Assuming its a relatively normal marine pumped gas blower motor you probably need -8's. If its relatively nasty with a solid roller, high RPM and high boost you may need -9's. You really need to do plug checks to verify the proper timing, A/F ratio and heat range. Also I personally do not like the use of projected tip plugs in any power adder motor i.e. blown, turboed or nitrous. They just don't shed the heat fast enough in my opinion. Some guys run them with success, but I still don't like the in an adder motor.