Cylinder Head Port Design - CHP How It Works

Part 2 Of Our Cylinder Head Roundtable. Our All-Star Cast Of Experts Chime In On The Most Intriguing Aspects Of Cylinder Head Port Design.

Stephen Kim May 4, 2010 0 Comment(s)
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Combustion Chambers
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."

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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."

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