Valve shrouding may be something you have heard of (or maybe not), but it and its effects are closely tied with both the seat and port design that works best.
This Tech Tip is From the Full Book, DAVID VIZARD’S HOW TO PORT & FLOW TEST CYLINDER HEADS. For a comprehensive guide on this entire subject you can visit this link:
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So what is valve shrouding? It is easier to see from a drawing than to explain, so take a look at Figure 9.2. When you have absorbed this concept, move on to Figure 9.3. This is the intake valve in the intake side of the infamous Weslake A-Series engine combustion chamber that I talked about in the Introduction. The point to note here is that the walls of the chamber are, for much of the valve’s circumference, close to the edge of the valve. So this setup shrouds that part of the valve, and flow is limited around that particular section. The green line represents the radius of the stock chamber wall, and the airflow produced by this is shown by the green line on the graph. Obviously, we cannot cut the chamber away where it is adjacent to the bore diameter (gray line), but elsewhere we can cut it to alleviate the shrouding.
We can look at the chamber wall as a continuation of the valve seat. At an angle of 36 degrees from the valve-stem, from the valve seat on up, the area around the valve is always as much as the curtain area. This represents a geometrical de-shrouded valve.
All this talk of shrouding raises a question: Is it possible to have zero shrouding within a head that still utilizes the largest valves possible? A hemi-style combustion chamber can provide just that. And it does so because the valves are always moving away from the cylinder wall as they open.
Many World War II aircraft engines had a hemi-style combustion chamber. These typically employed valve angles of as much as 90 degrees inclusive. This accommodated the biggest valves, but it also produced a very deep chamber (a hemisphere). Deep chambers were okay for the low-compression ratios used for heavily supercharged engines but were bad for high-compression use because of the high piston dome needed. In practice, it turns out that about the optimum angle for the intake valve from the bore centerline is about 18 degrees. For the exhaust, where shrouding is less important, the optimum angle is approximately 10 degrees.
Chrysler has been synonymous with Hemi engines from the 1950s and with good effect. It introduced the 5.7 in 2003 and a 6.1 version came later. The engine is a very-well conceived design, with heads that flow every bit as well as you expect a good design to do.
So far I have referred to “geometric” shrouding. It’s a good start to understand what shrouding is, but simply applying it without further thought about what might be going on may not be the way to go. If air entered all around the valve in a uniform fashion, geometrically de-shrouding the valve works every time. However, air is heavy stuff and tends to flow in a straight line. There is no point in de-shrouding a portion of the valve’s circumference if there is minimal flow there. So we must first understand where the deshrouding needs to be done to make the most use of material removed from the combustion chamber. This can be important; for every cc carved out less compression-ratio potential is available.
Written by David Vizard and Posted with Permission of CarTechBooks