Running an SU void of anything to smooth air flow around the carburetor mouth creates a constriction. This is caused by air being forced around a 90 degree angled edge immediately prior to the carb mouth, effectively reducing the flow throughout the induction system. Short velocity stacks are ideal for engines putting out power at higher RPM (over 7,000) or when fitting inside an air filter. These fit HS4 or H4 1.5” SU. Sold each.