There are a lot of little things that can fall into the background of engine performance testing. Everyone sees the 15-second dyno pull and the engine’s numbers, but not many get to see the time and effort it takes just to get the engine onto the dyno. It’s similar with the flow bench.
Sure, most people understand that more flow equals more horsepower, so they are aware that flowbench testing exists, but for the average enthusiast, that’s where it ends. Since you’re an EngineLabs reader, chances are you are more than the average enthusiast, so you are probably more aware of the cylinder head flow testing process.
The flowbench is a valuable evaluative tool in the engine builder’s arsenal, but like any other tool, you have to use it properly. What we want to look at here in the testing process is one of the fundamental principles of airflow, and one of the biggest factors in testing on a flowbench: the effect of a sharp edge on flow numbers.
Smoothing Out Flow
Usually, when looking at pictures of a cylinder head on a flow bench, you’ll see a short section of what looks like a header primary on the exhaust port and something that looks like a kid’s craft project surrounding the intake port. Both of these are designed to smooth out the airflow going into the port and give a more realistic flow condition into the port.
“On an engine like [the N/A 10.5 SBF] there can be a 40cfm difference between the reading of a bare port and the same port with a clay radius,” says Ben Strader of EFI University. “That sharp edge kills them.” The sharp edge he’s referring to is the junction of the start of the port and the intake flange mating surface of the cylinder head.
Right off the bat, you might be asking something along the lines of, “If that sharp edge kills flow, why not round it off?” If we operated the cylinder heads open to the atmosphere, then you would be on the right track. However, when the engine is assembled, the intake manifold butts up to the port with, ideally, a perfectly matched opening, making that sharp edge disappear.
Since the vast majority of cylinder heads aren’t tested on a flowbench with a manifold, that sharp edge needs to be “softened” so as to not introduce unrealistic turbulence into the test. That’s where the clay comes in. By creating a radius into the intake port, the turbulence induced by that sharp edge is eliminated, and you can more accurately evaluate the port’s flow characteristics.
To prove exactly what kind of real-world effect it has, once we completed flowing our stock LS5.0 cylinder heads, Strader kept the flowbench on, and pulled the intake clay off of the port. We saw an immediate 24.7cfm drop in the flow numbers at .700-inch of valve lift. Additionally, using the pitot tube wand, we saw an increase in turbulence inside of the port.
Then, aiming to quash some internet debates, Strader slapped (literally slapped) the clay he pulled off back onto the port, and immediately saw the port’s flow return to within one-percent of the flow numbers achieved with the carefully applied and sculpted port clay. “I’ve had people criticize how sloppy my clay radius is on the internet,” Strader chuckles, after proving that you don’t need a radius formed by Leonardo DaVinci himself, to achieve the desired results. The goal is to just break the edge turbulence, without inducing any new turbulence to the test.
Why Not Test With a Manifold?
At this point, you might be asking, “Why use clay at all? Why not just use the manifold itself on the head?” Again, that’s a great question. The fact is that sometimes an actual manifold is used, but isn’t an everyday occurrence. That’s for a variety of reasons from the availability of the actual manifold to the difficulty of fixturing the flowbench with a manifold attached.
“A lot of time when I’m flowing something high-end — like that N/A 10.5 engine — I’ll flow the head with the intake on it to get a real-world representation. With a really good manifold, you won’t lose anything, flow-wise,” says Strader. Flowing with the actual intake installed onto the head also has the benefit of measuring the behavior of the system as a whole, but is generally an extra step only seen in really high-end projects, when you are analyzing an entire system, not just a single component.
However, when you move to the exhaust port of the cylinder head, you don’t see a clay radius being used. That’s not because that sharp port-manifold flange doesn’t affect flow exiting the port (it does, significantly), but rather because a clay radius doesn’t provide the proper analog to the real-world system. Instead, a short section of primary tube is bolted onto the port to better simulate real-world flow coming out of the port.
The whole purpose of testing a cylinder head’s flow is to gather data on how the cylinder head will perform for its intended application. By applying the clay intake radius and the exhaust tube, a more real-world picture of the port’s behavior can be gotten by eliminating the turbulence created by high-speed air passing over a sharp corner. It also highlights why port matching an intake manifold to the cylinder head is so important when you are looking for every last horsepower.