Engineering and development
Tasked with designing rolling chassis for two cars while using as much as possible from the existing 502 sedan, engineer Fritz Fiedler designed two versions of a new ladder frame, one with the same wheelbase as the 502, and one with a shortened wheelbase. The long-wheelbase version was used in the 503.[2] Both cars used the steering system and a variant of the front suspension system from the 502; the 503 also used the 502's rear suspension.[3] As originally designed, the 503 used the 502's remote gearbox placement and shift linkage.[2] Both cars used the braking system developed for the 502, using drum brakes with vacuum assist.[3] From 1957 all 503s were fitted with discs on the front as a result of recommendations from John Surtees who had bought a 507. All 503s were configured for left hand drive except for three Coupés and three Cabriolets handbuilt for the UK market.
The perimeter chassis (so designed to keep driver and passengers safe in a collision) was constructed of sheet steel, as was the bulkhead, inner wings and some body bracing. The bodywork, bonnet, doors and bootlid were made of aluminium/magnesium alloy, the original aluminium having found to be too soft on the prototypes. The dashboard assembly was another hefty cast aluminium structure, with the cast glove box lid weighing as much as an ordinary dashboard on lesser cars. The side vents on the dashboard were discontinued in February 1957, the resulting hole being covered by a blanking plate being welded in which occasionally caused corrosion problems around the weld later on. It was intended that Baur should manufacture the bodies, and indeed they produced a prototype, but in the end they were hand built in house. Despite some claims by owners and auction houses, no 503 bodies were either designed or built by Bertone.
The Mark 2 version from September 1957 resulted in few outward changes. The chrome trim strip along the side remained straight at the rear, rather than kicking up at an angle. The ashtray moved from beneath the dash to adjacent to the speaker grill above it.
Both cars used the 3.2 L version of the V8 engine developed for the 502, but with two carburettors and with an improved lubrication system using a chain-driven oil pump. The 503's V8 had a compression ratio of 7.5:1 and yielded 140 bhp at 4800 rpm.[3][9] Some UK 503s were delivered tuned to 507 specification, either by BMW or by AFN Ltd, the BMW Concessionaires for the UK; these engines developed 150 bhp.
The 503 had sixteen-inch wheels and standard final drive ratio of 3.90:1,[2] A final drive ratio of 3.42:1 was optional.[3] Acceleration of the 503 in standard tune from standstill to 100 km/h (62 mph) has been estimated at 11–13 seconds since there are no reliable contemporary figures available; the top speed of the 503 is about 120 mph.[5] Some cars were ordered with uprated engines with increased performance from standard.
Also for the Mark 2 from September 1957, the 503's drivetrain was revised. The gearbox was bolted to the engine and the gear lever was moved from the steering column to the floor.[4][5][12][13] All RHD cars had floor changes.
