Designed by Aurelio Lampredi, the Fiat SOHC engine first appeared in the front-wheel drive (FWD) Fiat 128 of 1969.^[1] The in-line four-cylinder engine comprised an iron block with an aluminium cylinder-head containing a single overhead camshaft operating directly on both the inlet and exhaust valves in a reverse-flow cylinder-head configuration. The camshaft was driven by a belt rather than chain. The engine remained in production until about 2010 and grew in capacity over the years from 1100 cc (in the Fiat 128) to an eventual 1900 cc (in the Fiat Linea). The Fiat 130 2.9 L (2,866 cc) V6 engine, also appearing in 1969, although having crossflow cylinder head, is directly related to the 128 SOHC engine, but with a 1.20 upscale in bore and stroke. It was gradually replaced by the Pratola Serra engine series starting from 1995 (and in the 1100 cc guise, by the FIRE unit of comparable displacement), although it was also converted to use a multivalve DOHC head, giving birth to the Torque engine, used until 2005.

Design and production

Cut apart 1,581 cc version of the 128 engine

The engine was designed as a transversely-mounted FWD-only power-plant being the second engine/gearbox combination to exhibit the now standard transverse engine-next-to-gearbox layout with unequal length driveshafts (the first was the Autobianchi Primula, the Mini had its gearbox in the sump of the engine). The exception to the engines FWD-only design is its revolutionary use as a mid-engine drive-train in the Fiat X1/9. Also Tofaş has adapted 128 engines into its Fiat 131 based longitudinally-mounted, front-engine, rear-wheel-drive layout platform. Unusual for the time was the electrically driven fan, rather than it being powered directly by the engine.^[1] The 128 engine has five crankshaft bearings.

A prominent feature of the Fiat SOHC engine is its massively over-square proportions. This over-square design allows large (within the constrains of a reverse-flow configuration) valve sizes relative to engine capacity. A secondary benefit is the short crank throw reduces the accelerative forces on the con-rod allowing the engine to operate at significantly increased engine-speeds. The original engine's limit was only reached at 8000 rpm.^[1]

The 1.3 L (1,301 cc) version was turbocharged and used in the Fiat Uno Turbo. The 1.4 L (1,372 cc) unit was also turbocharged and first used in the MkII Uno Turbo, then in the Punto GT.

As with its bigger brother the famous Fiat DOHC (aka Fiat/Lancia Twincam) engine, the Fiat SOHC engine was shared with the Fiat subsidiary Lancia (appearing in 1,500 cc guise in the Lancia Delta/Prisma of the 1980s) as well as being used in the Fiat 128 derived Yugo.

The final incarnation of the 128 SOHC engine was modified to use 16 valves and double overhead camshafts. The engine (using the designation "Torque") initially featured the bore-spacing, bore and stroke of the 1.6 L (1,581 cc) 159/160 series engine with a new 16 valve DOHC crossflow cylinder-head. To incorporate the extra oil-return-paths and symmetrical head (as opposed to the asymmetrical SOHC head) the cylinder-block was slightly revised making the DOHC head non-interchangeable with the previous incarnations of the 128 engine. It powered the Brava/Bravo, Marea and Multipla Fiat automobiles as well as the second-generation Lancia Delta. It was then replaced with a version based on the 1372 block with a longer stroke, displacing 1.6 L (1,596 cc), ending production in 2005. The same stroke of the later version was also used in Brazil with the earlier Torque block, giving birth to an 1.8 L (1,839 cc) version, used in the Linea.

Engine specification

The Fiat SOHC engine was produced in a number of configurations differing in stroke and bore but maintaining a standard bore-spacing. The first generation of 128 derived engines used a bore and a stroke of 80 mm × 55.5 mm (3.15 in × 2.19 in) to produce a displacement of 1.1 L (1,116 cc). The bore was increased to 86 mm (3.4 in) (maintaining the 55.5 mm (2.19 in) stroke) to give a displacement of 1.3 L (1,290 cc) for the 128 Coupe 1300.^[2] This engine was later used in the X1/9. The original 1.3 L (1,290 cc) 128/X1/9 engine is the only one of the family to have used the 86 mm (3.4 in) bore. The second generation used bores of either 80 or 86.4 mm (3.15 or 3.40 in) with strokes of 55.5 or 63.9 mm (2.19 or 2.52 in) giving four possible engine capacities.^[3] The final generation of SOHC engines standardised on a longer 67.4 mm (2.65 in) stroke with either an 80.5 or 86.4 mm (3.17 or 3.40 in) bore (giving 1.4 and 1.6 L (1,372 and 1,581 cc) respectively). The 67.4 mm (2.65 in) stroke with the 86.4 mm (3.40 in) bore was subsequently used by the early Torque engines, while the later Torque units used an 80.5 or 86.4 mm (3.17 or 3.40 in) bore with a 78.4 mm (3.09 in) stroke.

Applications

List of vehicles using variations of the Fiat SOHC engine.

Induction

Camshaft profile

The camshaft was a main methods of varying the 128 series engines characteristics for different applications/markets. The high-revving oversquare design of the 128 engine lends itself to high-overlap high-duration cam profiles better than more traditional square/under-square engine designs without losing drivability.^[6] Though a beneficial characteristic for sports orientated engine the high-overlap high-duration cam design gives poor emissions output which was unacceptable under the tightened US emissions regulations. To remedy this (in addition to the introduction of MPI) camshafts for emission-restricted markets featured asthmatic low-lift, low-duration, low-overlap profiles totally unsuited to the nature of the 128 engine.

Footnotes