The intricacies of airflow dynamics through a cylinder head are some of the most difficult concepts to grasp in all of hot rodding. That made it the perfect topic to discuss in the inaugural installment of CHP's new "How it Works" department last month. As it turns out, we underestimated the sheer volume of meaty tech info our expert cylinder head designers not-so-willingly coughed up, so now we're making a call to the bullpen and heading to extra innings.
This month, we'll cover the aspects of head design we didn't get to address the last go-round, such as the effects of valve angles on airflow, combustion chamber theory, the relationship between cfm and power output, CNC machining, and how to scientifically determine an engine's airflow needs. Returning for battle is our razor-sharp cast of experts that includes Tony Mamo of Air Flow Research, Kevin Feeney of Racing Head Service (RHS), Al Noe of Trickflow Specialties, Tony McAfee of Dart Machinery, Darin Morgan of Pro-Filer Performance, Jason Neugent of Brodix, and Bryce Mulvey of Dr. J's Performance. At the risk out tooting our own horns too loudly, it's the next best thing to eavesdropping on a Pro Stock team's R&D department.
Darin Morgan: "There is no substitute for a low valve angle head design just as long as you raise the port high enough to make it work properly. Low valve angles let us increase valve size, decrease chamber burn time, reduce pumping losses, reduce detonation or pre-ignition sensitivity, reduce chamber shrouding, and achieve discharge coefficients higher than any other cylinder head designs. Low valve angle heads in the 7- to 12-degree range with port heights of 1.250 inches or more are the most efficient designs possible today. Pro Stock heads are 9- to 10-degrees for a reason. If you look at the most powerful normally aspirated engines, you will find an overwhelming percentage of low valve angle heads. When comparing the flow numbers of a 22-degree BBC head versus a spread-port head, we see that we need to look at the overall induction system design and package. A low valve angle head with low ports that flows high cfm numbers will not outpace a cylinder head with a low valve angle and high ports flowing the same cfm. With the early LS-series engines, there were underhood packaging considerations that the factory engineers had to consider. The short-side radius on the LS1 and LS6 heads is what we call hypercritical, and is not conducive to high flow above a certain lift point. They are great heads, but GM saw the light when they came out with the LS3 and LS7 cylinder heads. These heads have a low valve angle and high ports, not to mention a really nice chamber designs. These heads are the most efficient heads ever to come from GM, and unlike other OEM heads they have room for the high-performance enthusiast to expand upon."
Tony Mamo: "Flatter valve angles allow the valve to be unshrouded from the chamber walls faster than with steeper valve angles. That's certainly a perk, but it's not the end-all-be-all of cylinder head geometry. It's just another important piece of the puzzle. Also, there is a point of diminishing returns. There's a big improvement going from a 23-degree head design to a 15-degree head, but there isn't such a big difference when going from a 15-degree valve angle to a 12- or 13-degree angle. A better-designed 15-degree head could make more power than a 12-degree head. The real one-two punch is when you raise the entrance of the port and move to a flatter valve angle. That's when things really start happening regarding significant improvements in airflow and power production."
Al Noe: "Street heads with flatter valve angles tend to have more mid-lift airflow but sometimes aren't stable at over 0.600-inch lift, which can be undesirable. Valve location in the bore is more important than the actual valve angle, and a head with a flatter valve angle and optimum location will almost always make more power than a head with a steeper valve angle. Port shape, port floor, and roof location have to be optimized for the chamber and valve design. Not getting the valve diameter right for the intended engine size and rpm is one of the biggest mistakes head porters and engine builders make."
Tony McAfee: "The flatter the valve angle, the harder it is for a head porter to increase flow. A 10-degree valve angle in a standard SBC port would never flow any air. Low ports and a flat valve angle are a very bad combination. Once you start raising runners, however, flow improves dramatically. The advantage of a flat valve angle is that valves won't choke themselves against the bore as they open. Some production heads, like the LS7 castings, are as good as race heads. Their high ports combined with flat 12-degree valve angles means that air doesn't have to negotiate a 90-degree turn at the short-turn radius. Furthermore, having flatter valve angles and raised ports means that the combustion chamber is easier to design since so much of it isn't biased toward one side of the bore. Shallower chambers improve flame travel and efficiency."
Jason Neugent: "Diesel engines usually have a flat valve angle and are not considered high flowing heads. In most cases, when you stand the valve up to achieve the flatter valve angle, the intake port is elevated to complement the angle change. The angle that the intake port meets the valve can be critical. This elevated port is often beneficial in making a higher-flowing manifold as well. The shape of the roof and the floor can also be influenced by the type of fuel that is going to be used. Valve location is critical because of the valve's proximity to the bore wall. Having the exhaust valve close to the bore usually doesn't hurt the flow a great deal, but the intake valve needs to be farther away from the bore wall to promote decent airflow. The relationship of the valves to the bore is usually the deciding factor in valve size as well as valve placement."
Bryce Mulvey: "The main benefit of a flatter valve angle is that you can run smaller combustion chambers, especially with heads in the 12- to 15-degree range. Smaller chambers promote improved flame travel and combustion efficiency. Cross-sectional area in relation to engine size is more critical than valve angle. If you put 18-degree heads on a 427 small-block Chevy with undersized ports, then you swap them for 23-degree heads with properly sized ports, the 23-degree heads will make more power. Compared to a 23-degree head, the pushrod is moved over in an 18-degree head for a wider pinch point, which increases cross-section. Most 23-degree heads don't have this luxury. This also allows moving the valves over, which frees up space to run larger valves. Ultimately, the overall port design of an 18-degree head is much better than that of a 23-degree head. The flatter valve angle is just one of many variables that need to be optimized to design a high-performance cylinder head."
Kevin Feeney: "Valve angle is only one of many factors that contribute to flow. Air only likes to turn small amounts at a time because it's moving so fast through the ports. The height of the short-turn radius and its relative position to the port opening along with the valve angle all work together. If you were to simply flatten out the valve angle and not make any other changes you would most likely create more problems. It is important to create a 'happy' port by having all aspects of the port working together."
Darin Morgan: "A properly shaped combustion chamber is a balance of pressure recovery, wet flow, and flame travel. These three factors can make or break you. The pressure recovery dynamic is simply the chamber shape being an extension of the valve seat angles, and letting the airflow decelerate at the proper rate once the valves open. A chamber that is laid back too far is a disaster and causes a total loss of pressure recovery, flow control, and wet flow. When the valve opens, it's supposed to open evenly all around the valve. The SB2 heads have the best pressure recovery characteristics of anything I've ever seen. Something as simple as milling the heads to increase compression can kill pressure recovery on the short-side radius area and kill airflow. Likewise, peak volumetric efficiency will be reduced when the pressure recovery is undermined by a chamber that has been laid back too far. Every cylinder head is different, and with heads with low valve angles ranging from 7- to 14-degrees, it's very easy to design and manage all three factors. With very low valve angles, the chamber can come right off the valve seat like a venturi. With higher valve angles, you need to have a deep concave chamber to help pressure recovery. Deep concave chambers are more difficult to work with, but laying them back too far is somewhat difficult as well. Deeper concave chambers, like the ones found in 24-degree BBC heads, often have poor wet flow and reduced pressure recovery due to valve shrouding. Many times there is little you can do about it. Wet flow benches let us take these poor burning chambers and manipulate the seat angles, valve job, and chamber design in order to improve their poor wet flow characteristics."
Al Noe: "Combustion chamber design should come first, and then the port, valve, and valve job should be tailored to work with it. Combustion chamber shape is most dependent on the intended valve lift range and desired port 'cross talk.' Street heads that are designed for maximum airflow by 0.700-inch lift tend to be very de-shrouded around the valves at low lift, and tend to take on a bowl shape. The flatter the area between these intake and exhaust bowl shapes are, the more cross talk tends to occur between the intake and exhaust ports, which can help torque production. In high-lift race heads where port airflow stability at very high lift is desired, steeper valve seat angles are employed and the chamber becomes shaped more like a funnel and less like a bowl. They're very flat from the valve seat to the deck and laid back for maximum high-lift area. The amount of cross talk that is designed into this type of chamber can be crutched with cam overlap to accomplish the desired power curve."
Tony Mamo: "The ideal shape for a combustion chamber is a design that best complements the port design and port bias of both the intake and exhaust ports. This may take quite some time to figure out if you're a perfectionist and are constantly striving for more. Unfortunately, there is no blanket statement to be made concerning chamber design and shape, but most of the chamber designs I tend to arrive at all seem to have a figure eight design to them with purposely shaped approaches to both the intake and exhaust valve. I think that combustion chamber development often gets overlooked and it's actually an area I spend quite a bit of time working on, as I view it as an extension of the intake port and as the entrance to the exhaust. Both are very critical to flow and need slightly different shapes to optimize their respective jobs. You would be surprised how much airflow-and ultimately power-can be found in getting the chamber nailed just right. It can really help improve the flow curve of a head, which ultimately can separate a good head from a great head come dyno time. A good chamber design also usually shows itself in needing less ignition lead time for maximum power, and that's another perk. Less ignition advance is usually more reliable over the long haul, especially when using power adders such as boost and nitrous oxide."
Jason Neugent: "Chamber shapes will differ based upon the valve angles, plug location, and the valve seat angles. The combustion chamber needs to be the best compromise to complement these factors to support good airflow as well as proper wet flow. More than any other factor, the shape of the chamber determines how efficiently a motor burns the air/fuel mixture. That being the case, it's imperative to invest just as much effort into the chamber design as the port design."
Bryce Mulvey: "Getting the angles right from the valve seat into the chamber is one of the most important parts of the entire head. If you cut it too flat, you'll get wet flow separation and mess up pressure recovery. You'll also create vortexes that hurt flow as well. Most people go with a 45-, 30-, 15-degree valve job. I prefer a 45- to 35-degree cut right into the bowl so there are no flat areas for the fuel to drop out of suspension. The valve job really is the foundation of airflow since air has to negotiate the turn around the valve as it exits the port. We've seen heads pick up 25-30 hp just by changing the valve job and throat diameter. A common mistake is to undersize the throat diameter, so it should always be sized properly based on valve diameter. Combustion chamber shape is also important, but plug location is very important too. If you have a bad plug location within a good chamber, you will hurt power quite a bit. Likewise, if the plug is positioned too deep inside the head, fuel will huddle around the spark plug hole, which requires more ignition timing and increases pumping losses."
Kevin Feeney: "At RHS, our cylinder head designers believe the combustion chamber shape is probably the second most important part of the cylinder head. The shape of the chamber can greatly affect the spark timing an engine requires, and can cause a lack of fuel in areas of the cylinder, which will make for a real uneven burn in the cylinder. This severely wastes potential power. The chamber shape determines how the fuel is dispersed in the cylinder, and varies depending on the type of valve angle in the cylinder head that you're working with. For example, you would not use the same chamber shape in an 18-degree head as you would in a 23-degree head."
CFM and Power
Darin Morgan: "If you use a 235cc port flowing 340 cfm and they hurt power over a set of 210cc ports flowing 300 cfm, then the air velocity was to low in the 235cc heads. It's not always that simple, but that's the general idea. The 210cc port heads may have flowed less, but their air speed matched the engine combination more closely than the higher-flowing cylinder heads. This scenario shows the end result of installing a cylinder head that flows too much air without enough air speed, but what if we reverse the situation? If you install a 220cc head flowing 300 cfm on an engine that needs a 235cc port flowing 340 cfm, you will decrease the engine's rpm range and peak power.
"Air speed is the most important tuning factor when designing a port, intake manifold, or any induction system component. It's not the only one, just the most important. People are infatuated with cfm numbers because that's all they have to judge a cylinder head's worth. It's extremely frustrating for people in the cylinder head business. Blindly sacrificing air speed for airflow is a fool's errand. When a customer demands the most airflow possible they are not always correct in doing so! The problem is when customers fail to ask for the most airflow possible within the limitations of the air velocity and sizing constraints for his particular engine. This is and will continue to be the root of the problem."
"It's not often that a head with lower advertised cfm figures makes more power than a head with higher cfm figures, but it does happen. The reason for this is velocity just about every time. When you sacrifice air speed for airflow, you may be getting yourself into trouble. This of course depends on the engine combination you're dealing with and the airflow requirements of that engine. Today, professional port designers have been able to develop ports that do both. I have ports that flow a lot of air and have an exceptional velocity profile. I always try to get the most air possible, but never sacrifice air speed in order to achieve it. That is unless I can get away with such a trick, which is very rarely."
Kevin Feeney: "The public's fixation with flow numbers is a constant battle in the cylinder head market. At RHS, we stress to our customers that the quality of the flow is more important than the quantity of the flow, and this is mainly based on the balancing act between flow, velocity, and runner cross section. To properly design a port, you need to know what size the engine is, the target rpm band, and the application. At this point the taper in the port is determined to accommodate the criteria outlined. Ultimately, the taper will dictate what the runner volume will be. This is why the port volume on an engine will differ on another engine of the same displacement that's designed to operate in a different rpm band. It's not uncommon to have a properly designed port that does not yield the highest flow numbers on the bench, but makes more power and performs better on the track."
Darin Morgan: "Shaping a port for maximum airflow and velocity is a painstaking process. In addition to using a grinder, I often use tiny pieces of epoxy to add material where needed, and sand it down with sandpaper until my fingers start bleeding. It's a process that's impossible to repeat by hand, and considering all the hours that go into designing a head, you couldn't charge someone enough money to make it worth your time. That's where the CNC machine comes into play. Without a CNC, the port you just spent months designing couldn't be replicated. This repeatability is a big part of the reason why modern cylinder heads offer much better performance per dollar than the heads of just 10 years ago."
Bryce Mulvey: "A CNC machine only cuts the ports. It takes someone that knows what they're doing to create that port in the first place. Just because a head is CNC machined doesn't mean it performs as well as any other CNC head. It takes us three to four months just to design a port, and we might ruin five to six heads in the process. We then digitize the ports, cut the heads, and figure out which parts of the head are off the mark and correct the program accordingly. The tooling used to cut the head also impacts consistency. It takes a tremendous effort, but once the process is complete, you can replicate the port you spent hundreds of hours creating very precisely. Our goal is to get our CNC heads flowing within five cfm of our prototype ports, which is very hard to do. No one's going to be able to pay $6,000 for a set of heads, and the CNC machine allows you to pack $6,000 worth of performance for a fraction of the cost."
Darin Morgan: "To properly select the correct heads for any given application, you need to determine your engine's intended use (drag, endurance, etc), volumetric efficiency, rpm range needed for your target hp, valve area needed to achieve sufficient airflow for your target hp, piston and cylinder head peak cfm demands, target air speeds throughout the induction system, volume of the overall induction system, resonant tuning characteristics, and carburetor airflow requirements. Once you answer all these questions, then you can pick out the perfect cylinder head and induction system. It's complex and no simple three or four variable equation is going to give you a sufficient answer. It takes experience and a lot of mathematical evaluation of the entire system to come up with the answer. Port design is not as simple as most people would like to think. This is why you see that most of the top port designers have 25-plus years of experience under their belts. So how is a new up-and-coming engine builder or hot rod enthusiast supposed to make a well-informed decision? With education, a person can now learn the underlying variables and characteristics that govern the induction system. With today's porting schools and long-term tech schools, a novice can learn to design basic ports in one tenth the time it took my generation. One short-term crash course is my two-day Advanced Induction System Design School. The school consists of a decade of information crammed into 20 hours. Anyone who has been through it can tell you that it will leave your head spinning with enough information to keep you thinking for the next year. There are also software programs such as Engine Analyzer Pro and PipeMax that can show people their engine's all-important airflow demand versus their cylinder heads' expected airflow. With Engine Analyzer and PipeMax, I can run calculations to properly size certain areas in the port in as little as 30 minutes. Before this type of software came out, it would have taken a day to do all the calculations."