This formula works well for general conversation, but is a little lacking when it comes to the finer elements of valve job design. The problem is that the above formula only takes into account a valve job where the valve seat is at 90 degrees, and sits at the very edge of the valve. If you start moving the edge of the valve seat inward so that the smallest part of the valve job contacting the valve is, for example, .050 inch in from the edge of the valve, then that area will obviously be smaller. Additionally, by changing the angle of the valve from 90 degrees to something like 45 degrees, the distance from the valve to the seat is no longer a direct 1-to-1 correlation to valve lift. As seen in the illustration, when the valve is lifted .050 inch, the gap between the valve and the seat is only .035-inch wide. Obviously, there needs to be some corrections in the formula. The true formula for the valve curtain area becomes:

Ac = ∏ × Lift × cos (valve angle) × (valve diameter − [seat width × 2 × cosine (valve angle) ] )

Comparing the “general” formula to the “true” formula with a traditional 45-degree valve job, we see quite a difference in curtain area:

∏ × 2.08 × .050 = .327 square inch with the general formula

∏ × .050 × cos (45) × (2.08 − [.050 × 2 × cos (45) ] ) = .223 square inch with the true formula

Now comparing the .223 square-inch curtain area of that 45 degree valve job with the Ferbert’s 50-degree valve job we see a big drop in curtain area:

∏ × .050 v cos (50) × (2.08 − [.050 × 2 × cos (50) ] ) = .204 square inch

What all this boils down to is that the 50-degree valve job has almost 10 percent less area that the air gets to pass through. That explains in part why flow bench numbers show that usually a 50-degree valve job has smaller low-lift flow numbers and helps kill reversion.

The second thing about a higher angle valve job is, once the intake valve starts moving up in the lift range, the 50-degree seat actually starts flowing more than the 45-degree valve job and is able to get more air into the chamber when that piston is pulling hard.

Yes, most of us car guys avoid math whenever possible, but spending a few minutes with a calculator now and then isn’t so horrible, especially when you start figuring out ways to make more power. Rick created an Excel spreadsheet to make all that math a little easier to use (if using Excel, then you need to convert degrees to radians first, radians = (deg × ∏ ÷ 180). By using the numbers generated there, they deduced that the steeper angle valve job would make the engine think it had a shorter duration but higher-lift camshaft, since the air would get moving later but eventually pack more in.

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