Those Vortec heads not only flowed in the 230-plus cfm range (more than the iron Phase II Bow Tie racing heads of the time), they also had extremely efficient combustion chambers. Though the intake manifold bolt pattern changed a bit from the old-school small-blocks, aftermarket intake manifolds were instantly on the scene that would bring a basic 350 Vortec engine into the 375-plus horsepower range.
GMPP saw the potential in the low-buck but efficient head design and came out with two new versions, which were available over the counter of your local Chevy dealer or through any of the normal catalog companies. (The 185cc runner is PN 25534421, and its big-brother 225cc is PN 25534446). Rick and Randy picked up a pair of the big-port heads and began a thorough examination followed by a plan to modify the heads for maximum effectiveness.
Building a head for mass markets is one thing, but designing a head for a specific application is an entirely different undertaking. There are very few compromises to be made since there is only one engine for which it is destined to live on. There is only one specific power range for which it must create the perfect power curve. With that in mind, it was almost easier to design the runners than it would be for something that would be run on a variety of engines and conditions.
The act of designing a runner requires equal parts art and science. The science begins by understanding the engine cycles. It is actually easiest to begin talking about that at the point of top dead center on the compression stroke. Both valves are closed. The ignition just lit off the air/fuel mixture, which is slowly expanding. About 15 degrees after TDC, the flame front is kicking into high gear and building tremendous cylinder pressure. By the time the piston is about an inch down the hole, the fast-burning gas has done most of its work, and the piston is just along for the ride. In order to get a jump on getting all that burned exhaust out, the exhaust valve starts to creep open somewhere around 40 degrees before bottom dead center. Opening that valve early is called blowdown and is one way to help the efficiency of the engine. Since the gasoline is already burned but there is still high pressure in the chamber, that exhaust will start to move out the exhaust port before the piston starts coming back up. Once that piston is on its way back up, there are pumping losses as that high pressure in the chamber is still slightly trying to push the piston back down.
About the time the piston is two thirds of the way back toward TDC, the velocity in the header pipes gets high enough to actually help lower the residual cylinder pressure some. This is due to the fact that (science here still) as velocity increases, pressure decreases. Unfortunately not all of the pressure in the chamber has subsided by the time the intake valve starts to crack open just before TDC. This puts the exhaust gas in a dilemma as it wants to go out both the exhaust and the intake valve. Some of that exhaust gas does, in fact, get shoved back up the intake tract in a process known as reversion. Finally, that low pressure in the header pulls the remainder of the exhaust out just before the exhaust valve closes a few degrees after TDC.