Thanks for the replies so far !!

I would like to further open the discussion with some a few points of interest.

1. New commodore v6 engines are 10.2:1, and v8's are 10.1:1 and both run perfect on regular 87 octane fuel. I know they have knock sensors, but I can't imagine the knock sensors working flat out all the time to prevent detonation.

2. datoman_1000 has an a15 setup at 12.9:1. Have you run this engine yet datoman ?? What fuel are you limited too?

3. Datsun100ATurbo runs 12.5:1 using octane booster (100 octane total) - This isn't far from 98 octance.

4. I've read numerous times that higher rpm's can tolerate more compression without knocking.

This is why some people use the "Dynamic" compression ratio instead of a static one. Now this explains why some people have achieved very high static c/r (12:1 up to 15:1) through the use of very large cams.

For those people who are new to dynamic compression, it is the c/r achieved in real life while turning the engine over very slowly. The difference is simple - static c/r assumes the inlet valve closes at BDC. Dynamic c/r uses actual engine geometry and cam figures to compute a more accurate figure.

I've written an excel file to calculate dynamic c/r , static c/r and cranking pressure if anyone is interested.

Does anyone have any dynamic c/r figures that work ??

nzdatman: the compression was achieved mainly thru shaving the head, which the a15's original gx head. Mazda UC pistons, turned down and then valve reliefs cut in them.