GMPP Cylinder Head - High-Flow Happenings

The Bow Tie's 77cc D-shaped exhaust ports (bottom) feature flat floors for enhanced flow. Notice the cast Bow Tie logo. To aid identification during tech inspection at sanctioned competitions, the 206cc Bow Tie heads bear casting number 25534371C between the valvesprings.

The valve-seat widths are 0.060 inch on the intake seat and 0.080 inch on the exhaust seat. Both blend smoothly into the chambers with five-angle seat contours for maximum flow. The two seat angles closest to the bowls are so subtle we almost missed them until we picked them out with some blue Dykem and a stone. The valve-seat angles are 80, 75, 60, 45, and 30 degrees.

The fully assembled heads come with swirl-polished stainless 2.00/1.55 valves. The intakes have hollow stems for reduced mass, and the exhaust valve stems are sodium filled for added durability in endurance racing. Notice the reduced-diameter, back-cut stems on the intake valves for increased flow potential.

The exhaust valve seats are induction hardened for resistance to seat erosion with unleaded gas on the street or during long-distance endurance racing.

Beefy 0.450-inch-thick decks resist warping, add rigidity, and help ensure head-gasket sealing. The extra 3 pounds of metal mass is more than offset by the durability improvement.

Even though the diameter of the supplied valvesprings is 1.35 inches, the generous spring seats will easily accept 1.5-inch-diameter racing springs, making these heads readily adaptable to future equipment upgrades. The compact height of the valve-guide bosses is designed to allow cams with up to 0.530 inch lift without modifications to avoid retainer contact.

The supplied single valvesprings measure 101 pounds at installed height (1.78 inches), 275 pounds at 0.525-inch lift. These are the same springs (PN 12551483) used on '96 LT4 Corvettes and ZZ4 crate engines. GMPP says they're good up to 0.530 inch lift and work great with our test car's 0.480-lift hydraulic cam.

At 14.7 grams, the retainers (PN 10212808) are so light we thought they were titanium and used a magnetic probe to confirm they were steel. They're also from the LT4/ZZ4 parts shelf.

The Bow Tie's 71/416 screw-in rocker-arm studs are a nice strength upgrade over the press-fit studs used on Vortec 5700 truck heads. Dual bolt patterns accept both perimeter and center-bolt- style valve covers.