Thomson machined the block and bored its cylinders slightly, taking it from 4.000 inches to 4.093 inches. They matched the bigger cylinders with a slightly longer-stroke (3.725 inches) crankshaft, increasing the displacement 21 cubic inches, to 392. The crank was machined to accept Chevy big-block connecting rods, enabling the use of heavy-duty forged rods (along with new, forged pistons) to deliver the bottom end strength Denyer was looking for. Besides that, it was virtually impossible to find replacement Oldsmobile bearings for the standard rods.
Some of the other special ingredients in this street engine recipe include a custom-grind, flat-tappet hydraulic camshaft that delivers slightly greater lift and duration than the original, but it's not a "big" cam by any stretch of the imagination, with 202 degrees of duration at 0.050-inch lift.
Only the mildest of clean-up work was performed on the heads, too. It basically amounted to a thorough valve job and fitting them with hardened valve seats for worry-free pump-gas performance. The compression ratio was lowered from the original 10.0:1 to about 9:1, so that the engine would be more compatible with today's unleaded gasoline. The 300hp gross rating of the production J-2 was achieved with 97-octane leaded premium gas that was common in the 1950s.
From an appearance standpoint, the new-age Olds wears a set of aftermarket finned valve covers that are machined to accept reproduction "Rocket" spark plug wire hold-down brackets. The result is a nicely detailed bit of flash to accent the otherwise factory look.
Dyno details
The triple Rochester carburetors were sent out for rebuilding and refinishing, and came back looking better than new. While they were out, Thomson cleaned up the intake manifold and carefully rebuilt the intricate linkages.
With a fresh coat of Oldsmobile-green paint, the restored J-2 looked factory-fresh on the outside, apart for a few carefully selected dress-up parts, and was thoroughly modernized inside. Work moved to the engine dynamometer for break-in and testing.
Expectations were kept in check, as the larger displacement would be tempered somewhat by the low-flow cylinder heads and lower-compression, and lower compression generally means less power, because of reduced cylinder pressure. When the dust settled and the revs returned to zero, the triple-carb Olds delivered a 303-horsepower performance, with 414 lb-ft of torque.
On the face of it, 303 horses don't sound remarkable these days, but for an easy-cruising street rod that's not intended for competition in the Friday-night bracket series, it's a fine result. And, given the heads, intake, and exhaust manifolds were stock, and the camshaft was comparatively mild. The fact that the engine topped the original output, but with less squeeze in the heads and lower-octane unleaded fuel, was encouraging.
More impressive, however, was the robust torque, as the peak level is only part of the story. By only 2,500 rpm, the engine was making 406 lb-ft and held above the 400 lb-ft level through 3,800 rpm, and even then, it held in the 390-lb-ft neighborhood through 4,000 rpm. And because this mill isn't what you'd call an "rpm" engine, breathing was all done by 4,700 rpm anyway. (Another 1,000 rpm would do wonders.)
So, while it may lack that horsepower of, say, a new LS7 crate engine, this updated Golden Rocket delivers plenty of usable pulling power. It is also built with stronger parts on the inside for greater durability and carries the classic style on the outside that simply can't be matched in a modern engine.
Sounds like a success to us.
A forged crankshaft is used...
A forged crankshaft is used in the engine, but it was re-ground to accept Chevy big-block connecting rods. A longer stroke, 3.725-inch vs. the stock 3.687-inch, complements the larger bores to give the engine a 392-cubic-inch displacement.
One of the modern updates...
One of the modern updates to the engine was the addition of an adapter (right) that enables the uses of a conventional, spin-off oil filter rather than the original-style canister system.
A set of 7.000-inch-long forged...
A set of 7.000-inch-long forged H-beam rods from K1 Technologies is used. They're Chevy big-block rods and were selected for their superior strength over the cast rods.