Performance / Power Upgrades for your Harley-Davidson

What can be said about Harley-Davidson Performance?

There is an adage that goes something like: “An engine is nothing more than an air pump.”

The saying has been around forever and is usually understood to mean that because a Harley Davidson engine ingests air and mixes it with fuel for combustion, only to discharge it again out the exhaust pipes, it is merely pumping air in and out.

The term “volumetric efficiency” (VE) is used in measuring an engine’s performance as a guideline for establishing its ability to completely fill and discharge its cylinders with air and fuel.  Obviously the higher a Harley Davidson engine’s volumetric efficiency, the more power it will be able to produce.  It begins filling with air each time the cylinder moves down on the intake stroke.

When the cylinder volume is completely full it is considered to be 100 percent volumetric efficient.

This is where we get the term engine displacement: A five-litre engine would displace five-liters of air and fuel on each complete engine cycle if it was operating at 100 percent volumetric efficiency.

Of course, there are many factors that affect how well the engine will fill itself, such as intake and exhaust components, camshaft profile, and most importantly, engine speed.

As the engine speed climbs, the amount of time the cylinder has to fill decreases and because of this the engine doesn’t always fill completely.  As an example, our five-liter engine operating at 80 percent V.E would actually only displace four liters of air and fuel.  From the example, it is clear that in order to improve the power output of any engine (with a given displacement) we must improve the ability to flow air in and out.

In certain circumstances, it is possible to have a V.E that is greater than the engine displacement.
One example would be a supercharged five-liter engine that has twice the pressure in the manifold as the outside atmosphere. In this case, the engine airflow would be approximately double that of the normal five-liter and would of course, produce nearly twice the power output.

Modified Cam Selection Explained

Making the right modified cam choice is important, it changes your bike in more ways than one.

Sure there is almost always more power at the high end of the rev range, but what happens at the lower end is important too. Camshaft engineers design mod cams to perform well in the mid-range and more importantly top end but they often neglect to address all the idle issues. A mod cam has a number of specifications that one would need to study to understand the outcome – for example, lift, duration, timing spec, lobe separation, clearance duration, overlap, etc. to mention only a few. Most people only see the extra power and torque at the point of sale as this is important for them but fail to understand how the idle and just off-idle ride is compromised if the wrong cam is chosen. The loping effect (hunting) generally follows through while the bike is moving in gear slowly while the throttle is closed or close to closed.   What happens at idle almost always happens just off-idle too.

This would really only apply to or affect most female riders and those who move around carefully and slowly in car parks or do very slow-pack riding and or maneuvering. Getting away slowly and smoothly is very important for some riders, especially on an uneven surface.

Be sure to address the idle issues at the point of sale as it is not always possible to eliminate these issues during the tuning process. The lobe separation (lobe centers) on the camshaft spec compromises the idle, vacuum sensors, starting procedure, etc. Many people enjoy the loping sound at idle which is completely understandable and generally enjoy the result when the throttle is opened and the bike accelerates rapidly.  There are good modified camshafts available that generate good top-end power and don’t compromise the very low-end torque, take-off, and idle –  so you need to do your homework.

 

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