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Late Model Performance Cylinder Heads
As the economy continues to come out of its slump, cylinder head manufacturers are seeing a growing demand for new cylinder heads that can deliver race-winning performance.
By Larry Carley
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Whether the heads are upgraded aftermarket heads for traditional small block/big block Chevy and Ford applications, or hot heads for the latest generation of Chevy LS and Ford modular engines, people want the latest heads that make the most power.
Heads do make horsepower, but they don’t make it all by themselves. A set of killer aftermarket performance heads that are mismatched with a camshaft, valve train or induction system will never realize the engine’s full potential. It’s all about achieving optimum airflow and velocity within the rpm range where the engine is built to produce power.
That’s where many a racer has gone astray. They overbuild a street engine with race components that are designed to deliver high rpm power instead of building the engine to produce more low-end and mid-range torque. Even in all-out racing the engine with the biggest heads and biggest flow numbers doesn’t always win the race.
Is Bigger Really Better?
A lot of people are true believers in the “bigger is better” philosophy when it comes to selecting cylinder heads. Head manufacturers as well as those who port heads make a big deal out of promoting the huge cubic feet per minute (cfm) flow numbers they’re able to achieve using the wizardry of flow bench testing and CNC machining. Flow numbers are up, but the numbers can sometimes be misleading because of the way the numbers are measured.
Airflow depends on valve lift, valve area, air density and the test pressure. There are industry standard methods for calibrating flow benches and correcting airflow numbers to compensate for differences in air density. However, airflow values can vary somewhat from one flow bench to another depending on the methodology and equipment that are used to measure it. Most flow benches have a margin of error of around plus or minus one percent. On a cylinder head that flows 400 cfm, the margin of error could be plus or minus 4 cfm either way.
Because of this, comparing the airflow numbers claimed by one head manufacturer against those of another may not be the most accurate way of comparing performance. The only accurate way to compare one head against another is to test them on the same flow bench under the same conditions. Even then the airflow numbers won’t necessarily give you an accurate picture of which head will actually produce the most power on a given engine application.
It’s also important to note the valve lift at which advertised airflow numbers are calculated. How do their numbers compare to the actual amount of valve lift the cam and rocker arms in your engine will achieve? Big flow numbers at 1.00˝ of lift are meaningless if your cam and rockers only open the valves .850˝ or less. And on a street application, most engines won’t see much more than about .550˝ to .600˝ of lift.
Another point to keep in mind when comparing airflow numbers is that the valves reach maximum lift only once during the intake and exhaust strokes. Flow numbers at peak valve lift may be impressive, but what’s usually more important is how well the head flows at partial valve lift as the valves are opening and closing (which happens twice during every intake and exhaust stroke). Better flow characteristics at partial valve lift can have more of an effect on power and torque than big airflow numbers at peak lift.
Accurate cylinder head comparisons mean you should look at the entire flow curve from low lift to maximum lift. Once you’ve done that, you will have a much better picture of how the head will actually perform on the motor you are building.
Also, there’s more to airflow than big cfm numbers. Air velocity and swirl are also important. A port that’s sized properly for the engine displacement and rpm range will keep the air moving at higher velocity, resulting in more complete cylinder filling and more power. Air velocity affects both throttle response and low-end torque. That’s why heads with port runner volumes that are too large for a given engine application may not perform as well as a stock cylinder head.
Swirl helps route air into the cylinder more efficiently and promotes better air and fuel mixing for better combustion. Swirl and turbulence can also cause air and fuel separation in the combustion chamber, which is something that can be visualized by wet flow testing.
Valve angle (the angle of the valve stem with respect to the deck surface) also affects airflow, and here shallower is usually better. The configuration of the combustion chamber, valve shrouding and even the diameter of the cylinder bore are additional factors that also affect airflow and a head’s ability to make power.
The best way to compare cylinder heads is to bolt them on an engine and do a series of dyno test runs to see how they perform. The best head is not necessarily the one that makes the most peak horsepower in a narrow rpm range, but the one that delivers the best power and torque curves for the intended application. If you’re building a motor for a drag car, then peak high rpm power is what you want. If the engine is going into a circle track car, you want good throttle response and peak power in the mid to high rpm range. For a street application, a broad flat torque curve and lots of low and mid-range torque works best.
As Cast Or CNC?
There’s an ongoing debate as to whether or not ports in an “as cast” cylinder head flow as well as those in a CNC machined head. Many aftermarket performance cylinder heads that are sold with “as cast” ports are capable of delivering excellent performance out of the box with no additional modifications. The same goes for some stock heads (such as Chevy LS1 heads) that flow very well in their stock configuration. But there’s always room for improvement by either hand porting or CNC machining “as cast” heads to change the contours of their ports, bowls and combustion chambers for better flow. It depends on what you are trying to achieve.
Most CNC profiles are developed by grinding, testing and mapping various port configurations for a particular application. People who do this kind of work build up a digital library of port configurations that work well with various head castings, camshaft and engine combinations. This allows them to replicate a proven port configuration by entering the data into a CNC machine and removing the unwanted metal to modify the cylinder head.
Is the end result better than the best “as cast” head for a given engine application? It depends on the castings that are available and how well they flow compared to an optimized CNC port configuration for the same setup. With most high-end performance heads, a raw casting is CNC machined to specific dimensions for that application to deliver peak performance.
Bigger is better continues as the trend among Pro Stock drag racers. The 600 cubic inch “monster motors” of a few years ago seem almost puny next to the 850 and 900 cubic inch motors that are now commonplace on the drag strip. Sonny’s Racing now has a 1,000 cubic inch naturally aspirated motor that makes over 2,150 horsepower and 1,550 lbs. of torque. It’s hard to tell how far this trend will push engine displacements in the years ahead.
The bigger these motors get, the more air the cylinder heads have to flow to handle the added cubic inches. The availability of aftermarket engine blocks with wider bore spacing continues to expand, and with it the availability of larger cylinder heads from Brodix, Dart, Alan Johnson Performance Racing (AJPE) and others to fit these engines.
Another trend we’ve seen is that of reducing valve stem angles and raising the ports for increased airflow. This, in turn, often requires custom fabricated intake manifolds or special intake manifold castings to mate with the modified heads (ala Brodix, Dart, etc.). With these types of heads, installation is not a simple swap. Changing the head may also require changing the intake and exhaust manifolds, and possibly even the valve train configuration.
For small block Chevy applications, splayed valve symmetrical heads such as the Brodix BD Series heads are now available to replace siamese-style SBC heads. The symmetrical heads seal better and reduce the risk of head gasket failure in the hot spot area between adjacent exhaust valves on siamese style heads. The ports also provide a straighter shot at the valves than the ports on siamese heads.
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