In the early-'90s, GM introduced the Second-Generation LT1 small-block. It featured reverse-flow cooling and an aluminum cylinder head with a new port configuration. In essence, the company created the best small-block Chevy cylinder head ever put into mass production. It was released on top of the '92 LT1 Corvette engine and later on the '93 LT1 Camaro. The new port/chamber design that began life as an aluminum casting eventually carried over in a cast-iron version. Unfortunately, just as the LT1 concept began its ascension, GM executives decided to put an end to its production. This superstar cylinder head dropped off the performance map because both the aluminum and cast-iron heads lacked a coolant crossover passage, rendering them useless to any engine series other than the discontinued LT1.
The LT1 prospered from 1992 to 1996 in various vehicles, but its days were numbered. In 1997, GM released its Third-Generation small-block engine, the LS1. While the new Corvettes and Camaros were about to receive yet another cylinder head breakthrough, the engineers at GM's truck division knew that the LT1 port still had something to offer. Subsequently, the '96 truck engine series hit the market with the infamous LT1 port in cast-iron production. It was called the Vortec design and became the most efficient mass-produced small-block Chevy cylinder head ever created.
When Chevy enthusiasts discovered the capabilities of the Vortec head, it didn't take long to take advantage of its coolant crossover passages and fit them to First-Generation small-blocks. As if building the ultimate OE street-performance head wasn't enough, GM Performance Parts sweetened the deal even further by announcing that each head could be obtained fully assembled for approximately $250.
The cast-iron Vortec heads were released in 1996 and produced through 2001. Abetted by a newly redesigned coolant crossover passage, these heads could be bolted directly onto a First-Generation small-block engine with the help of some aftermarket parts. The intake face of a Vortec head is cut at a slightly different angle than the First-Generation SBC heads and features an eight-bolt-hole intake manifold hold down as opposed to the standard 12-bolt design, so a special Vortec intake manifold and gaskets are required to put the Vortec heads on top of a First-Generation small-block. Furthermore, the OE-production Vortec valvetrain requires a self-guided rocker system that will not exceed a maximum valve lift of 0.420 inches unless the valvetrain has been modified. For more details about Vortec head installation onto a First-Generation small-block, visit our Web site at www.chevyhiperformance.com or dig up a Jan. '03 issue.
Now that the history of the Vortec port/chamber design is clear, we can move onto this month's king-of-the-Vortec-head test. We ordered every example on the market in applications featuring 185cc intake runners or smaller; the idea being that a Vortec head is a low-cost street piece and should be tested in a street fashion. Our evaluations would be run across a 0.030-inch-over bore 350ci short-block featuring all remanufactured OE parts that had been balanced and blueprinted by Hye-Tech Performance. Hye-Tech offers this assembly for $900 less oiling, timing cover/chain, and camshaft, or fully outfitted for a few dollars more.
To finish it off, we secured a Moroso baffled oil pan, pump, and pick-up, and then moved to the valvetrain side with a Crane Cam setup featuring everything but valvesprings, retainers, locks, and rocker studs. Once the cam pieces were lubed and set in place, we installed 0.040-inch-thick Fel-Pro reusable MLS (multi-layered steel) head gaskets. Since we'd be swapping five sets of heads, these reusable gaskets would cut the interchanging parts cost and offer consistency between tests.
We began with the OE Vortec casting that had only been valvetrain-modified by Scoggin-Dickey to accept valve lift up to 0.600-inch. We fastened them to the block using ARP head bolts covered in thread sealant. After torquing them to 65 lb-ft, we added a 1 5/8-inch primary pipe, long-tube Hedman Headers; proceeded to install the rest of the valvetrain; and lashed the valves on the self-guided rocker arms. We covered the assembly with center-bolt Proform valve covers.
At this point, the Edelbrock dual-plane RPM Air-Gap intake manifold was ready to go on. With silicone under the front- and rear-lifter valley rails and the Fel-Pro gaskets in place, we lowered the intake on top of our Vortec heads and tightened the eight fasteners in a cross pattern. We topped it off with a 650-cfm Speed Demon mechanical secondary four-barrel carburetor and finished the assembly with a Pertronix HEI distributor sprouting matching wires held in place by Made For You Products wire looms.
To even the testing ground for all of the heads, intake runners of 185 cc's-or-smaller would ensure every head was a true street contender and offer consistent torque at all engine speeds. All tests would be on 91-octane pump gasoline, which is why our Hye-Tech mule motor had its valve-relieved Speed-Pro pistons sitting 0.010-inch in the hole. When combined with the 0.040-inch-thick head gasket and 64cc to 67cc combustion chambers, the compression ratio would range from 9.7:1 to 10:1. With our camshaft specs, the motor was set up to pull 14 inches of vacuum at 800 rpm while running 36 degrees of total advanced timing. The key to running a 10:1 compression ratio with iron heads lies within the spark curve, which is why we used a box-stock Pertronix distributor set-up with a rather slow advance. By keeping timing out of the cylinders at high load/low rpm, the motor stayed out of detonation and made power across the entire rpm curve. Two other variables that we'd monitor and steady for consistency were the engine oil and water temperatures. Hot oil reduces friction, so we kept ours at approximately 180 degrees while the water circulating through the block and heads was kept as close to 160 degrees as possible. With all our variables under control, we were ready to begin.
The first test called for a baseline run using the OE Vortec heads. While the factory GM versions can only handle 0.420-inch valve lifts, we used modified valvetrain Scoggin-Dickey versions that accommodate a valve lift of 0.600-inch. No port modifications were made and the valves and valve job were untouched. The OE Vortec heads come fully assembled with 3/8-inch rocker studs and require a center-bolt valve cover. We used polished Proform rocker covers that proved sturdy from test to test and looked good as well. The only area of concern here is that some valve covers, like our Proform pieces, may have breather interference issues with certain rocker designs. The internal baffles can be removed and upgraded using an external breather design if clearance becomes an issue.We were surprised at how well the OE Vortec cylinder heads performed against the various aftermarket versions. Our first pull delivered some good numbers, but the dyno showed a reading of 633 cfm being consumed by the little Mouse. While our Speed Demon carburetor was rated at 650 cfm, we felt there may be a bit more power in a 750-cfm Speed Demon application. We watched the numbers climb: An amazing 364 lb-ft of torque at 2,000 rpm rocked the dyno screen and the motor was well on its way to posting a peak torque of 431 lb-ft at 4,000 rpm. In kind, the horsepower number was 391 at 5,400 rpm. So the OE Vortec really shined and left little room for the competition to claim a spot in our winner's circle. The average power numbers of 400 lb-ft of torque and 293 hp are good enough to move a 3,200-pound car to a solid 13-second quarter-mile time.
|Advertised Airflow Measured at 28 inches of H20|
After testing the untouched OE Vortec versions we wanted to see how lightly hand-ported pieces would do in comparison. The heads were the same part number as the previous ones, but touched-up using a Standard Abrasives porting and polishing kit. The actual amount of material removed was very minimal because no carbide cutters were employed. We rounded sharp edges to help create a smooth, flowing port. On the Superflow flow bench, our hand-ported parts delivered roughly 5-15 cfm more flow than the stock version and made us curious as to what the dyno would have to say.We fastened our ported Vortec castings to the Hye-Tech motor using the same Fel-Pro MLS head gaskets and ARP head bolts. Then we hooked the fuel line up to the 750-cfm Speed Demon carburetor and set the timing at 36 degrees. Our first pull delivered a slight gain in low-speed torque, posting 369 lb-ft at 2,000 rpm and a peak of 434 lb-ft at 4,000 rpm. So our home-porting job had moved the powerband up 100 rpm and delivered a peak horsepower number of 394 hp at 5,500 rpm. Though not much of gain, we felt it was still worth noting for grind-happy enthusiasts. In all, the average power numbers showed a gain of 3 lb-ft of torque and a loss of 1 hp.
|Advertised Airflow Measured at 28 inches of H20|