Designed by Aurelio Lampredi, the Fiat SOHC engine first appeared in the front-wheel drive (FWD) Fiat 128 of 1969. The in-line four-cylinder engine comprised an iron block with an aluminium cylinder-head containing a single over-head 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 the mid 1990s and grew in capacity over the years from 1100 cc (in the Fiat 128) to an eventual 1600 cc (in the Fiat Tipo/Tempra). Also appearing in 1969, the Fiat V6 130 engine 2866 cc, although no reverse-flow cylinder-head, is directly related to the 128 SOHC engine, with a 1.20 upscale in bore and stroke.

Design and production

The engine was designed as a transverse mounted FWD-only power-plant being the first engine/gearbox combination to exhibit the now standard transverse engine-next-to-gearbox layout with unequal length driveshafts (the Mini had its gearbox in the sump of the engine). The one exception to the engines FWD-only design is its revolutionary use as a mid-engine drive-train in the Fiat X1/9.

A prominent feature of the Fiat SOHC engine 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 engine was developed for forced-induction use in the Fiat Uno Turbo. It was then further refined for the Uno Turbo Mark II, and later for 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 utilise 16 valves and dual cam shafts. The engine (using the designation "Torque") featured the bore-spacing, bore and stroke of the 1,580 cc 159/160 series engine with a new 16 valve DOHC 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. Development of the Torque engine took place in the US rather than Italy, whilst Fiat was in partnership with GM. The "Torque" engine (178.B3 - 182.A4) powered the Brava/Bravo, Marea and Multipla Fiat automobiles as well as the second-generation Lancia Delta (never sold in the UK), finally ending production in 1999.

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 stroke of 55.5 mm and a bore of 80 mm to produce a displacement of 1116 cc. The bore was increased to 86 mm (maintaining the 55.5 mm stroke) to give a displacement of 1290 cc for the X1/9. The original 1290 cc 128/X1/9 engine is the only one of the family to have used the 86 mm bore. The second generation used bores of either 80 mm or 86.4 mm with strokes of 55.5 mm or 63.9 mm giving four possible engine capacities. The final generation of SOHC engines standardised on a longer 67.4 mm stroke with either an 80.5 mm or 86.4 mm bore (giving 1372 cc and 1581 cc respectively).

Applications

List of vehicles utilising variations of the Fiat SOHC engine.

 Fiat

• Fiat 128: 1969-1985
• Fiat Regata: 1985-1990
• Fiat Ritmo/Strada: 1978-1988
• Fiat Punto Mk I: 1993-1999
• Fiat Uno: 1983-1989
• Fiat Tipo: 1988-1995
• Fiat Tempra: 1990-1995
• Fiat X1/9: 1972-1989

 Lancia

• Lancia Delta: 1979-1994
• Lancia Prisma: 1980s
• Lancia Dedra: 1990-1998

 Zastava

• Yugo 55
• Yugo 60
• Yugo 65
• Zastava 101
• Zastava 128
• Zastava Florida
• Yugo sana 1.3 1.4

 Tofaş

• Tofaş Şahin/Doğan/Kartal

 Induction

 Production

The Fiat SOHC used the full gamut of induction techniques through its long production run. The original 128 and 138 series engines originally used a single down-draught carburettor. The carburettor was normally of progressive twin-choke design (two asymmetric sized chokes operated progressively) such as the Weber DMTR and DATR of the X1/9. Multi Point fuel Injection (MPI) was introduced late in the 138 series to help meet lowered emission requirements in the US. The original MPI system comprised a large cylindrical plenum running parallel to the engine with individual runners to each inlet port and a single throttle controlling air-flow into the plenum. This plenum was pressurised in the Mk I Uno Turbo. The plenum was slightly redesigned to a more angular "box" shape for the 3rd Generation engines and was again pressurised for the Uno and Punto GT Turbo models. For the 3rd Generation engines the down-draft carburettor was replaced with a Single Point fuel Injection system (SPI) in the base-level models. The SPI system mounts on a manifold similar to that of the carburetted models and looks, as well as acts, much like a single-choke carburettor with a single throttle plate but no venturi and a centrally mounted single injector.

Tuning & modification

Carburettor

Traditionally both down-draft (DCNF & IDF) and side-draft (DCOE) twin-choke carburettors have been used to extract more power from the 128 derived engines. IDFs are rarely used due to an incorrect orientation of the float-bowls (causing possible surge/starve issues under cornering), the correctly orientated DCNF being the preferred down-draft carburettor in this application (also having a much lower profile than the IDF). For ultimate power the DCOE carburettor is used even though the float-bowl direction matches that of the IDF (i.e. incorrect). The aforementioned carburettors are usually used in pairs (effectively giving one independently tuneable carburettor per engine-cylinder), though applications where a single DCNF/DCOE are used to feed a shared manifold do exist. DCNF's tend to be either 36 mm or 40 mm, the larger 44-DCNF is deemed too large for the engine. Both 40-DCOE and 45-DCOE are commonly used where space allows.

 MPI

Tuning of the early MPI systems has previously been difficult as the electronic control systems are analogue making them difficult to "re-map". Low-cost DIY Fuel Injection (FI) controllers (such as MegaSquirt) have made it easier to alter the characteristics of the early MPI systems and release similar potential as the DCNF/DCOE carburettors. It is possible to "chip" the later digital MPI systems though aftermarket FI controllers again give greater flexibility.

Throttle bodies

Individual FI Throttle Bodies (TBs) are available for mounting to DCNF and DCOE manifolds as well as custom-built FI systems which, with aftermarket controllers, give the most flexibility and control over fuelling out of all the options.

 Motorcycle carburettors

Motorcycle, or "bike" carburettors, being a modern and cost-effective alternative to DCOE/DCNFs, have been adapted to work with the 128-derived engines. They potentially offer better control over fuelling than DCNF/DCOE, but their use on the Fiat SOHC engine is still in its infancy.

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. 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.

Production

Tuning and modification