Posted by DRHP on December 08, 1998 at 01:16:57:
In Reply to: Re: 7.4 GEN VI Marine Heads posted by Dave G. on December 03, 1998 at 10:32:33:
Dave, I spoke with the author of the book that you mentioned 1 1/2 years ago. We discussed the flow potential of the Vortec head, I thought it would provide adequate flow for 454's to around 4800 RPM - 502's to 4500 RPM. The power requirement that is imposed by hull drag goes up dramaticly with boat speed, the wetted surface area is exposed to surface tension shear that results in extreme drag. So the engine must develop a torque curve that matches this load. The philosophy that I've developed is that maximum torque is needed at max hull speed, this means that the volumetric efficiency (which goes hand in hand with the torque curve) must peak at high speed (5000 to 5400 RPM normaly), to do this the intake tract must have adequate flow capacity to provide cylinder fill, this means rectangular OEM heads(335 CFM at .550" stock - up to 365 CFM ported) or specialized aftermarket oval port heads (just tested a 572 with max port Dart ovals - 736HP at 5600 and 740# at 4700). This is the means to developing MAX speed power, if you're not interested in top speed optimization then use the small stuff! Also when using high flow heads, a fairly large cam profile and single plane intake manifold with a 1050 Dominator carb is reqiured to tune the combination for high speed work. Hope this helps - DRHP