anything you want. lol seriouselly they recommend 9:1 or less. for the mini blowers 144 CID 172 CID I would stick around 9:1 because they dont produce a ton of boost and when your NOT in boost the extra compression will help your economy and N/A powerband. like when your cruising on the highway or taking off from a light real easy.
the more boost the less compression you want, technically. if you can get the boost really cold, like with an intercooler you could run more compression, safer, but roots blowers dont usually get used with intercoolers (aftercoolers.. whatever)
With a 671 blower I would say around 8:1 maybe less if your going to overdrive it. the 144/172 series is already 200%~ OVERdriven to make the boost you want.
hope this helps... | 
