber with a thin coating of light grease. Use a 100cc burette to fill the chamber through the drilled hole with a thin fluid, then record the amount used to fill the chamber.

Knowing the combustion chamber volume is important for accurately determining the mechanic

Compression Ratio Considerations

Following are several points to consider when determining an engine's mechanical compression ratio:

* Match the mechanical compression ratio to fuel octane, cam timing, volumetric efficiency (VE), combustion chamber design, gearing, and total bike weight.
* Aluminum heads generally tolerate higher compression than iron heads.
* Power-adder applications (supercharged, turbocharged, and nitrous) require less compression.
* At high altitudes or with low VE, increase the compression ratio.
* Dual spark plugs may allow a higher compression ratio for a given fuel octane.
* Higher compression requires an earlier exhaust valve opening.
* A lower compression ratio requires a later opening exhaust valve.
* A low compression ratio benefits from a higher flowing exhaust port.
* Higher compression offsets some negative affects of a large low-velocity intake port.
* Higher compression requires less ignition timing.
* Good fuel atomization reduces the potential for detonation, thus allowing a higher compression ratio.
* Higher compression improves throttle response.
* Minimizing combustion chamber heat loss is most important with low compression.
* Higher compression may require a high-torque starter motor.

Dual spark plugs reduce flame travel and unshroud the plug, which can minimize the potential for detonation. Dual plugging is especially helpful with high-dome pistons that shroud flame travel and large-bore engines where the combustion flame has a long distance to travel. The second plug should be located high enough in the chamber for good combustion efficiency and sufficient clearance between the piston and plug electrode. The engine will typically require 5-8 degrees less ignition advance with dual plugs. Test and tune to determine the optimum spark advance for your engine.

Dual spark plugs reduce flame travel and unshroud the plug, which can minimize the potenti

Maximizing Efficiency

Assuming an engine is properly designed for optimized cylinder fill, then the combustion process, scavenging, and ring seal become keys to making power. Improving combustion efficiency is important for optimizing power. Good combustion chamber design will optimize combustion and thermal efficiencies. An engine with optimized combustion efficiency is identified by crisp throttle response, improved fuel efficiency, and a low brake-specific fuel consumption (BSFC). A compact chamber, relatively flat or slightly dished piston top, and centrally located spark plug will provide for short, unobstructed flame travel and rapid combustion. Additionally, a large, tight squish band is important for proper turbulence. High turbulence will more thoroughly mix the air/fuel mixture into a more homogenous mixture that will burn more quickly and efficiently, thus improving power. It will also require less ignition advance, run on lower-octane fuel, and be less prone to detonation and pre-ignition.

The optimum squish clearance (clearance between the squish band areas on the piston and cylinder head) varies depending on the engine components and use of the bike. In general, tighter is better, but not so tight that it impedes combustion. In addition, the piston can never be allowed to touch the head during engine operation. Crankcase, rod, cylinder, and piston expansion must be considered as an engine warms to operating temperature. An engine built with aluminum cylinders can have tighter squish clearance than an iron-cylinder engine because aluminum expands more than iron. Some factory crate engines have been measured with as much as 0.072-inch squish clearance, which is way too much.