Baseline: Stock Oval-Port Heads, Lunati Hydraulic Cam
Starting off in truly mild form, we baselined our initial configuration using stock GM large oval-port heads. GM Casting No. 353049 was used on some '73 cars and trucks and reputedly ranks among the best-flowing factory oval-port heads. We scored a set of the relatively rare castings at Cylinder Head Exchange, a Southern California core bank.
Also installed for the baseline was a mild (for so many cubic inches) 233/241 degrees duration (at 0.050) Lunati Voodoo-series hydraulic flat-tappet cam, an Edelbrock Performer intake, a Holley 750 carb, and Hooker headers with 2-inch primaries.
One thing to watch out for on Rat motors when running a hydraulic cam is premature valve float. With their canted valves, long pushrods, heavy valvetrain, and high 1.7:1 rocker-arm ratio, big-block Chevys skate on the fine edge of upper-end distress. Spring pressures satisfactory in a typical small-block may not cut the mustard on a Rat motor. Yet running overly stiff springs with a hydraulic cam can cause the lifters to bleed down overnight, leading to noisy lifters and possibly impacting lifter longevity.
Another problem is the declining quality of hydraulic flat tappets combined with most modern street motor oils' lack of the zinc-based extreme-pressure lubricants needed to keep flat tappets happy. Some old-school builders don't like running hydraulic or hydraulic roller cams in any serious Rat motor, opting to move up to a serious mechanical flat tappet or roller grind. Nevertheless, most guys are running hydraulics on milder motors nowadays, so that's what we started off with.
To avoid durability problems on the dyno and speed up the test process, the stiffer springs specified for the Lunati hydraulic roller cam we intended to run next were installed on the flat tappet. However, the engine was first run in with EOS additive on the outer springs only. This procedure worked OK on the dyno, but (as noted above) running stiffer springs than those officially specified for a given cam may not be the ticket for a street driver. Instead, consider going with a slightly softer but modern high-tech ovate valvespring (such as Lunati PN 71817).
Speed-O-Motive uses a DTS dyno for testing. Its design enabled us to easily pull from 2,000 to 6,000 rpm for all tests in this series, which were run with 35 degrees total advance in a locked-out MSD distributor on 91-octane pump gas. Speed-O-Motive also used the conservative SAE J1349 net dyno correction factor; while about 5 percent lower than we're used to, it's more representative of real-world in-car operating conditions.
Speed-O-Motive pressure-checked our GM 049 castings before treating them to a performance three-angle valve job. The castings' semi-open chambers are superior to early bathtub chambers, yet-unlike later peanut-port castings-they still have fullsize 1 3/4 x 2-inch intake runners.>>>>>>
Right from the get-go, we knew we had a winner on our hands: Generating 576.2 lb-ft and 480.5 hp, even with obsolete factory castings, this engine would make a great towing package in a heavy vehicle.
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