David Vizard's How to Port & Flow Test Cylinder Heads by David Vizard

David Vizard's How to Port & Flow Test Cylinder Heads by David Vizard

Author:David Vizard [Vizard, David]
Format: epub
ISBN: 9781613250952
Publisher: SA Design


Valve Seat Forms

The more rounded and streamlined a seat and the approach and departure areas are, the more efficiently it’s likely to flow. Figure 8.5 shows about how much the average flow efficiency of the first 0.250-inch lift of an approximately 2-inch-diameter valve varies with seat design. Although a hypothetical knife-edge valve seat as on cylinder number-1 gives the largest throat area, note that its efficiency is pathetically low at 45 percent. Number-2 is really the first seat that can actually be used, and at best this is 56 percent efficient.

By streamlining the underside of the 45-degree seat, the situation is further improved when a 30-degree top cut is applied as on number-4. For number-5, a surface is present to constrain at least one side of the jet of air/fuel as it enters the cylinder (or exits for the exhaust). With suitable attention to detail, the flow efficiency of an intake valve in the first 0.100 to 0.150 inch can closely approach 100 percent while the exhaust, at higher lift values, can actually exceed 100 percent by virtue of nozzling. That is, the seat and port is starting to emulate a nozzle similar to a venturi of a rocket nozzle. All this only comes about by virtue of a well-developed form before and after the seat.



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