Ever since the small-block Chevy was introduced 50 years ago, engineers, racers, and backyard hot rodders have delved into its cylinder heads in the quest for more power.
Chevrolet did an admirable job through the mid-1960s by continuously refining and revising the heads, as displacement, horsepower, and torque all increased. But while Chevy had the lead for the first decade or so, the aftermarket started nipping at its heels. By the '70s, several companies began offering their own cylinder heads for the engine, and the trend has increased ever since.
In the last dozen years or so, the advent of high-volume CNC-ported aluminum heads brought cylinder head performance to a new level. Aluminum heads, while not as cheap as comparable iron versions, could be easily mass-produced and delivered out-of-the box performance that a week's worth of grinding couldn't match on a set of double-humps.
GM Performance responded with the 23-degree, aluminum Fast Burn head. Its 210cc intake runners and raised intake valves made it capable of supporting more than 500 hp. More than flow bench numbers, however, the Fast Burn head worked well on both street and competition engines. Fast Burn heads also serve as the focal point of GMPP's Fast Burn 385 crate engine--a 350-cube small-block that produces around 400 hp.
Still, it was hard to beat some of the aluminum heads at their own game, until late last year, when GM Performance Parts introduced the Vortec Bow-Tie head, an all-new, iron small-block cylinder head that promises to deliver the performance of a CNC-ported alloy cylinder head at a fraction of the cost. While GMPP didn't create a fanfare when the first few heads trickled out of the foundry, the company plans to get much more vocal about them now that they're being produced in quantity.
GMPP tells us a complete, bolt-on set of Vortec Bow-Tie heads should cost about $700--about half of the cost of aluminum heads. The obvious catch to these new high-flow heads is their material--dense cast iron rather than lighter aluminum. It's a weight penalty that GMPP is hoping everyone from the weekend cruiser to the weekend oval track racer will overlook for its performance and affordability.
By the way, these are not the same Vortec heads introduced several years ago (PN #12558060). This is an all-new design that also carries the Vortec tag."This is an inexpensive head that flows as good as any CNC-ported aluminum head," says Bill Martens, of GM Performance Parts. "People on a budget, especially racers, will get a lot of bang for the buck here."
Indeed, the racetrack was one of the heads' initial application intentions, as GMPP sees grassroots and "claimer" circle track racers gravitating toward the budget-minded performance that the heads offer.
But what exactly is the performance? Compared to previous GM small-block cylinder head designs, the Vortec Bow-Tie has all-new ports that are taller and narrower, the idea being to increase velocity while maintaining volume. The heads are available in "small-port" (PN# 25534421) or "large-port" (PN# 25534431) versions, with the small-port heads boasting 185cc intake runners and 65cc exhaust ports; and the large-port heads cast with 215cc intake ports and 84cc exhaust ports.
Out of the box, the heads come with 66cc combustion chambers and 0.450-inch deck surface thickness. GMPP tells us a camshaft with up to 0.530-inch lift will work with the heads, too. Assembled heads come with 2.00/1.55 valves, too. Additional details include mounting provisions for early-style valve covers or the later-style center bolt covers, as well as mounting holes for earlier six-bolt intake manifolds and later-style four-bolt Vortec manifolds.
On the dyno
Proving the theory that the taller, more narrow intake ports were the key to the Vortec Bow-Tie heads' performance fell to McLaren Performance--a third-party vendor which spent more than 500 development hours and made nearly 110 independent dyno pulls on various engine combinations that included the new GMPP heads.
The test engine was a ubiquitous GM Performance ZZ4 crate engine--the seeming standard of Chevy crate engines. From the factory, it comes with 10:1 compression, a forged crank, cast pistons, and a steel hydraulic roller camshaft; with 0.474/0.510 lift specs and 208/221-degree duration specs. Aluminum cylinder heads are standard on the ZZ4 and feature 58cc chambers, 1.94-inch intake valves, and 1.50-inch exhaust valves. The valvesprings have a 1.5:1 ratio.
With the standard configuration, the ZZ4 is rated at 355 hp and 404 lb-ft of torque. A Hot Cam package, which includes a 0.485/0.490-lift and 245/254-degree duration cam and 1.6-ratio rockers, boosts horsepower by approximately 25 hp. We got a peek at the test sessions toward the end of the development cycle and we were granted looks at numerous dynamometer results. The test sessions included numerous combinations, including three different intake manifolds, five cylinder head designs, two camshafts, two different-ratio rocker arms, and two carb spacer heights.
The heads that were tested included the two versions of the Vortec Bow-Tie, as well as the GM's Vortec truck head, Fast Burn aluminum head, and the stock aluminum ZZ4 production head. When it came to the intakes, the stock ZZ4 dual-plane manifold was used, along with the Vortec single-plane and dual-plane manifolds.
Some tests were made with the stock cam and 1.5 rockers, while others were made with the GM hot cam and 1.6 rockers. A 650-cfm carburetor was used for all dyno sessions.McLaren also put examples of the new heads on the flow bench, producing some interesting results (see sidebar, "Fun on the Flow Bench").
The test results we received from McLaren showed average horsepower for the multitude of dyno pulls made with each engine configuration (i.e. stock ZZ4, ZZ4 with large-port Vortec Bow-Tie heads, etc.)
Baseline runs were made with the stock ZZ4 engine, including the stock cam, 1.5 rockers, dual-plane intake, and no carb spacer. In this guise, the ZZ4 averaged 328.5 hp. It made 335 hp with the truck heads and averaged 348 hp with both sets of Vortec Bow-Tie heads, as well as GM's aluminum Fast Burn heads. So, right off the bat, the iron heads were proving their worth. While we expected to see an increase in power, simply due to the Vortec Bow-Ties' increased capacity over the stock ZZ4 heads, we were intrigued to see that the new iron heads pretty much followed the power curve of the aluminum Fast Burn heads.
The Fast Burn head has the same combustion chamber size and valve sizes as the new iron Vortec Bow-Tie, yet the less-expensive--though heavier--iron heads delivered comparable performance.
We don't have the space to list the results of all 108 dyno pulls, however our examination of the test data show similar results with various combinations of intake manifolds, carb spacers, and rocker arm ratios. The best averaged performance achieved with the test engine and Fast Burn heads was 371 hp, with a configuration of a single-plane manifold, 1.6 rockers, the stock camshaft, and a 2-inch carb spacer. The power peak was 5,500 rpm.
When it came to the best performance with the Vortec Bow-Tie heads, the results included 391 hp with the small-port heads and 386 hp for the large-port heads. The engine power curve peaked at 6,000 rpm with the Vortec Bow-Tie heads.The 391 hp average peak achieved with the small-port heads included the engine equipped with the single-plane Vortec intake, 1.6 rockers, the hot cam, and a 2-inch carb spacer. The same configuration was used with the large-port heads to achieve their 386 hp average.
So, why the 5hp difference with the smaller-port heads? Displacement, mostly. The large-port heads could use a larger-cube engine to make the most of their capability. Regardless, the new iron heads showed that they had the flow to keep up with--and surpass--aluminum heads.
The test session didn't compare aftermarket heads with the new Vortec Bow-Ties, nor were the iron heads ported or machined in any way. They were out-of-the box examples. We're interested to see how these low-cost alternative heads would fare against the competition, as well how airflow will improve with a little careful grinding.
The initial impression is not only encouraging, but downright impressive - especially for those of us on a tight engine-building budget. And for "spec-head" circle track engines, this latest version of the famed Bow-Tie iron head family seems just about perfect.