Never mind. You answered while I was typing!
Thats exactly it. By using a short reach adapter in a plug hole for a long reach plug, you effectively add volume to the cylinder and comp goes down.Bob... I don't know either but I would guess it's the larger volume/area in the chamber because of the plug hole. I do know that if you cc the heads you will know what the deal is in relation to comp. numbers.
Eric I was trying to find something on the net to calculate the dang stuff... I used a syringe to figure my old ones... Not sure how close that was but they came up to 34cc (260 heads).I out 14 mm x 6.35 mm into a cylinder volume calculator and it spit out 3.91 cc