Getting Some Heads
Perhaps the most striking feature of the 409 is its unique cylinder heads and their staggered valve arrangement that in turn mandates a distinctive set of scalloped valve covers. (The best-looking, best-fitting, currently available aftermarket covers are made by Moon, as shown in the lead photo.) The weird combustion chambers displace 9 cc; like a flathead, most of the effective chamber volume is in the block. McCord made the best head gaskets (note the past tense), but CH still has a stash of them.
Standard 348 heads use 1.94/1.66 valves and are generally not useful for performance applications. High-perf 348 and low-perf 409 heads have 2.06/1.72 valves; if larger 2.19-inch valves are installed, their intake runners typically flow about 225 cfm stock and are capable of supporting a 450hp engine. High-perf 409 heads have 2.19/1.72 valves, flow 255-260 cfm on the intake side, and can support up to 520 hp as delivered; expect to shell out $500-$800 for a good set of cores.
High-perf Z-11 heads are in a class by themselves: Capable of supporting 600 hp, their raised intake runners feature a nearly contemporary straight-shot valve bowl approach and flow about 300 cfm stock-better than many modern performance big-block cylinder heads. Would you believe $10,000 per pair-if you can find a set!
Adding a large valve seat relief (arrows) ups flow. Longer 396/454 valves fit but need sho
Intake runners, from left: 348 and low-perf 409, high-perf 409, and Z-11. All port entries
Stock 348/409 stamped rockers look like Rat rockers, but modern big-block parts won't fit
Not the Same Old Grinds
Hydraulic and mechanical flat-tappet and mechanical roller cams are readily available. CH will have dedicated 348/409 hydraulic roller cams and lifters by the time you read this. Although they appear to fit, standard small-block roller-lifter guideplates are a tad short for the 348/409 and may bind in a running engine.
Because of the slanted deck, the exhaust valve opens toward the cylinder wall: Passenger-car blocks are limited to 0.650-inch lift max; the truck blocks with the large exhaust notch take up to 0.730-lift. An oversize 1.88-inch valve hits at only 0.300-lift! But you don't want to go back to an old "resto" grind that exactly duplicates early '60s specs, either. Even the 409/425hp and 430hp Z-11 mechanical cams were ground with a 113 1/2-degree lobe displacement angle (LDA). Stick that in a 3.55:1-geared, 4,000-pound sedan-ugh!
The timing chain sprockets interchange with small-block parts, but the chain is longer. CH
CH's 499hp 474ci engine uses an Isky Z-33 grind, essentially the 425hp mechanical cam (0.560-inch lift, 250-degrees duration at 0.050-inch tappet lift), but ground with a 110-degree LDA and installed on a 107-degree intake centerline. This wakes that baby right up. The next step up is a mild Isky solid roller that specs out at 0.620-inch lift, 248-degrees 0.050-inch tappet lift duration, 110-degree LDA, and 107-degrees intake centerline. Isky No. 3105 valvesprings work with the mechanical flat-tappet cam. Use Comp Cam's No. 916 small-block Chrysler spring and 10-degree titanium retainer with the roller cam, along with heavy-duty screw-in rocker studs.
Only the Z-11 heads use screw-in rocker-arm studs and guideplates stock. Every other 348/409 has pressed-in studs. Starting in November 1961, GM went to pinned, hardened studs on the high-perf heads. At 1.75:1, the stock rocker arm ratio is even higher than the late big-block's 1.7:1. The stock rocker stud's top 3/8-24 threads are like a small-block, but the rocker arms and balls are unique to the 348/409. CH highly recommends moving up to its full-roller "real 1.75:1 ratio" 409 rockers, as current replacement stamped steel rockers are of poor quality (the tooling is worn-out and the heat-treat is garbage).