Nismo 615 posted by A10WartHog on December 08, 2013 at 07:36 PM
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The general rule is the amount of CC's is the amount horse power at the flywheel. You want the number to be 80% cycle duty of the total. Your goal of 400 RWHP is more than possible for 615cc to support it. They max out around 500HP to the wheels. 615cc minus the 18% loss to the wheels.
Also I never dynoed my Z, but I am running 15PSI on 615CCs with all the supporting mods stock turbos. Definitely in the HP range your goal is.