Harley Davidson Performance | Power Upgrades

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-litres 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-litre engine operating at 80 percent VE would actually only displace four litres 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 VE that is greater than the engine displacement.

One example would be a supercharged five-litre 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-litre and would of course, produce nearly twice the power output.