The Air Gap name describes the fact that the runners and plenum are separated from the effects of hot oil splashing against the bottom of the manifold by an integrally-cast "hallway." Greater intake charge density is the objective, while the increased carburetor height allows more efficient runner designs. Nonetheless, these manifolds fit beneath virtually any production Chevy hoodline. With the single-plane Victor Jr. in place atop the iron heads, in Test 2, the 383 churned out 384 hp at 5400 rpm and 395 lb-ft at 4,000 rpm, a gain of 5 hp and a loss of 10 lb-ft. Though not an extreme example, this is a typical result of what happens when you trade a dual-plane manifold for a single-plane design.
After installing fresh head gaskets, engine owner Dale Snoke installed the aluminum heads on the 383. Thanks to the smaller 64cc combustion chambers, compression jumped from 9.0- to 9.8:1. Though we're well below any concerns of detonation even with 91-octane pump gas, know that a 1-point increase in compression ratio typically calls for 1-3 degrees less ignition timing advance. However, this requirement isn't as important when swapping from iron to aluminum heads, due to the reduced surface temperatures caused by aluminum's rapid heat transfer characteristics. In our case, it's a washout, and we left the total advance at 37 degrees BTDC with no detonation problem.
While it's true that head bolt threads must be lubricated for 100 percent assurance that they're properly torqued, the fact that they thread into the coolant jacket beneath the deck mandates sealant to prevent water leaks. Teflon pipe sealant safely pulls double duty, though a light coat of oil between the bolt heads, washers, and cylinder head eliminates potential frictional drag that could yield undertorqued fasteners. Is there any difference in the torque value for an aluminum head versus an iron head? Don Johnson answers, "Nope, you torque the fastener, not the head. The bolts go to 70 ft-lb, in three steps, regardless of head material."
With the aluminum heads in place, the previous, 8.050-length pushrods (arrow) were too long, so they were swapped for a set of Elgin 7.860-length pushrods (PN PR-777). Remember the pushrod guides used with all aluminum heads require hardened pushrods. Iron heads can get by with nonhardened pushrods as long as guideplates are not employed.