{"id":47757,"date":"2017-12-27T13:20:34","date_gmt":"2017-12-27T21:20:34","guid":{"rendered":"https:\/\/www.racingjunk.com\/news\/?p=47757"},"modified":"2017-12-27T13:20:34","modified_gmt":"2017-12-27T21:20:34","slug":"cylinder-heads-a-primer","status":"publish","type":"post","link":"https:\/\/www.racingjunk.com\/news\/cylinder-heads-a-primer\/","title":{"rendered":"Cylinder Heads – A Primer"},"content":{"rendered":"
\"Cylinder
We take a look at cylinder heads and what their specifications mean.
Image from video<\/figcaption><\/figure>\n

The cylinder heads on your engine are designed to seal the combustion chambers and move gases from one side (intake) to the other (exhaust). How well a cylinder head does this basic task determines whether it\u2019s good, fair or poor. There is a variety of numbers that cylinder head designers and manufacturers will throw at you in spec sheets, such as valve diameter, valve guide diameter, valve angle, port volume, combustion chamber volume and more. What do these numbers mean? RacingJunk is going to tell you, and then we\u2019ll show you several heads side-by-side and compare their numbers.<\/p>\n

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Let\u2019s Talk Materials<\/strong><\/p>\n

\"Cylinder
Cast aluminum heads are lighter, but they have problems with heat dissipation.
Image from video<\/figcaption><\/figure>\n

No matter the material used, every head on the market today is cast. Unlike many other engine components, I can\u2019t think of a single engine cylinder head that is forged. The biggest difference between cylinder heads is the materials used in the casting of the head. Iron and aluminum are the two major options. In high performance engines, aluminum is normally used because it\u2019s lighter but still has much of the heat resistance of iron. If you\u2019re racing, you want aluminum.<\/p>\n

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Combustion Chamber Volume<\/strong><\/p>\n

\"Cylinder
A comparison of combustion chamber volumes. The photo on the right shows a head with a 72 cc combustion chamber and the photo on the left shows one with a 124 cc chamber.
Images courtesy Dart Heads<\/figcaption><\/figure>\n

Combustion chamber volume plays a big part in determining engine compression and total engine displacement and, as a result, engine performance. The larger the combustion chamber in the cylinder head, the lower the compression but the higher the displacement. If you go with a smaller combustion chamber, you\u2019ll have less displacement but more compression.<\/p>\n

Why does that matter? Because the size of the combustion chamber is going to have a say in how you time the engine. Larger combustion chambers need you to advance the timing significantly in order to ensure that the whole air-fuel mixture that\u2019s been compressed burns. Conversely, a smaller combustion chamber means the air-fuel mixture will burn more quickly and you can use less timing.<\/p>\n

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Calculating Combustion Chamber Volume<\/strong><\/p>\n

\"Cylinder
Image courtesy Brodix<\/figcaption><\/figure>\n

Shops will measure combustion chamber volume by turning the head over and installing two valves with oil or Vaseline on the sealing surfaces, followed by covering the combustion chamber with a perfectly flat piece of (usually) Plexiglas with a small hole drilled for the high spot in the chamber.<\/p>\n

\"Cylinder
Filling the chamber to measure combustion chamber volume.
Photo by Enilda Aguilar<\/figcaption><\/figure>\n

They then carefully fill the volume created with something like denatured alcohol from a graduated container until it reaches the bottom of the drilled hole. The volume of liquid that has been poured from the graduated container is the chamber volume. If you\u2019re buying a new head, these calculations will be made for you.<\/p>\n

\u00a0<\/strong><\/p>\n

Port Shape and Size<\/strong><\/p>\n

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Spec sheet for the Dart Eagle S\/S Vortec SBC Iron Cylinder Head. Refer to this image for measurements mentioned in the text.
Image courtesy Dart Heads<\/figcaption><\/figure>\n

The shape and size (numbers D1 and D2 above) of the head\u2019s intake and exhaust ports will determine the intake and exhaust manifolds that are used, because they have to match as closely as possible in order to not play havoc with the air-fuel mixture\u2019s flow into and out of the cylinder.<\/p>\n

\"Cylinder
This head has rounded exhaust ports. You can\u2019t use a manifold with square ports on this head.
Image courtesy Dart Heads<\/figcaption><\/figure>\n

Note:<\/strong> It\u2019s important to remember one important thing when it comes to port shaping and sizing. Like a carburetor or injector, bigger isn\u2019t always better. Sure, a bigger port has more volume for airflow, but the speed<\/em> of the airflow through the port is almost as important as how much air flows, especially in street performance applications. Lots of air flowing slowly into the chamber isn\u2019t going to do much for power output. A little less air flowing faster will make more power. The proper balance needs to be struck here.<\/p>\n

\"Cylinder
This Dart head uses exhaust ports that are more square. A manifold with round ports won\u2019t perform properly on this head.
Image courtesy Dart Heads<\/figcaption><\/figure>\n

Stick an intake manifold with 1.7\u201d inch round ports on a head with intake ports that are rectangular and measure 1.09 inches by 1.975 inches, and a goodly portion of the mixture\u2019s flow is going to be interfered with and slowed down. This will cause improper mixing and burning of the air-fuel mixture, causing a reduction in power output.<\/p>\n

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Valve Sizing<\/strong><\/p>\n

The size of the intake and exhaust valves plays a significant part in deciding how much the head will flow. You want an intake port with a volume significantly higher than the combustion chamber volume. Higher intake port volume means that most of the air-fuel mixture will have entered the combustion chamber by the time the intake valve closes.<\/p>\n

Conversely, you want an exhaust port with a volume as close to the combustion chamber volume as possible to facilitate evacuating\/exhausting the chamber after the burn has concluded. This way the burned air-fuel mixture is (hopefully) completely evacuated from the cylinder prior to the intake valve opening and the chamber filling with a fresh air-fuel mixture charge.<\/p>\n

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Flow<\/strong><\/p>\n

Here\u2019s the meat and potatoes of the job of the high performance cylinder head: Move more air in and out of the engine in less time. In the real world, this measurement\/specification will greatly depend on the flow qualities of your intake and exhaust systems, but manufacturers and machine shops will use the measurement when tested with \u201cideal conditions\u201d equipment known as a bench flow tester\/flow bench. This won\u2019t tell them how the head will perform when installed on your engine, but it will give an idea of how well the head will hold up its side of the airflow maximizing bargain.<\/p>\n

\"Cylinder
Flow testing requires removing the valve springs and using lighter ones that won\u2019t break the flow bench.
Image from video.<\/figcaption><\/figure>\n

In the chart above, the intake and exhaust flow numbers show that the valves have been lifted .500 inch off the seats and that the machine is pulling 28 inches of water\/mercury worth of vacuum to make its measurements. As you can see, the intake flows almost one third more than the volume of the combustion chamber. This ensures that the chamber is completely filled with fresh air-fuel mixture on each cycle. The exhaust port being able to flow more than the volume of the port ensures that the cylinder is as fully evacuated as possible between cycles; thus more power can be developed.<\/p>\n

\"Cylinder<\/p>\n

Note: This is important. When comparing head flow numbers, make sure you\u2019re working with oranges and oranges, and not oranges and tangerines, by making sure the amount of vacuum pulled during testing is the same; otherwise, there\u2019s no way you can realistically compare the flow numbers. <\/strong><\/p>\n

\"Cylinder
The head is prepped with the lighter springs and installed on the flow bench to begin testing. Here the technician is testing to make sure the valve can be properly opened and closed once the test is underway.
Image from video<\/figcaption><\/figure>\n

Machine shops and head specialists will take multiple flow readings with the valve stepped open by .050 inch so as to see what effect modifications they\u2019ve made to the valve and port have had. There\u2019s a new tool out recently that allows for flow readings to be taken at eight places around the diameter of the valve. This lets the machinist smooth out and maximize the airflow through the port with the valve at varying lifts.<\/p>\n

Note: <\/strong>Avoid looking just at peak flow numbers. Unless you\u2019re racing full out all the time, like on an oval or circle track, you need to also look at the lower lift flow numbers. If acceleration is life in your type of racing, you need to look more closely at the flow numbers for low lift values as these will impact more directly on your specific application.<\/p>\n

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The Valve Plays a Big Role in Airflow through the Head<\/strong><\/p>\n

\"Cylinder
Image by Enilda Aguilar<\/figcaption><\/figure>\n

This is basically a no-brainer, but I\u2019ve got to mention it and how to overcome the problems valve stems cause. Lots of force is being dissipated where the valve meets the valve stem, so this joint has to be strong. The problem is, a beefy stem to seat will cause disruption to the airflow. High performance valves will be manufactured with a \u201cback cut\u201d on the stem to improve airflow around the valve. This back cut is a reduction in the diameter of the stem just above the transition from the valve seat to the stem.<\/p>\n

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Problems with Flow Testing<\/strong><\/p>\n

\"Cylinder
Rick Touchette developed the \u201cPressure Differential Valve\u201d to obtain the most accurate head bench flow readings possible. The tool comes with software that graphs out airflow measurements to help the machinist\/engine builder maximize airflow through the head.
Images courtesy rtstooling.com<\/figcaption><\/figure>\n

The current \u201cstate of the art\u201d has the head specialist\/engine builder lowering a measuring device called a pitot into the port and then incrementally opening and measuring the airflow through the port. The problem with this is that the pitot disturbs the airflow in the port, and real world\/”as installed” flow numbers can\u2019t be obtained.<\/p>\n

\"Cylinder
The information provided by the P-D Valve software from RTS Tooling.
Image courtesy rtstooling.com<\/figcaption><\/figure>\n

Rick Touchette of Rick Touchette Specialty Tooling (RTST<\/a>) came up with a unique fix for this problem: He put the pitot into the sealing surface of the valve itself. This takes the pitot out of the airflow and removes it from the airflow equation. He also created a locking collar that allows you to raise and lower (open and close) the valve by .050 inch increments.<\/p>\n

The collar is also notched so the valve can be rotated a precise amount, so that the pitot in the valve can be rotated precisely through the diameter of the valve seat and accurate airflow readings can be taken at multiple points around the valve. This ensures that any porting or polishing you\u2019re doing has a positive effect on airflow through the head.<\/p>\n

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Other Important Information the Spec Sheet Will Give You<\/strong><\/p>\n

\"Cylinder
Comparing port shapes. The top image is of an Edelbrock Performer RPM E-TEC 200 head with rectangular ports and the bottom image is of the Dart Iron Eagle Platinum head with Vortex ports.
Image by Enilda Aguilar<\/figcaption><\/figure>\n

Aftermarket heads and manifolds will typically have intake and exhaust ports that are sized and shaped differently than those on OEM heads. For this reason, most head spec sheets will call out a part number (esually either their own or Fel-Pro’s, since F-P is the largest maker of intake and exhaust gaskets) so you get the ones that fit the best. Notice in the chart above, it says that a Vortec style exhaust manifold must be used. This is because of the shape and size of the exhaust ports.<\/p>\n

Milling is important because it has a hand in determining compression ratio and head bolt length. Above, you can see that the Dart Vortec head in question has been milled .060 inches which corresponds to 60cc and that a Flat Milling technique was used.<\/p>\n

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Valve and Piston Angle<\/strong><\/p>\n

Another important piece of information that can be found on the spec sheet is the valve angle and the type of piston that can be used. If a head has a valve angle of 23 degrees, for example, it can use any \u201c23 degree aftermarket piston.\u201d Valve angle is the angle the between the valve flat when closed and the cylinder bore.<\/p>\n

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Comparing Head Specifications from Three Popular Brands and Models<\/strong><\/p>\n

\"Cylinder
The Brodix IK200
Image courtesy Brodix<\/figcaption><\/figure>\n

So who makes the best cylinder head for the most applications? Some people prefer Dart; others prefer Brodix. Trick Flow Specialties and Edelbrock are also popular brands with engine builders and racers. Let\u2019s take a look at one model from each and see how they compare. They are:<\/p>\n