Flow bench testing appears simple on the surface, but it is not. Cubic feet per minute (cfm) statistics have become the industry’s exclusive focus since they are so easy to manipulate, neglecting other data concerning the cylinder head’s performance. You may think that flow bench testing would be simple. However, this is incorrect and will not help you on race day.
Flow bench testing and what you need to know about your engine’s cylinder head and induction system are examined here.
Cylinder head designers and head porters ignore flow numbers, but this does not indicate that flow figures are unnecessary. This nuance is frequently lost throughout the translating process. These numbers are significant and have a purpose. Just not in the sense that most people think of it. Data on airflow provides an understanding of how much power a vehicle can generate. They do not, however, ensure that you will get additional power.
According to industry experts, flow figures alone are meaningless and cannot be utilized to assess a cylinder head’s power potential. In a highly complex system, flow numbers are simply one of many variables that can be used to study the port’s features. The RPM range, acceleration, and power of ten different ports with the same flow statistics can vary drastically.
When designing the correct port/manifold design for an engine’s combination, the intricacy of arithmetic, aerodynamics, thermodynamics, and physics come into play. It takes years of practice to become proficient in these techniques.
Cylinder heads may be near in volume, yet their flow curves can differ dramatically. Many assume they may increase their power by purchasing a high CFM flow rate cylinder head. To achieve flow numbers, make everything large and mount it on a large bore. On the other hand, engine builders will be unable to use the results of the tests if they do not also have access to the relevant variables.
A slew of variables complicates cylinder heads’ performance. It doesn’t matter what the peak airflow of a ported head is, as if it doesn’t consider factors such as the engine’s size and rpm range as well as the location of the valves.
More essential than airflow is a slew of other issues. How far away from the cylinder wall do the valve heads extend? What’s the valve diameter to bore diameter ratio, for example? What is the airflow to valve size ratio? What is the port’s diameter, taper, and radius on the short side? When it comes to engine performance, the answers to these questions aren’t as straightforward as comparing flow numbers, but they are the ones that matter.
The CFM value is the last item to look at when flow testing a cylinder head. To make horsepower, there are three laws: air speed, air speed, and airspeed. The flow bench tells you this, and these are the three rules. A port must have a certain amount of airflow, but it must also have a certain average velocity and peak velocity. Then the runner must be tuned harmonically on the system and on the bench to achieve this. Properly built ports can move large amounts of air.
The most critical component of the induction system is the valve and seat configuration, which governs the amount of air that enters the cylinder.
The ultimate goal when it comes to Flow Bench Testing, is to maximize the effectiveness of that particular location. There will be no air movement when the most significant air is flowing. To get the best of all worlds, a compromise must be made between the mean air speed, peak airspeed, and airflow. An engine builder can’t make an informed judgment without all those data. It’s common for people to shop around for CFM numbers since they don’t understand what goes into them.
A cylinder head has been placed on a flow bench, and the results are in. “What’s that?” The discharge coefficient is the most crucial factor to consider.
The discharge coefficient fully uses the available surface area possible. It’s impossible to extract gasoline and circulate air through the engine without having a high discharge coefficient. This system’s average air speed must be between 246-268 feet per second. At the port’s narrowest point, the maximum speed should not exceed 285-310 feet per second, depending on piston speed and other parameters. It’s impossible to escape the pinch point in two-valve, pushrod engines. You know you’re safe if you keep the air speed of the pinch point below 300.
Remember that more fantastic ports don’t necessarily mean more power at the higher frequencies. The best power comes from the correct port size.
This information is critical for the cylinder head designer and flow bench tester. An experienced veteran can tell you if the cylinder head will hit its objective or not by taking a few measurements. If the measurements are correct, you can put a cylinder head on the flow bench to see if it has the flow potential to achieve what you want.
420 cfm can be obtained in various ways, but only one is correct. Cylinder heads are made to fit each engine model. The engine’s specific application necessitates an induction system design tailored to that use. A jet boat engine, a truck, tractor pull engine, or a drag engine all have different induction systems, even if they produce the same power at the same RPM. Because of how each engine operates, it will have another cylinder head.
A jet boat reaches a certain RPM and holds it there. The RPMs of a truck and tractor pull engine can be held at 9,600 and then brought back to 9,000 RPMs. A drag engine must shift up and down through the gears to reach top speed.
Three completely different motors will have various runner links and ports, valve diameters, and chamber dynamics because they are all designed for multiple purposes.
It is complicated to change a drag race engine or an engine that must accelerate through multiple gears, requiring a specific combination of components. Your airspeed, camshaft, and manifold are either on or off. It would help if you aimed for air speeds as high as possible while running the flow bench, such that it’s on the verge of traveling too fast or nose-diving. At that point, the fuel curve would rise sharply. It would consume more gasoline at the top to help the engine regain speed at the bottom of the gear change.
An advantage of the truck pull engine is that it doesn’t have to accelerate through the power range because it is wound up. The internal components don’t require a lot of energy to accelerate until the driver reaches 9,200 RPM. The ports can be slowed down a bit. It will be able to generate more power as a result.
Again, assuming the same engine, let’s say its pump speed is 8,500 revolutions per minute (RPM). To obtain the most air speed and cylinder fill, I’ll cam it and shrink the ports back to the point where the airspeed is around 20% below the critical maximum for the choke. Tune the induction system to that particular frequency at this stage.
An acceptable flow rate is required for the cylinder head to meet the needs of its intended function. As mentioned above, different port sizes, runner lengths, manifolds, and camshafts would be required for each engine.
There’s more to it than just airflow. Valves have a finite amount of time in which they can be open without affecting performance. To put it another way, the timing of the valve events must match the rest of the engine.
Cylinder heads are no different. Cylinder heads should not be judged only on their airflow capacity. Airspeed, port cross-section, port volume and form, and the connection between throat and valve seat size are significantly more critical considerations. Even if you spend all your time working on the flow bench to correct these faults, you will never be successful. It’s a good idea to ensure you understand all the relevant information before purchasing a new engine part or reviewing data from a flow bench test.