Transitional knock is something that no one seem to have come up with a sure fire answer yet, and this has been worked on by many people, for many years.
Personally mine went away when I switched to de-natured, and ac-43 plugs, gapped at .032. (I was running '45s previously, with isoprophol) There is a trade off, as the de-natured does not cool as well.
My theory on what causes this knock deals with hot spots, and spark, and equal air distribution.
Theory 1.-Hot spots: They develop in, and around the middle cylinders (2&3). Could be the plug electrode, as well as valves. When the alchy hits them, it steams, and shorts the plug electrode out. when the electrode finially fires, (after the seteaming action stops) it does so with a richer charge in the chamber, producing knock.
Theory 2- Steam, and steam vapor. Actually the same thing as far as semantics go, but a fully dispersed alchy charge, from hitting too much heat at once, may cause transitional knock by releasing its energy early in the compression stroke. Same as gas, and pre-ignition. The steam vapos acts like a spring, sort of, pushing back on the piston as it rises to it's full height.
Theory 3- Air distribution. There is no way I can think of to get more of the alchy charge to the 2-3 cylinders to cool them more than the others. If this were possible, then the hot spot problem wouldn't exist. Instead we spray the alchy in, giving the 'cooler' cylinders a bit too much alchy, and the hot ones not enough. As you get into the run, the temperature difference between the 'hot' and 'cool' cylinders get even greater, I suspect. As a partial remedy to this, I installed a hemco plenum, in the hopes that the more even air flow (air that is carrying alchy) would help address the difference in cylinder temperatures.
Theory 4- Unless your car is running right, good spark, good fuel, etc., you should not be spraying. Bad pcv's, bad turbo seals, that are letting hot oil in will cause knock, as well as fuel pumps that can't keep up, and ignition systems, and plug wires that fail under load. Any of these things could be causing knock that could be mistaken as 'transitional' knock.
I honestly don't know if the Hemco helped, because I didn't have a scanmaster right after it's install, and when the alchy system on my car went down, I got the scanmaster, and changed plugs, changed gaps, and switched to the de-natured, all at once.
http://www.loveridgemachine.com/RJC_Buick_Products/rjc_buick_products.html
Here is a link to a company that makes a plenum spacer that evens out the airflow. It costs much less than a Hemco, and according to them, works a bit better. I would buy it. The Hemco reduced my boost pressure by 2 psi, and I was able to turn up the wick to the previous levels, without knock. The entire jury is still out, but I believe that the product works.
BTW my signature shows alum. heads, but I will not be running them until next week. I was using stock, cast heads when I made the changes stated above.
I'm hoping the alum. heads will help with shrouding the plug, and keeping the spark from going out, as well as keeping the hot spots down a little bit. I'll still be spraying, and I'll let you all know how this works out with pump gas.