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Performance That's Right for You

Evaluating Dyno Charts

Cam Basics

Porting: The Black Art

Power Transfer to the Rear


Exhaust Systems

Corrected Compression Ratio

Performance That's Right for You

by John Hildreth

So you want to add a little spice in your riding life and have decided you need more horsepower out of your Harley. Where do you begin? Many people start with a 1550 cylinder kit. Unfortunately, they may not realize that a 1550 kit simply increases the displacement size of a motor by replacing the stock cylinders with larger ones. Increasing the displacement means other components must also be upgraded to take advantage of the new motor capabilities. To realize full benefits, a 1550 kit needs to be accompanied by performance heads, cams, air cleaner, ignition and exhaust systems, and either fuel injection mapping (if injected) or performance carbs or a carb kit (if carbureted.) All these products enhance the motor's performance by increasing airflow through the motor.

But how do you evaluate the wide variety of claims from manufacturers of performance products? First off, you need to know which enhancements will best suit your riding style. Be realistic with yourself. If you only ride in traffic and on the highway, you require a different configuration than someone taking his or her bike out to the drag strip, and a drag racer needs something entirely different from a road racer. Generally speaking, though, a motor with wide power band is far more rider-friendly for street and highway riding than a peaky, high horsepower motor. Before making your evaluation, you need to understand a few basics about calculating horsepower, cylinder head flow, and the importance of velocity and valve size, in order to be able to compare manufacturers' claims, and to help you decide what the best package is for your needs.

One way to calculate horsepower is by measuring the cubic feet per minute of air (CFMs) that is moving through a motor at any given RPM. You measure this movement using a flow bench, and performing a simple calculation. The calculation formula is: 1 CFM at the intake valve = .27 horsepower per cylinder, at the crank, as measured at 25" of water. For example: If, from the air cleaner to the exhaust valve, your motor configuration is moving 158 CFMs, then you have the capability of 43 hp per cylinder, or a two cylinder total of 86 hp at the crank. If that flow is increased to 230 CFMs, the horsepower rating changes to 62 hp per cylinder, or 124 hp total -- a 45% boost of 38 hp.

A motor's power is directly related to the amount of air/fuel mixture it can draw, retain, and expel throughout the ignition and exhaust cycles. Reducing the resistance that the air/fuel mixture encounters improves cylinder filling, combustion, and elimination of exhaust gases, thus increasing the horsepower. Hand porting of intake and exhaust ports by a professional is the most efficient way to reduce resistance and to increase the CFMs. For instance, stock Twin Cam heads flow approximately 181 CFMs at .500 valve lift (25") with the stock 1.84" valves. Hildreth Performance Heads increase this flow to 235 CFMs (25"), a significant increase achieved while still maintaining the stock valve size.

CFMs are not the only factor affecting performance, and just increasing the number does not mean an increase in a bike's ride-ability. One of the first things a rider notices when they purchase cylinder heads with increased CFMs obtained by larger ports and bigger valves (1.9 to 2" intakes), is that there is a significant loss of power in the lower (1,000 - 3,000) RPM range. Large valves and hogged-out ports can easily produce high flow numbers, but this increased volume of air and fuel can't remain properly atomized without the velocity provided by smaller valves and ports. Atomization is necessary to ensure that the air and fuel remains efficiently mixed for a more complete burn during combustion. High port velocity is essential for maintaining this atomization, thus producing wide, useable power bands, good fuel economy, and excellent throttle response

Performance for your Harley can be tailor-made to satisfy most demands. But it takes a little homework on your part. When embarking on the hunt for performance, evaluate what you expect out of a performance package; then talk with your dealership, engine builder, and friends; and finally, compare the numbers and claims of manufacturers. Once you are comfortable with a choice, go for it!

© 2004 Hildreth Performance

Last Updated on 06/14/2004