SAC Engineering South Africa

Steves Auto Clinic - Engineering

Specialising in new, used, refurbished, gas flowed, big valve & race cylinder heads.

CYLINDER HEAD – VOLUMETRIC EFFICIENCY

The Truth About Your Engine’s Performance

There is a saying that has been around forever that “an engine is nothing more than an air pump”. It is understood this way because an engine takes air in and combines it with fuel to create combustion, only to release it again out of the exhaust system – creating the perception that it is simply air in and out.

However, this process is vital to achieving maximum performance within your engine and is measured as “volumetric efficiency” (VE). Simply put, volumetric efficiency (VE) measures the ability of an engine to fully fill and empty its cylinders with air and fuel to produce maximum power i.e. the higher your engine’s volumetric efficiency, the higher its power output.

Your engine (cylinder) starts filling with air every time the piston moves down on the intake stroke and a 100% volumetric efficiency is only achieved when the cylinder volume is entirely full i.e. a 5 litre engine that is operating at 100% volumetric efficiency will displace 5 litres of air and fuel during each completed engine cycle. This is where we get the term engine displacement.

Obviously, there are factors that affect how well your engine will fill itself i.e. camshaft profile, intake and exhaust components and most importantly engine speed. As the engine speed increases, the amount of time the cylinder has available to fill reduces and this results in an engine that does not always fill completely. For example, if our 5-litre engine is operating at 80% volumetric efficiency it would only displace 4 litres of air and fuel.

When considering the above it is clear, that if you want to improve the power output of an engine (with a given displacement), you must improve its ability to flow air in and out.
In some cases, it is even possible to have a volumetric efficiency that is better than the engine displacement. A good example of this would be a supercharged 5-litre engine that has double the amount of pressure in the manifolds as the external atmosphere. In a case like this, the engine’s airflow would be approximately double that of the normal 5 litres’ and it would naturally output nearly twice the power.

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* All manufacturer’s names, numbers, symbols & descriptions are for reference purposes only. It is not implied that any part, conversion or modification is the product of these manufacturers.