Originally Posted by Chip

As per what I said above mate.

high rpm -> high cr
low rpm -> low cr
low boost -> high cr
high boost -> low cr



yours is high rpm and high boost (doesnt even spool till over 4Krpm so certainly isnt low rpm) so is "middle" cr as a result, it doesnt need to be low compression because you arent making boost at low rpm, and it cant be high compression because you wouldnt be able to get any timing into it in the midrange.

If yours was low (ie under 8) you would lose loads of power on race fuel, if yours was high (ie over 9) you would lose out on road fuel.
Where Mark has put yours is a good compromise for the two fuels.


FYI (with regards to my comments)

Under 8:1 = low compression
8 - 9 = normal compression
Over 9:1 = high compression



There are other factors too of course, such as the flow characteristics of the head and the cams that are used which will also effect things, but unless he is doing anything very wild in those respects they dont really need to be covered by this thread particuarly.