The ball-stud hemi (known internally as A279 and affectionately as the BS Hemi)[1] was an automobile engine designed by Chrysler Corporation in the late 1960s.[2] It never entered production.[3]
Intended to deal with the troubles created by the low-production 426, of which only about 9,000 were built from 1966 to 1972),[4] as well as the different architectures of the higher-volume 383 cu in (6.3 L) and 400 cu in (6.6 L) B and 440 cu in (7.2 L) RB V8s, the ball-stud hemi was to be suitable for high-volume manufacture at low cost while generating high performance.[4] Chrysler hoped to replace three block and two head designs with the ball-stud design.[4] It was to be based on a low-deck block and available in 400 cu in (6.6 L) (4.34 by 3.38 in (110 by 86 mm))[5] and 440 cu in (7.2 L) 4.32 by 3.75 in (110 by 95 mm))[5] displacements[4] (and Chrysler considered a 444 cu in (7.3 L),[4] which could have used stock 4.34 in (110 mm) 400 cu in (6.6 L) pistons),[5] while the new valvetrain would cut both weight and cost,[4] as well as making it possible to fit it in a greater variety of models.[4]
The A279 initially used the B-block head bolt pattern, which "badly hampered" its exhaust ports,[1] forcing the use of a serpentine flow around some of the outer bolts[1] This was changed late in development,[1] but not before some tooling based on the original design had already been created.[1] This meant the head was not truly hemispherical, but had more intake port area than exhaust (3.575 in2 (23.06 cm2) versus (2.488 in2 (16.05 cm2)).[1]
Its intake valves were nearer the intake manifold, exhaust valves nearer the exhaust pipes.[1] The staggered "poly" arrangement improves airflow by "reducing valve shrouding and eliminating the sharp runner turns" of inline valve arrangements.[1] It also had equally spaced intake ports (similar to the 428SCJ), thereby achieving "more consistent mixture distribution" than the Wedges.[1] Chrysler used a 14 mm (0.55 in) spark plug, rather than the typical 18 mm (0.71 in), in an effort to fit it as centrally as possible in the combustion chamber.[1]
The intake and exhaust valves were the same size as the 426's,[6] the 2.25 in (57 mm) intake canted at 15° from the bore centerline, the 1.94 in (49 mm) exhaust at 6°.[6] By contrast, the B-block's were 2.08 in (53 mm) and 1.74 in (44 mm).[6]
Compared to the wedge, the ball-stud block had two additional clean-out holes to remove casting sand, because the large bore made coolant in the water jacket very significant,[6] and an additional oil drainback hole at either end of the block, due to demand for lubrication of the rocker arms.[1] Suggestions to increase the size of the oil pump pickup to 3⁄8 in (9.5 mm) (compared to the 426's 1⁄2 in (12.7 mm)) never passed the endurance testing stage.[6]
Pistons were to be cast aluminum.[6] Compression ratio was targeted as 10.5:1; on the surviving engine, it was measured at 9.8:1.[1] Forged connecting rods used the Wedge's 3⁄8 in (9.5 mm) hardware, as opposed to the Hemi's 7⁄16 in (11.1 mm), but would nonetheless have been considered high-performance parts.[1]
The intake was to be a single Carter ThermoQuad, of greater flow than any previously used, on a dual-plane intake manifold,[5] while dual four-barrels were never even considered, in the face of toughening emissions standards.[1] A split-level (vertically split, rather than horizontally divided) intake was also experimented with.[5]
Chrysler hoped to have the ball-stud in production sometime between 1971 and 1973.[4] In testing, it proved able to outperform the single four-barrel carburetted A134 440, and lagged behind the eight-barrel A102 426 Street Hemi.[7] About one year of development was put in before the project was stopped, in late 1969.[4] It was a victim of increasing demand for emissions controls and a reduction in emphasis on performance, as the "horsepower wars" wound down.[4] Moreover, Chrysler was suffering "severe financial stresses" that nearly brought the company down,[8] so the new production tooling and facilities were now uneconomic.[4]
Estimates of the number built vary from three to twelve.[4] Only one is known to survive.[4]