Albert,
It depends. If you still have the OEM plastic surge tank, then 9 lb would be as high as I'd go to keep the pressure relatively low and protect the tank, while providing protection against boiling. The plastic tanks have a nasty habit of leaking around the welds. The 2 small hose ports on the back of the tank can be especially prone to leak, as can the weld at the filler neck. Since you've got a C9, I suppose it normally runs about 204 F and gets up to 215 or so on long steep grades. A 7 lb cap would increase the boiling point to about 233F while a 9 lb would yield about a 240F boiling point.. The C9 will be very close to derate about 235. Based on that, I'd go with a 9 lb cap to provide a bit of margin as you can reach 235 on some grades in the hot summer if you don't manage your RPM and temp gauge closely. If you've replaced the plastic with a metal tank, then 13 lb should give you good service and margin.
Steve
