To lower restriction even further we have gone to a 5 bolt mounting design. This design eliminates the pass thru boss needed for the 6th bolt and allows us to open the cross sectional area even further. Some 5 bolt manifold designs allow "gasket creep" which causes long term gasket failure due to thermal cycling. To eliminate this, we leave a gasket retainer boss for the 6th bolt location during final machining. This boss captures the gasket and eliminates gasket creep.