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!
These machining tools allow the Vortec cylinder heads to accommodate high-lift camshafts and larger valvesprings. All three tools can be purchased from Summit Racing for just over $100. |
When replacing the factory alvesprings, it is a good idea to remove the factory press-fit rocker studs and replace them with more durable screw-in versions. The Vortec heads come with excellent positive valve-guide seals for both the intake and exhaust. |
McKenzie Racing used the Comp Cams cutting tools and an electric drill to open the spring seat and machine down the valve guide and valve-guide boss. |
Machining the valve guide boss (A) provides additional clearance for the tighter inner diameter of the dual valvesprings. The second machining process requires opening the valvespring seat (B). When enlarging the spring seat, only remove enough material to match the original seat depth. Removing too much material will alter the installed height and could ruin the head. |
The 11/32-inch Arbor cutting tool is used to remove material from the top of the valve guide to provide additional retainer clearance with valve lifts of more than 0.420 inch. |
Before the spring seats are enlarged, it's a good idea to use a valve-height micrometer. Once the seats have been enlarged, recheck the valve height to make sure the spring-seat depth has not been altered. |
McKenzie used a 1/2-inch drill bit to enlarge the pushrod holes in our Vortec heads. Be sure to drill at the same angle as the factory design. This will increase the inboard pushrod clearance when using 1.6:1 rockers. |
Once the heads have been machined and cleaned thoroughly, the valves can be reinstalled and matched to the proper-sized Teflon oil seals. |
If you plan to work on more than one set of cylinder heads, the PowerHouse valvespring installation tool is worth its weight in gold. The tool is listed under PN POW351015 and can be purchased from Powerhouse for $129. |