I just recently found out about this forum and am very impressed so far with the topics and discussions.  I decided to post a question.

Quick background is that I am designing a head for a V twin from a clean sheet of paper.  It is a DOHC, 4 valve design.  I have a mechanical engineering background and I have experience with flow simulation software and would like to utilize it to design the proper port sizes and shapes for this engine. The engine has 4.25 bore and 4.00 stroke. The revrange should be 7000 to 8000.  I would like to see it make peak power well above 6500.
Most of what I find for literature exists for 2 valve designs.  

What equations /rules apply for max port velocity at 28 inches H2O ( intake and exhaust)for a 4 valve design (2 intake, 2 exhaust?

Any other suggestions on good port design practices would be appreciated. 

Phil is right, the thing to consider also is the total valve(Gauge Valves) area of the design & this will govern what the port area etc will be. Everybody has there own idea's on things like this, but for me it is better to design the ports with the intention of porting to the correct size & shape to get the engine characteristics required. In other words start with the smallest ports in the design stage & test from there to get what you need, allow ample port wall thick.

A number of sources quote a figure of 50% or so of the speed of sound. This is based on average piston speed, the temperature of the inlet charge (which determines the speed of sound), and the flow area. The flow area might be the port area, the area under the seat, or through the seat depending on who is quoting the figures. There is at least one good SAE paper on the subject.

I think these rules of thumb are very usefull to get one started on a design. Simulations may have their place, but eventually you will have to slug it out with a good 2d or 3d cad program to figure out what will fit within the bore, how you will run the ports, how to get the spark plug in, and how to get enough cooling for the exhaust. This is what will make or break a design, and there will be lots of compromises.

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