Weve been getting a ton of mail with questions about how and when to modify the Vortec small-block iron headsso much mail that we decided to condense all the Vortec information into one concise story that should answer almost any question you have about this powerful iron cylinder head.
The Early Days
In 1996, GM incorporated the port design from the very successful iron LT1 engines used in the Impala SS, which was improved over the aluminum LT1 Corvette and Camaro engines. In essence, the Vortec head uses the iron LT1 chamber intake and exhaust ports in a standard-water-flow iron head. These production Vortec heads were used in the new Vortec 350ci engine intended for ½-, ¾-, and 1-ton Chevy pickups. Amazingly, this mundane, production-based 1.94/1.50-inch valve head performed better than the iron Bow Tie head. The deal of century occurred when GM Performance Parts decided to sell these heads as complete units for under $500 per pair, ready to bolt on. For you junkyard hunters, be forewarned that some ¾- and 1-ton trucks came with Vortec heads equipped with a specially hardened exhaust seat that kills low- and mid-lift flow. Casting number 10239906 should be avoided.
Chevy enthusiasts quickly discovered that these heads are capable of 400 hp right out of the box when teamed with a performance intake and a camshaft. But there are several changes that must be made before bolting this head onto your engine. Well run through all of them here and offer a few recommendations based on our experience.
Bolts, Springs, and Such
There are several important requirements dictated by the use of the Vortec head. The most important detail is that the Vortec uses an eight-bolt intake-manifold pattern that is different from the standard 12-bolt pattern. The Vortec requires a specific Vortec intake manifold available from both GM Performance Parts and Edelbrock. On dozens of dyno-tests, we have had the most success with Edelbrocks Performer RPM and the RPM Air Gap models. Professional Products also offers a specific Vortec intake. The intake ports are also significantly taller than typical small-block ports. While companies such as Jim Pace offer modified heads that will accept a standard 12-bolt intake, this requires a manifold with at least a 0.325-inch-tall sealing surface above the top of the intake port.
Most street intakes require welding and machining to create the proper sealing area. This is both expensive and time-consuming. Using a dedicated Vortec manifold is the best choice and probably less expensive.
The Vortec head also requires the use of center-bolt valve covers, and the good news is there are several styles to choose from. If you decide to use Corvette-style covers, you will have to trim the driprails inside the covers with a die grinder if you opt for roller rockers. Crane offers narrow-body rocker arms designed to work with center-bolt valve covers, but these are more expensive than standard roller rockers. Stock Vortec heads come with pressed-in 3/8-inch rocker studs and do not use guideplates. The rocker arms are guided (also called rail rockers) and employ a pair of guides or rails that center the rocker arm over the valve. This eliminates the need for pushrod guideplates. GM Performance Parts, Crane, and Comp offer self-aligning stamped-steel and roller-rocker arms that will bolt right on. Or, you can modify the heads to add screw-in studs and guideplates and then use conventional rocker arms, but be forewarned that this can add over $200 to the cost of the heads. Do not use guided or self-aligning rocker arms with heads that use guideplates, since this will cause pushrod bind and excessive wear. Also be aware that 1.6 rockers move the pushrod very close to the pushrod hole. Elongating the pushrod hole should be considered essential when using 1.6 rockers.
This leads us to valvesprings, which require the most attention in a performance application. Since this is an OEM iron head, GM designed the valve guide to also serve as the inside diameter (id) locating device for the valvesprings. The guides large diameter acts like a sleeve that keeps the single-wire valvespring located >> properly. The guide is also machined at the top to mount a positive-style, 0.560-inch-diameter valve-guide seal. All of this conspires to limit total valve lift to a safe figure between 0.420 and 0.430 inch. The limitation is the clearance between the bottom of the retainer and the top of the valve-guide seal.
Accommodating more valve lift requires modifications to the valvespring seat and valve-guide areas of the head. The best route is to increase the spring-seat diameter from the standard 1.25 inches to 1.440 inches. Both Comp and Crane offer tools that use a valve-guide arbor to position the cutter. The Comp Cams cutter increases the spring-seat diameter while simultaneously reducing the outside diameter (od) of the guide boss. The Crane tool also cuts the valve-guide-seal diameter at the same time. Comp sells a separate tool that cuts a new guide seal and reduces the guide height to create additional retainer-to-seal clearance. Both cutters machine the top of the guide down to a 0.530-inch diameter, which is the standard size for positive-type aftermarket seals. This requires purchasing new valve-guide seals, since the factory uses a 0.560-inch guide to locate the seals. Another way to go is to have a machine shop cut the guide boss diameter to around 0.760 inch and reduce the seal height and use either the Crane H-11 tool steel spring (PN 99846) or the Comp spring (PN 981).
Once these machining operations have been completed, you have the choice of several aftermarket valvesprings to match your cam profile. These machining operations will produce enough clearance to allow between 0.525- and 0.550-inch valve lift before running into valvespring coil-bind. Given a 1.440-inch valvespring seat diameter, Comp and Crane offer several single and dual springs that will work with the stock valves.
The Vortec heads offer a wealth of power potential for their extremely low price. In several tests of these heads, weve been able to make 400 hp and excellent torque with a relatively mild camshaft. If you include the cost of machine work and better valvesprings, the price does escalate a bit, but 400hp potential without porting with a stock set of iron heads is hard to beatespecially for so few dollars. Small-block street performance doesnt get much easier or cheaper than this.