Flow benches and cylinder head performance
For many years, flow benches have been an integral part of the engine builder’s tuning arsenal, allowing quantifiable data about port flow rates to be obtained. Although the principles of operation have not changed drastically over the years, the latest generation of flow benches and associated ancillaries are capable of providing greater levels of insight into cylinder head performance than ever before.
One area that has long been of interest to cylinder head tuners is the analysis of flow velocities within a port and around the valves. For decades, pitot tubes have been used to take spot pressure readings from inside the ports, and most flow benches come supplied with a port to accept a pitot.
A pitot tube can be used in various ways, either by being placed in a static location to measure flow velocity at that point or moved around like a wand, to try to identify areas of high or stagnant flow. While this makes them a useful tool for obtaining a general idea of flow velocity within the port, however, they have their limitations, the most notable of which is the fact that inserting a pitot tube into a port introduces an obstruction that can interfere with the airflow and skew the accuracy of readings.
This problem has led some head tuners to develop methods of taking pressure measurements from within the ports in such a way that there are no obstructions to the flow. For example, the author has seen one example of a test head where the tuner drilled a series of holes (known as pressure taps) around the valve seating area, with each attached to its own manometer. This allowed pressure readings to be taken without any impact on the flow conditions around the valve as there was no obstruction present.
The data it generated was also interesting, giving a snapshot of how pressures differed around the valve and changed with valve lift. It also allowed for comparative testing between different port designs.
There is one big disadvantage with this approach though, in that the heads that were drilled in this way could not be used for anything other than testing.
This brings us to a second novel approach to assessing differences in flow across the valve. It involves using a specially made valve that incorporates a pressure tap in the valve head. The tube from the pressure tap then runs up the valve stem to a tube that can be connected to the pitot port present on many flow benches.
As the head is run on the flow bench, the pressure at the point on the valve where the pressure tap is located can be recorded, either manually or via a flow bench software package. Once the pressures at that point of the valve have been recorded through the range of valve lifts, the valve can be rotated to record at a different point; adding an indexed collar on the valve stem means this can be done in a repeatable fashion.
The end result is a full pressure map of flow at the valve head throughout the valve’s entire lift range, providing an invaluable insight into a particular design’s flow characteristics.
These two systems just go to show that, although CFD and complex flow visualisation systems are the favoured approach these days for large racing operations, there is still plenty that can be learnt from a good old-fashioned flow bench.
Written by Lawrence Butcher