It's A Spring Thing

The one goal that most engine builders share is making the most power for the least money. Since cylinder heads are typically the bottleneck of an induction system, that's where the smart money usually gets spent. This often leads the determined enthusiast to shop through multiple aftermarket cylinder head catalogs until a head that flows big-time air is found for a reasonable price. In 1996, Chevy introduced its Vortec cylinder head on the L31 truck engine and it didn't take long for enthusiasts to bolt them on their engines and find out how good they really are. The cast-iron Vortec heads offer a 170cc LT1-derived port, making them great performance pieces for street engines when tons of torque is key. As the displacement of the engine increases, the torque numbers go through the roof and the horsepower figures follow.

CHP recently bolted a pair of Vortec heads on a 383ci (see "Twister, Part III" on page 96) and made 486 lb-ft of torque at 4,600 rpm and 449 hp at 5,300 rpm. These power levels were achieved using a stock iron Vortec cylinder head featuring 64cc combustion chambers and 1.94-/1.50-inch intake/exhaust valves. The fully assembled factory heads can be purchased from any GMPP dealer including Scogggin-Dickey, Burt Greenwald Chevrolet, and Friendly Chevrolet. Before you go bolting these babies on your favorite small-block, there are a few things you need to consider. The factory Vortec heads use a light, single-wire valvespring with a damper that generates between 70 and 80 pounds of load on the seat. Because these heads are meant to work with a 1.5:1 rocker, their tall valve guides can only handle maximum valve lifts of around 0.420 inch. While the factory-assembled heads are a great choice for the enthusiast looking to build a stout street engine, there are some modifications you should consider for both power and durability.

If you are willing to take on the additional work, the Vortec heads are capable of unleashing 500 lb-ft of torque. Depending on the size of your small-block, the Vortec's 64cc combustion chambers will require a 12-24cc dished piston to lower the compression level down to a pump-gas-friendly ratio. We've used a 1.6:1 rocker with the GMPP Hot camshaft with excellent results, but found that the larger Comp Cams XE 282HR hydraulic roller with 1.6 rockers requires some head modifications to accommodate the taller lift and required bigger valvesprings.

Installing a high-lift camshaft in conjunction with the Vortec cylinder head requires several minor modifications. Increased valve lifts require stronger springs. Comp Cams recommends that its PN 986 double valvespring be used with its XE 282HR camshaft. The Comp Cams PN 986 >> valvespring features an installed height of 1.750 inches, a 1.430-inch od, 0.697-inch id, and a seat pressure of 132 pounds. The larger spring and increased lift figures require opening up the spring-seat diameter of the cylinder head and machining down the valve guide and valve-guide boss to clear the retainer. To keep things simple, Comp Cams has designed an easy-to-use tool that allows you to machine all the necessary areas at home.

Once the heads are machined and the new springs are in place, it's a good idea to replace the weak press-fit rocker studs with screw-in types. Be careful--the type of studs or rocker arms you choose will determine whether or not guideplates are needed. The GMPP screw-in studs don't require guideplates when used with rail-type rockers. ARP screw-in studs require guideplates. To prevent binding, do not use rail-type rockers with guideplates. It's also wise to enlarge the pushrod holes to a 1/2-inch diameter for increased clearance. This is required when using 1.6 rocker arms to prevent scuffing the pushrods against the pushrod hole in the head.

The "Twister Part III" dyno-test shows that the larger 383ci small-block benefited when the valves spent more time off the seat. However, the increased valve lifts would have created a valvetrain stability problem and required stronger valvesprings. When ordering the proper camshaft for your particular application, it is crucial to use manufacturer-recommended parts. But once that's accomplished, these heads are ready to rock and roll!