GMPP Cylinder Head - High-Flow Happenings

Hey, GMPP, one casting revision we'd love to see is a slight outward repositioning of the No. 5 spark-plug hole to ease tight clearance issues between the header tube and spark-plug boot. A mere 0.055 shift would do the trick. A little hammer work saved the day for Dale's small-tube headers.

The guys at Superior Automotive randomly sampled the combustion-chamber volume and recorded 63 cc. That's 2 cc smaller than the 65cc GMPP specification. This is no concern to street and strip users, though class racers should double-check theirs to ensure legality. The volume measurement was taken with the recommended AC MR43LTS spark plug.

After the steam-hole locations were transferred from the 400 head gasket to the heads, a 11/48-inch drill bit is used to make the holes. Beware, you must drill at a 30-degree angle as shown in order to correctly intersect the water jacket. If you mistakenly drill straight down, you might break through into the head-bolt holes which will then become filled with coolant and leak into the valve covers during operation.

The 400 block requires specific head gaskets with matching steam holes (Fel-Pro PN 17031). Head gaskets for nonsiamese-bore Gen I small-blocks will work if a 400 gasket is used as a template to mark the many 11/48-inch steam-hole locations for drilling.

There is some conflicting information regarding the high-flow Vortec Bow Tie's intake-port volume. Some factory literature calls it 203 cfm, the GMPP catalog calls it 206 cc, and the tech sheet shipped with the heads calls it 215 cc. Superior got an on-the-money 215cc reading. Again, this is of little consequence to all but class racers who may be subjected to teardown inspection. Be advised, GMPP also sells a small-port version of the Vortec Bow Tie with 185cc intake/66cc exhaust-port volumes as PN 25534421 fully assembled (PN 25534351 bare).

On the Superior Automotive Super Flow SF600 test bench, intake port flow was 256 cfm-and climbing-at 0.550 lift. The exhaust port flows 157 cfm at 0.500 lift. Both measurements were taken at 28 inches of water.

The chassis dyno confirmed the added potency of the Vortec Bow Ties with gains of 12 hp and 17 lb-ft of torque over the polished and relieved heads they replaced.