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GM Bow Tie Vortec Small Block Engine - Blue Thunder 355

This 632hp Small-Block Sports New GM Bow Tie Vortec Heads

Ro McGonegal Jun 1, 2005

Call it new tech by old NASCAR guys, conceived and executed by the long-time Winston Cup hands at Blue Thunder Racing Engines, chief among them, engine builder Harold Grady. About a month before Daytona Speed Week, Grady contacted Hank the Crank Bechtloff and the two formulated an engine package that would be showcased at the event--one that would include a lot of things that no other engine could claim, and one that would be built by seasoned NASCAR suppliers and bright thinkers.

Hank furnished the entire 9,000-rpm rotating assembly with the help of Carrillo's Jack Sparks and CP Piston's Barry Calvert, and Paul "The Rocket" supplied the box-and-box forgings, the latest Nextel Cup gimmick, which is lightweight and rigid to help the rings seal, and Fel-Pro engineer Greg West and Product Manager for Fel-Pro and Speed-Pro's Ron Rotunno provided the trick MLS head gaskets. Speed-Pro's Scott "Lord of the Rings" Gabrielson did the rings, and Harold Elliott applied his special coatings to all of the appropriate surfaces. Before the rotating assembly left The Crank's for the Blue Thunder shop, Automotive Balancing massaged the assembly. Additionally, HTC's representative in the southeast, Marty Cimaglia, oversaw the build and the testing of this project. Dig on that 1.8 hp-per-cubic-inch output, and you know that everyone involved dipped into their trick bags for this fierce small-block.

By nature, the motor was generously over-built, and therefore pricey due its extra-stout rotating assembly and the inclusion of nearly every subtle enhancement known to a naturally aspirated engine. Scrupulous attention to detail combined with things you can't see make a difference between just a motor and a killer motor that runs on the edge. And despite this mighty Mouse's intense racing flavor, it teems with ideas that could be applied to a small-block street engine.

The motor was constructed with the care and precision prevalent in all such extreme-duty applications. In this case, the bullet was destined for the Blue Thunder Super Late Model running under FASCAR (Florida Association of Stock Car Racing) rules. The restrictions include a 362ci (maximum) iron block, the first GM Bow-Tie Vortec big-port cylinder heads put into use (cast-iron and out-of-the-box stock save for thoughtful valve job; no changes allowed to runners, bowls, or combustion chambers), any flat-top piston design, an unaltered intake manifold-specific Holley carburetor (again without modification to alter airflow save for removing the air horn) and a mechanical fuel pump. Compression ratio, camshaft phasing, the rotating assembly, oiling system, and all the ways to reduce frictional drag are open to interpretation. With a 12.0:1 compression ratio and swilling Sunoco 110 race gas, this 355 made 632 hp at 7,500 rpm and 502 lb-ft at 5,900 rpm. Here's how it happened:


Grady began with a cast-iron, two-bolt main bearing block fitted with HTC steel billet main caps on the center three races that are secured with ARP studs. To strengthen the assembly, he also installed deck plugs. Then he prepared the block's lubrication system by plugging off the top of the engine, restricting oil to the top, and by the distributor boss.

"We plug the oil galley that goes around the distributor and feed it from the front. We also plug the hole behind the cam bearing and the one in front of the distributor. Then we drill a hole under the front cam bearing in the groove. This is something I learned from Ben Barnes."

Because Grady wanted to use a ring diameter of 0.040-inch, he held the bore at 4.039 inches. The idea is to make the most power possible with a flat-top piston so that the engine would seal exceptionally well, be able to pass a lot of air, and see as much compression as possible with the new GM Bow Tie Vortec cylinder heads. A common 350 block was stress-relieved and sonic-tested before the honing process. Grady used the same length studs used on the engine to secure the honing plates. By using a fine lapping compound under the spacers and washers, Grady makes sure that hardware doesn't spin when he torques the plates down prior to the honing procedure. If these pieces move upon the bolt-stretching sequence, they will likely distort the cylinder walls. Grady finishes the honing process with a Sunnen J83 stone. It's critical to use the same MLS gasket as you use on the engine and spray both sides of it with WD-40 (for extended reuse) so it won't stick.

Hank the Crank's billet arm is a jewel. Besides a Teflon-based oil-shedding coating (see sidebar), it hosts everything Hank Bechtloff has learned in 45 years of making crankshafts--though son Scott mapped out and executed the actual build. The 4340 steel assembly has been rifle-drilled to reduce weight, bringing the race-ready total to just 39 pounds.

Hank laid out the counterweights so the weight would be off the ends of the assembly and so it would not flex at high revs. He used a very small amount of heavy metal to retain balance, but only in the end counterweights, and in this way it was able to keep the oil-cutting surface of the counterweights absolutely smooth. Further, the shape of the counterweights and their placement is critical to gain horsepower. Hank employed a 1.8882-inch crankpin rod-bearing diameter (referred to as the Honda or Quad-4), the most popular size for Busch cars--and if you hurt it, you can easily go down to the 1.850-inch Nextel Cup size.

The mains are 2.300-inch (from 2.448-inch diameter main to a 2.2985-inch diameter) early-Chevy size to reduce the bearing speed with Clevite bearing spacer (MS1110HX). The crank has also undergone Accurate vacuum plasma ion treatment (the last of four heat treatments), a process that increases fatigue life 20-25 percent.

Grady likes Clevite products (PN CB1663-V) because he feels that they bed-in better than others. He buttons up the lower end with HTC steel billet main caps. For you hip street rats, HTC can supply an American lightweight forged steel crankshaft that costs $1,000 less than the billet unit.

The stroke was finalized at 3.470 inches and realized via 6-inch-long Carrillo steel connecting rods that weighed 525 grams.

The caps are secured with 3/8-inch Carr bolts that have a tensile strength of 285,000 psi. They use a 2.015-inch big end, and the rods have also been rifle-drilled to allow oil to pressure-feed the pins at all times.

The 100-gram Precision Products piston pins are 2.250 inches long with an 0.868-inch diameter, chamfered for wire locks, made from 9310 steel, and are Casidiam-coated, which is the critical ingredient when you run steel against steel as there is no bushing in the small end of the rod.

Featherweight 390-gram CP pistons have a flat top, a cam-shaped skirt, and flat-ring lands (note coated surfaces).

Ring seal is enhanced with a series of 0.062-inch gas ports (arrows). Scott Gabrielson broke out some special low-drag 0.043-wide plasma-moly top rings with a 4.040-inch diameter followed by a 0.043-inch second ring. Both are back-cut but only the second ring has a reverse bevel taper (RBT), and then some special 3mm SS50 stainless oil rings run with 10-psi tension. With these rings, the correct finish on the cylinder walls and the closer tolerance available with the piston coating, Grady sees an extra 35 hp.

Original L31 Vortec casting.
The new heads carry PN 25534371 and were the only ones in use at press time, but will be available through GMPP dealers by the time you read this.

New GM Bow Tie Vortec
Hank credits Lannie Barnes for securing them for Blue Thunder through the Scoggin-Dickey Parts Center. The 4371 castings have a much thicker deck than the previous Vortecs (see side bar).

The L31 exhaust port (top) looks like old tech;

GM Bow Tie port has definite D-shape, is raised up, and has a 77cc volume.

Grady puts them to the block with Fel-Pro 1144 MLS multi-seal gaskets. The 1144s have a stainless steel inner core and a super-bonding agent on both sides (less than $100 each). They feature a 4.100-inch bore size, a compressed thickness of 0.040-inch, and a volume of 8.5ccs. ARP studs secure the heads.


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.

"Invisible" Power
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.


Scoggin-Dickey Parts Center
Lubbock, TX 79424
CP Pistons
Irvine, CA 92614
Wilson Manifolds
Ft. Lauderdale, FL
Lakewood, NJ 08701
Muskegon, MI 49443,
Vector Dampers
South Gate, CA 90280
Stef's Fabrication Specialties
Lakewood, NJ

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