by Tony » Mon Dec 19, 2005 5:49 pm
It sounds rather like vortex shedding may be the cause. I have never heard a head whistle, but here is my theory.
When you have an un-streamlined obstruction in moving airflow, the flow can alternate, first one way then the other around the object. This is caused by flow separation which produces a cyclic pressure variation in the wake. The faster the airflow, the faster the frequency of the fluttering wake.
Vortex airflow meters work that way by measuring the actual frequency. It is also why a flag flutters in the breeze. A very large amplitude slow waving motion at very low wind speeds. In a hurricane, a flag would be almost rigid, but the last inch might flutter so fast it was a blur.
To the sailors here it is "luffing" of the sail. To pilots it indicates incipient stall.
What you probably need is more streamlined valve guides, teardrop shaped, especially on the trailing edge. That hopefully should reduce flow separation. Try packing some modeling clay behind the valve guides to make a long streamlined teardrop airfoil shape, see if that stops it.
I suspect violently turbulent and unstable flow in the port caused by the guide is not going to help the flow numbers. Reducing the port area slightly may (?) actually lift the flow numbers.
This is all completely abstract theory, I have never tried it.
Also known as the infamous "Warpspeed" on some other Forums.