Engine Flow Numbers – Bigger is not Always Better!
On the surface, flow bench testing seems straightforward, More flow = More HP. Many in the industry have become solely fixated on cubic feet per minute (cfm) numbers, placing importance on a number that is easily manipulated and ignoring other data about a cylinder head’s performance.
Here is a deeper look into flow bench testing for better understanding of cylinder head and induction system selection for an engine.
Experienced cylinder head designers and head porters often tell people not to pay attention to flow numbers, but that does not mean that flow numbers don’t matter. This distinction is often lost in translation. Flow numbers are important, and they do have meaning. Just not the meaning most people attribute. Airflow numbers give insight into the potential for power. However, they don’t guarantee you will make more power.
Flow numbers are just one variable used for various equations to analyze port characteristics in a very complex system. You can have 10 different ports with the same exact flow numbers, yet they can vary wildly in RPM range, acceleration and power.
The complexity of cylinder head set up and flow bench testing comes in the form of math, aerodynamics, thermodynamics and the physics it takes to design the correct port and manifold design for a particular engine combination. These processes take years to master.
By the Numbers
Cylinder heads often can be close to one another volume-wise, but they can vary drastically in flow curve. People mistakenly believe that if they buy a cylinder head that flows more air and has a big CFM number, they’re going to make more power. It’s really easy to get flow numbers – just make everything big and put it on a big bore and you’re done. However, the testing data will be useless to an engine builder without knowing all the other variables you need to know.
The factors that determine the performance of a cylinder head are complex. A head that is ported without considering air speed, the size of the engine, the rpm range, the location of the valves, and a dozen other parameters isn’t going to be the best head, regardless of its peak airflow.
The experts will dig deep with other questions like – How far are the valve heads off the cylinder wall? What’s the ratio of valve size to bore diameter? What’s the ratio of the airflow to the size of the valve? What’s the size of the port, what’s its taper, how high is the short-side radius? The answers to these aren’t as simple as comparing a flow number, but they are what really make a difference in an engine.
The flow bench tells air speed, and there are really a key component for making horsepower especially for a street car – air speed and velocity. If the head is designed properly, it will automatically flow a lot of air.
Valve and seat configuration is the most important area in the whole induction system because it controls the air entering the cylinder.
Flowing the most air is easy, but it won’t have any air speed. So there is a give and take there and a balance that has to be struck between the mean air speed, peak air speed and air flow. Without all that information, an engine builder can’t make a reasonable decision. People go shopping for CFM numbers because they don’t know all the underlying things that go into it.
The peak velocity should not exceed, depending on piston speed and other factors.
What you have to keep in mind is bigger ports don’t necessarily make more top-end power. The right size port makes the most top-end power.