I don't know about primary tube but if the collector is too small (where it doesnt allow the motor to achieve it's natural torque peak RPM) it will limit top end power....(so to me) that could likely be more than 5% of the potential peak power. Stahl was also likely talking about an open header/race situation which is obviously very different from a street/strip type application. In other words the peak torque limit of a too small collector could very likely lower the HP trajectory/rate of rise/curve from that RPM all the way to peak....that being said.... (cont'd below)