Combustion chambers on the Vortec heads are listed at 64 cc, but GM tells us most heads are actually around 61 cc. The intake port was borrowed from the LT1, and the 1.94-inch intake and 1.50-inch exhaust valves allow the head to breathe. As it turns out, the head was designed to flow very well so the truck engines could make good power for pulling heavy loads, trailers, and the like. It's been a standard on pickup trucks and SUVs for the last few years, gaining little recognition.
It was never a Corvette standard like the L98, nor is it being touted as a small-block head for the next millennium like the Fast Burn. The Vortec cylinder head is the sleeper in the bunch. Satisfied, we yanked the L98s, and hauled the iron Vortec heads to the dyno cell. After setting timing at 35 degrees, we made a couple of pulls to check jetting, and were rewarded with as-advertised horsepower and torque numbers (355 hp, 405 lb-ft). For the dyno testing, we also added a pair of Hooker headers (designed for a '69 Camaro), and Flowmaster threechamber mufflers. We began with a baseline run on the ZZ4 as delivered, with the addition of GMPP plug wires, AC plugs, and a Demon 750-cfm carburetor. We decided that back-to-back testing would be the most effective way to find out, so we got all the stuff we needed for the test and headed to the Westech Performance dyno to make some pulls. We wondered how much more power we could get from swapping to GM's bottom-dollar Vortec cast-iron head, and also their new "Fast Bum" aluminum heads. To complement the cylinder head design, GMPP incorporated a steel roller tappet camshaft with. The valve seats are radiused and include valve stem seals to help keep the oil where it belongs. Also included are lightweight valvespring retainers that weigh half as much as the previous design. The aluminum angle-plug cylinder heads have screw in rocker studs, 1.94-inch intake and 1.50-inch exhaust valves and 58cc combustion chambers that yield a crisp and responsive 10:1 compression ratio. Topping the ZZ4 is the aluminum L98 cylinder head that was available on the pre-LT1 Corvette. The engine arrives with an intake manifold, HEI distributor, and many of the little details crate engines normally ship without. The retail price of just over $3,000 makes it an affordable, reliable, and available performance engine with a 12-month warranty. The ZZ4 has a nice combination of features, including 355 hp and 405 lb-ft of torque.
We decided to look at the cylinder heads offered by General Motors Performance Parts (GMPP) on the most popular of their small-blocks, the ZZ4 350. The fair prices, nationwide availability, and serious horsepower produced by these engines have made them the preferred choice of street rodders and racers alike. General Motors really hit a bullseye with their series of "ZZ" smallblock Chevy engines. Testing the three best small-block Chevy heads from GMPP
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