Vortec Bow-Tie Brief
There are two versions of the new GM Performance Parts Vortec Bow-Tie cylinder heads: This one, PN 25534371 (bare) and PN 25534431 (complete), with the larger ports, and another with smaller ports, PN 25534351 (bare) and PN 25534421 (complete) with 185 and 65cc volume, otherwise, the architecture and improvements over the L31 design are the same. We will test the smaller version very soon in a dynamometer exercise. We were able to get an exclusive look at the big-port heads just as they were used in a NASCAR-type application.
Out of the box, they feature revised intake and exhaust ports and are machined for a 2-inch intake and 1.55-inch exhaust valves. The deck surface is 0.045-inch thicker than the L31s, and it has 65cc combustion chambers, a 206cc intake, and 77cc exhaust ports (see Flow Chart). The head is also machined for 3/8-inch screw-in rocker studs and a large valvespring pocket, and it will accept up to a 0.530-inch lift camshaft without modification. These castings are identified by the Bow-Tie logo below the exhaust port and the Vortec logo on top of the intake port area. The GM logo is also cast into the bottom of the intake port runners.
All concerned will be delighted to hear that the head includes intake manifold mounting holes for both early-model six-bolt and late-model four-bolt Vortec design. Mandated GM intake manifolds include a raised runner-type PN 10051103 (six-bolt type) or Vortec design PNs 12366573, 12496820, 12496821, 12496822, and 12499371 (four-bolt type). Though production cylinder head and intake manifold gaskets are acceptable, the hot squeal is Fel-Pro (PN 1142 for the MLS and PN 1289 for the intake). As per Scoggin-Dickey, pricing is: small port 4421 complete, $499.95; small port 4351 bare, $269.25; large port 4431 complete, $539.95; and large port 4371 bare, $293.25.
Put Yur Coat On!
Harold Elliott knows better than most that the reciprocating parts must have an edge--a very slick edge--to compliment the engine builder's expertise and make his product a winner instead of an also-ran. He treated all the key elements of Harold Grady's bullet with a variety of friction-frying stuff. In a very brief explanation, this is how it goes:
Elliott coats the crankshaft counterweights with Teflon-loaded SL-11 in order so that the weights break the surface tension of the crank material so the oil won't stick to it and attempt to build up on the surface. The same goes for the connecting rods. He also uses SL-11 on the windage trays and the scrapers inside the Stef's dry-sump pan.
What he applies to the piston skirts, rod, and main bearings is called HM-30, which has is composed of a high percentage of molybdenum and makes the surface area much more durable in high-pressure areas. This allows the engine builder to tighten up his clearances as in the piston-to-wall area that will keep the piston straighter in the bore and aid ring sealing. Less clearance on the rods and mains means less oil leakage that will not collect in those areas, thereby not taxing the oil recovery system as much.
The HS-44 coating used on the tops of the pistons has a ceramic base, and therefore has very good heat-reflective properties. Keeping combustion heat away from the piston under the rings greatly improves the life of the piston and allows the builder to tighten the piston-to-wall clearance even more. Elliot does not stop there. Since the chambers, exhaust ports, and piston domes act as heat sinks during the combustion cycle and pull valuable power-producing heat from the area, he applies HS-44 to these surfaces as well. This coating is also applied to the tubular exhaust system in the interest of reducing under-hood heat and in presenting a very slick appearance.
Strict rules tend to initiate creativity and engender the NASCAR racers' credo of "being competitive." In this case, we're not talking about bending the rules or exploiting their interpretation, but simply using the most effective (and legal) means to reduce power-robbing friction, increase parts longevity, and enhance power output. In no particular order, the arsenal includes the gas ports that are drilled beneath the top ring land to ensure the best and most effective cylinder wall seal. This venerable helper is worth about 20 hp. The right rings, gas ports, and cylinder wall prep can release as much as 40 hp. Coatings and treatments for piston tops, skirts and pins, bearing surfaces, and crankshaft counterweights yield another 25 hp. The smooth counterweights are good for 5 to 6 hp, the small main journals 6 to 10, and the small rod journals 4 to 6 hp. Hank figures that the HTC Dyna/Rev special center counterweights are worth 4 to 6 more hp.