Friday, February 1, 2019

Valve Potential Flow

We all fall prey to it occasionally; working on a project in which we have a preconceived idea of the results, but when you come near the end, they appear disappointing at best.  What did I do wrong? Can I fix it? When is “it will have to do” good enough?  Or, the ever-present option: do I need to start over?

But then sometimes when you find yourself in such a position, you need to take a moment an re-evaluate those expectations you had when you started. 

To make a long story short, I recently ported a set of head castings that I don’t often deal with, using smaller valve sizes than what I would normally select (for reasons I will not go into).  Once ported and flow tested, I was left wondering what I might have done differently.Then I remembered the actual valve sizes involved and it occurred to me that perhaps I expected more flow than I should have given the “smallish” valves.

For years I have had a chart that I produced for my own use, utilizing information from SuperFlow’s instruction manual for common valve sizes which provides “maximum potential flow” for the given valve size.  When I consulted the chart, I realized that I was dealing with a valve that was not on my chart.  The good news, however, was that once I did the math, I found that my work was completely up to snuff. 

But of course, never one to take small victories and move on, I decided that it might be a good time to update my chart.  And when I say update, I mean completely re-vamp it, put it into Excel and make it “interactive” so that with absolutely minimal input, all the data will be calculated for any valve size. So, if you have Microsoft Excel on your computer, you should be able to follow the link and download the “chart” and add valve sizes as needed.  

If you do not have the Excel program on your computer, you should still be able to download the chart though you will not be able to add sizes and will be limited to the valve sizes that I have listed.
This chart enables you to input the valve head diameter along with the stem diameter and everything else will be calculated for you.  Column A is a short description of where the valve is commonly used.  Column B is the valve head diameter and column C is the stem diameter.  Once the figures are entered into columns B & C, the rest of the fields will fill in automatically.  Column D gives the result of subtracting the square of the stem diameter from the square of the head diameter.  For the most part this figure can be ignored (though it is used by the program for further calculations).  Column F is the net valve area (which takes the area displaced by the stem into account).  From there the columns alternate between the potential flow at the given “valve lift to diameter ratio” and then what that valve lift is in inches. 
A quick word about lift to diameter ratio.  Many of those who do porting work find it most convenient to test flow at every .050” or .100” of valve lift, while others do all their testing at lift to diameter ratios. In the Harley industry, the lift to diameter ratio is not used often, though the concept is simple. The ratio is the derived by multiplying the valve diameter by a percentage; commonly 10%, 15%, 20%, etc.  This is expressed as: .10d, .15d, .20d, (you get the picture).  In other words, a 2.0” diameter valve would have a .25d lift of .500”.   The lift to diameter ratio has a couple of advantages, in that it allows one to compare the flow efficiency of two ports using different size valves rather than just the raw CFM of flow (where the larger valve will nearly always show up as the winner). This would be handy in a shop that ports a wide variety of head types. Also important, though often overlooked is the .25d flow.  At .25d (where lift = ¼ of the valve diameter) the “curtain area” of the valve is equal to the valve head area.  At that point, the amount of valve lift is no longer the primary deterrent to flow.  In porting, this has important implications when deciding what portion of the head needs improvement.

Hope this may be of use to some of you …

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