Different Octane question

[Hoople]

What do you consider low compression? My Evo has a CR of 8.5-1 and runs fine on 87 octane. The current 96ci engines are 9.2-1, so they're iffy for 87 octane and H-D recommends 91.

Your getting lost in the understanding of Compression Ratio. Compression Ratio is a DESIGN Ratio and is not an absolute number.

Example:
Engine A has a Compression Ratio of 8.5 to 1
Engine B has a Compression Ratio of 8.5 to 1 also.

But when I take my Snap on compression Gauge and measure the actual Dynamic PSI I get, Engine A = 145 PSI and Engine B =165 PSI. Why is That?
Of course piston ring sealing methods come into play but the main reason is cam shaft design. There is an ABSOLUTE max PSI that you can achieve
using Pascals principles of pressure but dynamic numbers will always be lower.