Sorry, heavier flywheels do not build boost faster, boost is dependent on airflow, not engine rpm. Lighter flywheels allow for quicker engine acceleration, that might be what you're thinking of.
A heavier flywheel keeps more of the engine's inertia so the rpm's don't go down quite as quickly between shifts. Also, a heavier flywheel allows for a better drag launch and all around better streetable clutch. The ACT is a good unit, although Shep is complaining it slips a bit, but then again he makes 1000 whp, so we don't have his problems.
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