Flow testing has become a hotly debated subject in the rotary world. The mailing lists, and forums are full of discussions, arguments, and full out attacks, mostly debated by individuals who have never even built an engine, let alone, had to prove its performance on a dynamometer, or a race track. Unfortunately, these discussions are fueled by a few uninformed "Engine Builders" who feel the need to justify their back yard approach to engine building.

The fact is that flowbench testing is a very important part of engine building, and anyone who is not using a flowbench to develop their ports/manifolds is missing out on some very valuable information.

Let's consider a few obvious points.

Everyone interested in increasing horsepower will discuss, or at least consider flow.

Everyone talks about it, even the guys who claim that flowtesting is a waste of time. "I need bigger venturis because they flow better", or "A K&N filter flows better than stock." We all agree that increasing flow is a good thing, but, if you can't measure it, how do you know it actually flows better?

Even though you can't see, feel, or hear how well a port flows, there are those who claim that measuring the flow is not valid.

If taking an actual measurement of the flow is not valid, I don't know what is. Imagine taking the same approach to the rest of the engine build.

How would you set apex seal clearance without measuring the seal and the rotor housing with a micrometer? You cannot see the difference between a seal that is 3.1436" long, and one that is 3.1446" long. You also cannot see the difference between an exhaust port that flows 270 cfm, and one that flows 240 cfm.

Anyone who tells you that they can do either of the above is either lying, or delusional.

To take this concept to its most ridiculous extreme, imagine that you ask your engine builder for a quote, and he holds up two fingers and says "It will take a stack of bills about this high..." Ones? Twenties? Euro's? This high... WTF?

As ridiculous as that seems, it is no different than randomly grinding an intake runner, and then proclaiming "It flows more now!"

In many cases, it is quite possible to enlarge a passage and make it flow worse!

Let me give you a few examples.

1. Port matching the intermediate runners on a stock 12A intake manifold. This absolutely ruins the flow. Additionally, the velocity is reduced, and so both high, and low rpm power is reduced.

2. Removing the injector boss in a 6-port intake manifold. Reshaping, and reducing the height of this slightly will improve the flow a considerable amount, but if you go too far, the flow will decrease, and you will end up with a port that flows less than when you started.

3. Cutting a large radius on the roof of an exhaust port. (12A, or 13B) A generous radius is helpful if the exhaust port is close to stock size, but if it has been widened considerably, a large radius will absolutely kill the flow. This is one of the most sensitive areas of any of the ports in a rotary engine, and you can lose 10% to 15% flow in a hurry. The difference between a .050" radius, and a .070" radius can easily cost you 25 cfm.

So how do I know this? I measured it! There is no other way that I could know. I found out that these things didn't work because I did what looked right, and then measured the results, only to find that I could not have been more wrong.

The first time that I tested a stock 12A intake manifold with port matched intermediate runners, I couldn't believe the results. I actually spent the next half hour looking for leaks in the flowbench! Over the years, I have checked for leaks a few more times after seeing results that I couldn't believe. The point is that you just can't guess where the air wants to go, and just to make sure we are all on the same page, if you are not measuring, you are guessing.

This is the nature of airflow, and this is the reason that Boeing, NASA, F1 teams, and auto manufacturers still use a wind tunnel to develop their designs. These companies have entire teams of engineers with more airflow knowledge, and experience than we can imagine, and they still go to the wind tunnel to prove, test, and optimize their designs.

If that doesn't convince you that measuring is far superior to guessing, nothing will change your mind, and you should not read any further.

This is our flowbench. We designed and built this specifically for our own needs , rather than adapting a unit designed for testing piston engine heads. With this unit, we can test the flow of intake ports, exhaust ports, carburetors, throttle bodies, and even complete exhaust systems. We have many different fixtures for testing a variety of pieces. This picture shows the unit with the exhaust port fixture attached. (The flanged tube in blue.) The intake fixture is much more complex because we flow through a complete chamber which consists of a modified rotor, and rotor housing. This allows us to accurately simulate the flow patterns that exist inside the motor. It also allows us to add an induction system so that we can see how much an intake manifold, carburetor, or even an air filter will affect the total intake flow. To learn more about how a flowbench works, check out the Superflow website.

For all the excitement surrounding flowtesting, a flow bench is actually a very simple device, and the "technology" used for measuring airflow has been around since the 1800's.

When we flow test a part, we have to have a way to quantify the results, just as if we were measuring the length of a 2X4 with a measuring tape. The flow potential of a port, carb etc. is described as the amount of air, in cubic feet per minute, that will flow through it with a given pressure differential. (Normally referred to as the test pressure.)

Pressure differential is just another way of saying that there is greater pressure at one end than the other. This is as simple as sucking Pepsi through a straw. When you suck on the straw, you are creating a partial vacuum, the result being that the pressure is greater at one end as compared to the other. Once you create a pressure differential, you have flow.

The first thing that is needed to test airflow is a pressure differential, or more simply, a way to move a lot of air. A flowbench normally has several small vacuum motors, much like what is used in a standard shop vac. (My first flow bench was actually built from a large shop vac.)

This vacuum source can be used to either suck through, or blow through the piece being tested. Either approach is valid, as it is the pressure differential that determines the flow, not the absolute pressure.

The airflow is measured by passing the air though a calibrated orifice, and then measuring the pressure differential across the orifice. This "calibrated orifice" is normally just a thin metal plate, with a round hole in it (Pretty high tech huh?) and most flow benches have several orifices of varying diameters so that different flow ranges can be accurately measured.

Since the pressures that we are dealing with are normally quite small, they are measured with a manometer, rather than a gauge. A manometer is a very simple device which uses the force of gravity, and a fluid of known mass to measure pressure.

For a better understanding, have a look at our Intake cycle article.

What follows is a pictorial guide to porting, and flow testing an exhaust port.

The first thing to consider is that the horsepower output of any internal combustion engine is determined more than any other factor by the airflow potential of that engine. To describe it simply, all the power comes from the heat energy of the combustion of air and fuel. Getting enough fuel into an engine is not a problem, because fuel is quite compact, but along with that fuel, we need an appropriate amount of air to go with it. Filling the chamber with air at 9000 rpm is quite a task, because we only have 5 thousandths of one second to do so! Even at idle, the intake cycle only lasts 6 hundredths of a second!

As you can see, we must have very efficient flow passages if we are to get a reasonable amount of air into the engine. By now it should be obvious that filling the engines chambers with air and fuel is not a simple thing, and it is somewhat complicated by the fact that we cannot see, feel, smell, or even guess how efficient our flow path is. Luckily, we have the flowbench which will tell us exactly how efficient our flow path is. Without this tool, you are left to guess, and you will spend a great deal of time chasing your tail.

A picture is worth a thousand words right? We recently disassembled a 12A for a local customer who felt that it was in need of a freshen up. We were asked to do what we could to improve the power while we had it apart, and so we decided to lighten the rotating assembly, and spend some time on the ports.

We never know what to expect when we get inside of a competitors piece, but this motor gave us a pleasant surprise. The ports looked a bit rough, but flowed fairly well. This seemed like a great opportunity to give a "guided tour" of the porting and flowtesting process.

Just to make things more interesting, I had a rookie do the portwork. Having cut, and flowed countless exhaust ports myself, I normally get to the completed port without much testing, which wouldn't make for an interesting article. Nathan on the other hand did his first set of complete exhaust ports just a few weeks ago.

Note: Nathan has been working here for about 8 months while going to school. He currently has a bachelors in mechanical engineering, and is taking further schooling while working here building carburetors, machining and prepping engine parts, and double checking my math.

Not knowing exactly which areas require attention, he actually lost flow three times on the way to completing the port. This helps illustrate the importance of measuring the flow rather than guessing. The only guidance I gave was letting him know that the port roof would need to be widened, and to make sure that the inlet radius on the roof was not too large.

So, with very little experience, a digital camera, and a die grinder, I turned him loose.

	This shows the port before "tweaking" It looks a bit rough, but flows 230.6 CFM which isn't bad for a 12A exhaust port. This port was definetely cut by an experienced engine builder.
	This shows the port after widening the roof by "squaring" the corners. Going on looks alone, one would think that it would flow worse because of the square top, but in fact the flow jumped up to 249.6 CFM for an increase of 8.2% Take a good look at the two pictures. What you see is a very small increase in port size that resulted in a substantial increase in flow.
	This shows the port after further squaring and widening of the roof. (If a little is good, a lot must be better!) The widening was done only at the port window, and the flow dropped to 238.3 CFM which is a 4.7% decrease. Again, a very small change in port shape resulted in a substantial change in flow. From experience, I can see that the radii at the corners of the roof are too large, but going on looks alone, one would think that a large radius is helpful. The flowbench tells us otherwise!
	This shows the outlet side of the port after extending the port window shape into the runner. This reduced the corner radii, and brought the flow back up to the same 249.6 CFM that we had before.
	Knowing that having too large a radius on the roof of the port would hurt flow, Nathan removed some material from the flat portion of the roof, and guess what...We lost flow again!!! This time the flow dropped to 240.1 CFM. Having the flowbench to test the ports can be quite frustrating, but at least we know that we lost flow, and must do something to correct it. Note how slight the difference is between the two ports.
	This shows the port after laying the top back slightly to increase the radius on the roof. BINGO! The flow has now increased to 260.9 CFM which is an increase of 13.1% over our original port.
	At this point, the existing port window shape was maintained, but was smoothed up for another flow test. The flow increased by 1 CFM. Even though it looks much better, the increase was only 4 tenths of 1 percent.
	And finally, the arc of the port roof was flattened slightly, and everything was blended in. The end result is 263.8 CFM which is an increase of 14.4% over our original port. The port will now get a bit more clean up just for the sake of cosmetics, and it is ready to go.

Below is the original port, and the modified port shown side by side for comparison.

Note that only the top half of the port was changed. Any work done on the port floor would have shown little or no improvement, and substantial widening of the port floor would have hurt the flow badly.

So what's left? Just one simple question.

Do you want your engine builder to be measuring, or guessing?

Thanks for reading.

PY