Frank Talk About Flow Numbers
I often hear people say that flow numbers don’t matter, and people should not pay attention to them. This is a gross oversimplification and to be quite frank, these statements are dead wrong! Experienced cylinder head designers and head porters 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 the translation. Flow numbers are important, and they do have meaning. Just not the meaning most people attribute. Air flow numbers alone give insight into the potential for power. They don’t guarantee you will make more power. What professionals in the industry really mean to say is, “Flow numbers alone don’t matter and cannot be used to judge the power potential of a particular cylinder head design.” 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. I will let you in on a little secret: achieving mind numbingly high flow numbers is easy. So easy, in fact, that I can show anyone how to do it in one day. What I can’t teach in one day, is how to incorporate experience in conjunction with the math, aerodynamics, thermodynamics and the physics it takes to design the correct port and manifold design for a particular engine combination. These processes took me 20 years to master.
Average Air Speed
Average air speed in the induction path is tuned to a specific level. The, “induction path,” as I refer to it, is the intake manifold and port as one unit. Even a slight variation in the average air speed in the induction path will alter the power band. You can have 100 different port manifold combinations with identical flow numbers, but the average air speed can vary greatly. Having the average air speed too fast or too slow always results in an under-performing engine.
Peak air speed
Peak air speeds occur at precise areas in the induction path, and are used to tune or accent the power band at different engine speeds. If these areas in the port have their peak air speeds too slow or too fast, the resulting effects will have profound changes on engine power and acceleration. The engine could either stop accelerating due to sub-sonic choke or worse yet, not accelerate because the speeds are too slow.
Port Geometry (shape)
Port shape is unbelievably important. I can’t stress enough how the shape of a port can change its characteristics on the running engine. You can have 100 different complex geometric port shapes that all flow the same, yet only a few will stand out as being more powerful than the rest. How efficiently the port uses its area in relationship to its flow can be measured, but that’s not all there is to it. Certain shapes carry the air fuel mixture more efficiently than others, and this fact is not aerodynamically intuitive. As a matter of fact, it’s the opposite of aerodynamically intuitive! Just because a port looks pretty, swoopy, smooth, or aerodynamic, does not mean that it will operate well on the running engine. The terms “smooth” and “pretty” should not be in the general lexicon for describing ports, because neither has anything to do with how they perform! There are other dynamics to take into account, such as pressure waves and fuel suspension in the air stream. Particle flow and air flow are two separate entities. Port shape and air speed addresses them both. To sum it up, air flow quality is just as important if not more so, than air flow quantity.
Wet Flow (otherwise known as the air fuel mixture)
This goes hand in hand with port shape, air speeds and every other aspect of port design. Everyone looks at air flow but forgets the fact that air alone is not the working fluid, even in a fuel injected engine! Smokey Yunick was the first to refer to the air fuel mixture as the “working fluid”. I found this definition not only poignant, but accurate in its description and have used it ever since. Very few consider or even know about the fact that particle flow and air flow are separate entities. Although they are separate entities, they are interdependent. Particle flow depends on air flow, port shape, manifold design and air speed to carry it to the cylinder in the most homogeneous state possible. That’s the whole goal isn’t it? Get as much air and fuel into the cylinder as you possibly can, and have it evenly mixed so it burns quickly and completely. In the real world, attaining a totally homogeneous mixture is impossible. Just like every other aspect of engine design, it’s a give and take situation where the best compromise is the best solution. Here is a little known fact: a great majority of particle flow in a port turns 60-90 degrees instantaneously. The air however does not.
The “Mach Index” of a port will not show up on any flow bench unless you know how to measure it. Flow numbers give absolutely no insight whatsoever into how a port will perform in a given RPM range. The Mach Index however, gives us an indication as to what RPM range a port design is best suited. Low Mach Index ports limit the engine speed in which peak VE can occur. Attempting to operate a low Mach Index port at a higher RPM range than it was designed for, only results in decreased VE across the entire RPM range, and an engine that won’t accelerate. A 23 SBC port is a perfect example of an ultra-low Mach Index port, while a Pro Stock port is the best example of an ultra-High Mach Index port. 90% of ports designed for racing are somewhere in between these two extremes. This is one reason why head porters raise the ports higher off the deck in order to get a straighter flow path. The “Mach Index” is a way of rating how well the port handles high air velocities and how efficiently it carries the air to the cylinder. Two ports can have identical flow numbers, yet vary 1000RPM+ and more than 100 horsepower! A port with the correct Mach Index for the engine combination, can efficiently fill the cylinder in the intended RPM range. If the Mach Index is too low, it will cause the engine to “nose over” at higher engine speeds, and accent the lower portion of the power band around peak torque. If the Mach Index is too high, it will cause a reduction in low end torque and mid-range power accenting only the top end of the power range.
Combustion Chamber Design
Here is a little known fact: the chamber is part of the intake port. The chamber walls are an extension of the intake port design and effect how it flows dynamically in the running engine. As the intake valve opens, pressure recovery takes place. To put it simply, this means that the air exiting the valve is being slowed down at the correct rate. The flow bench will flat out lie to you in this case, because a poorly designed chamber can flow more air than a proper one! People forget that the intake pressures we deal with on a flow bench are 5.5 times less than what you’re dealing with in the running engine. The dynamics of the running engine cannot be fully simulated on the flow bench. This is one of those instances.
Exhaust Port Design
I have bad news. You can not design an exhaust port on a flow bench the way you do an intake port. If you’re designing exhaust ports to flow the most air possible, you’re destroying the engines ability to blow the cylinder down and exhaust properly. If you design an exhaust port to flow the most air possible on the flow bench, the power will be lower, fuel signal will decrease, and the engines acceleration will suffer. Professionals understand that designing an exhaust port on the flow bench has nothing to do with how much it flows, but everything to do with how it flows and at what air speed. The exact size, shape, and air speed of the port in conjunction with how smooth it flows, are the primary points to consider. Personally, I care very little about how much the exhaust port flows. It’s at the very bottom of my list of importance. I don’t try and make the exhaust port flow as much as possible, because I have been down that path hundreds of times and it’s a dead end street! The pressures the exhaust port has to deal with in the running engine are hundreds of times greater than the flow bench could possibly simulate. Ask yourself this: why would you design a port using 1 PSI test pressure at 70°F, when it’s actually flowing at 500PSI+ and 1300°F?
Bore diameter has a great deal to do with the performance of the cylinder head. Not only how much it flows, but more importantly, how it flows. Most modern performance heads used in Drag Racing are designed around the largest bore possible. For a Big Block Chevy that would be a 4.625 bore. The reason for this is simple: with larger bores we can increase valve diameter, valve efficiency and produce more horsepower at the same or higher engine speeds. A big problem arises when people attempt to use these heads on much smaller bores. If a cylinder head design was optimized around a 4.625 bore and someone uses it on a 4.500 bore it shrouds the valves, decreases air flow, and consequently destroys the port’s air speed characteristics. This totally destroys the entire power band from start to finish! You must utilize a head designed for the bore you’re using with smaller ports and valves. Yes, the head will flow less, but it will make more power and accelerate down the race track much faster!
Testing Regimen and Setup
When comparing flow numbers, the majority of people never consider the testing regime or setup used to attain these numbers. No one asks if the flow bench or data acquisition system was properly calibrated. Most just take the numbers at face value, because most are not aware of how a simple flow bench setup can greatly affect the flow curve. Take for instance, bore size. Very few advertise what bore diameter they used to flow the heads. I have seen flow numbers that where totally untrue because the heads where flowed on a bore size not found in reality. I have seen a company who flowed SBC heads on a 4.250 bore. That’s not achievable in a 4.40 bore space SBC. That 4.250 bore added 12 CFM of air flow across the entire flow curve! Were they trying to defraud the general public? Actually, no. The person who tested the head for this new company had just purchased his first flow bench, and didn’t realize what he was doing. I would like to say this is an uncommon occurrence, but I can’t. Using incorrect flow testing setups by novices, or people who just don’t care, is common. Professionals however, know better and go to great lengths to mimic the engine the heads will be used on. Otherwise the data is useless. Another factor to consider is that flow benches vary in accuracy, and not all flow benches are equal. I go to great lengths to accurately calibrate the flow bench at Reher Morrison. I have ten scientifically calibrated flow orifices in 50-100 CFM increments that calibrate the entire flow curve for every flow range. I do so because if my testing is inaccurate, ALL my data is inaccurate, and that’s a disaster! All the other variables such as air speed, power potential, Mach limits, and many others variables, are calculated and measured on the flow bench. If my numbers are incorrect, I am lost.
The truth of the matter is this: flow numbers are about the least important factor you should utilize when purchasing a cylinder head. Looking back over everything I have stated here you can see there is a lot more to cylinder head intake manifold design than most know. What’s really scary is the fact that I left out about 65% of the other variables that go into cylinder head and intake manifold design because this would turn into a book instead of a short editorial. You may be asking yourself,” How does a racer know which head would fit his engine combination”? Bottom line is, they can’t. People on the internet, racing buddies, friends or novices have your best intentions at heart when they speak up and give you recommendations regarding your engine, cylinder head and manifold choices. They are also the least qualified to make that determination. This type of “internet” research almost always ends badly! I help about ten people a day pick out the correct head manifold combination for their engine. Sadly, I am confronted by the fact that more than 50% of these people had to spend their money twice because they got bad advice! Dropping $3000.00+ on a set of heads is hard enough without having to do it twice! What type of cylinder head and intake manifold you need should be left up to professionals? Professionals whose life long discipline has been to research, develop, design, manufacture and test cylinder heads on every conceivable engine combination possible. Professional engine builders like Reher-Morrison and cylinder head designers like myself work out what cylinder head and intake manifold packages make the most power for a wide range of engine combinations ranging from reliable 7500rpm bracket engines to 11000rpm small blocks. We do the testing of camshafts, intake manifolds, heads and hundreds of other components on a daily basis. Why would anyone want to spend their own money to test what we have already tested?
We have already invested huge sums of money, time and research so you don’t have to!