Of course, according to Chevrolet, the new intake manifold bolt configuration, gaskets, and attaching hardware were "an improvement in sealing technology." On the other hand, our insiders tell us it was done so the intake manifolds screws could be installed and tightening by a machine-something that wasn't possible with the early configuration; you decide on that one.
The Vortec AdvantageThe oldest hot rod trick in the book is to port and polish the intake ports of small-block Chevy heads and then stuff in the biggest valves available. But the increase in high-rpm performance that provided was usually accompanied by a loss on the bottom end. That's because there is often a compromise in performance for one end of an engine's rpm band for the other. Most rodders have had experience with an engine that pulls hard at lower speeds but seems to run out of breath on the top end. Or an engine that would wind happily to red line but wouldn't pull away from a stop sign without bogging and falling on its face.
In simple terms, what happens is this: Small carburetors, ports, and valves help maintain velocity in the intake tract, and as a result the cylinders fill effectively on the intake stroke at low engine speeds. On the other hand, at high rpm, the carburetor, ports, and valves aren't big enough to provide the quantity of air and fuel the engine requires, so it runs out of steam.
The other scenario is an engine with large carburetor, ports, and valves. While that combination will flow a large enough volume of mixture to sustain high-rpm operation at low engine speeds, the velocity is so low the cylinders don't fill well; as a result, low-end operation suffers. What makes the Vortec heads so unique is the engineers at Chevrolet have found a way to improve performance at both ends of the rpm range.
The size of an intake port is usually measured by the volume of liquid it will hold. In the case of '86-and-earlier production heads, that volume was around 160 cc. By comparison, production Vortec heads will hold 170 cc. The assumption would be the bigger ports would gain top-end performance and lose some at the bottom end, but in this case that's only half right.
By redesigning and raising the ports, there is a larger radius, a more gentle turn, for the incoming mixture to make on its way into the cylinder. The end result is that Vortec heads have ports that are big enough to increase high-rpm horsepower and efficient enough to maintain mixture velocity for excellent low-rpm torque. When used with a relatively mild cam and a performance manifold, stock out-of-the-box Vortec heads will easily make 400 hp. These heads are the reason so many of Chevrolet's crate motors produce such impressive horsepower and torque figures.
Vortec VariationsAs we said earlier, the intake bolt pattern on production Vortec heads is unique and as a result a Vortec-style intake manifold (or fuel injection system) is required. However, there is a version of Vortec heads available from GM Performance Parts that has both the late eight-bolt and early 12-bolt patterns. But before you start to visualize that early dual-four intake manifold on Vortec heads, there's some bad news: The intake ports won't align. The higher ports on the heads result in a gap between them and the top of the manifold runners.
Vortec ManifoldsWhy GM Performance elected to put the early bolt pattern in these heads when the ports don't align is a matter of much debate. Early on, the production Vortec pickup engines had a reputation for developing leaky intake manifold gaskets with high mileage. Improved gaskets are said to have solved that problem. Some speculate that by using the 12-bolt pattern in the GM Performance version of the Vortec heads, the problem was prevented. In any event, Vortec heads require high-runner manifolds, no matter which bolt pattern is employed.
Another unique feature of Vortec heads and manifolds is that there are no exhaust heat passages in either. In addition, '96-and-later engine blocks don't have a thermostat bypass in the block (see accompanying photos). With some intake manifolds, a bypass hose can be run between the water pump and the 5/8-inch pipe fitting in the manifold.