Flow benches wet-flow testing
Traditional flow benches for measuring cylinder head port flow rates are a useful tool for determining how efficient a particular port design is in terms of basic airflow characteristics. However, they are limited in that they do not show how a particular geometry affects other factors such as mixture suspension or distribution within the port.
Aspects such as these can be investigated using modern CFD techniques, although this is still a very specialised field and an avenue not available to many engine builders. This has led some tuners and cylinder head manufacturers to investigate methods of physically visualising port flow conditions through a process known as ‘wet flow’ testing; illustrated in the video link below.
The goal with any intake port development, in addition to maximising flow, is to design the port so the fuel is vaporised or mixed with the inlet air stream as uniformly as possible. This is difficult to accomplish because even the tiniest fuel droplet has significantly more mass than air, which means it is much easier for air to turn a corner or move around a valve seat than fuel. Thus it is not uncommon for fuel to fall out of suspension and collect on the port walls, ultimately entering the combustion chamber in a stream.
This can be a particular problem where the geometry of the inlet tract is far from optimal, for example in the case of V8 engines with a centrally mounted four-barrel carburettor, such as those in many drag and stock car series. Obviously this is not conducive to efficient combustion, so any improvements in port design that help keep fuel in suspension will aid performance. This is where wet-flow testing can provide significant benefits, allowing for fuel distribution in the inlet to be assessed and the impact of changes on behaviour analysed.
The mechanics of wet-flow testing are straightforward. The intake port is pressurised and liquid is introduced into the air stream using an atomiser. A clear plastic cylinder sleeve allows the operator to observe and record the behaviour of the fuel droplets in the valve bowl and combustion chamber. The liquid is then separated from the air and captured in a recovery canister.
The liquid that is introduced needs to be of the same specific gravity as the fuel that will ultimately flow through the ports, in order to produce behaviour as close to the real operating conditions as possible. In addition, the liquid contains a fluorescent dye, visible under ultraviolet light, to allow for its movement to be easily tracked and recorded using still or video imaging.
The resulting information can provide an invaluable insight into areas of potential improvement in a cylinder head’s design. For example, one head manufacturer found considerable real-world performance gains between one of its heads optimised using traditional flow bench techniques and one using wet flow. While both heads produced very similar ‘dry’ flow figures, the one developed using wet-flow techniques produced better numbers on the dyno, thanks to improvements in fuel atomisation and distribution.
The testing method is still in its infancy, but beyond basic port development it has also proved useful in the area of carburettor design, particularly with the four-barrel types that are common in US racing. No doubt as more engine builders and tuners are able to adopt wet-flow testing practices, they will find even more applications and make useful gains with components that were already thought to be ‘optimised’.
Example of a two-valve head being tested using the ‘wet flow’ technique
Written by Lawrence Butcher