Fiat SOHC

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Designed by Aurelio Lampredi the Fiat SOHC engine first appeared in the front-wheel drive (FWD) Fiat 128 of 1969. The inline 4-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 being 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).

Contents 1 Design and production 2 Engine specification 3 Applications 3.1 Fiat 3.2 Lancia 3.3 Yugo 4 Induction 4.1 Production 4.2 Tuning & modification 4.2.1 Carburettor 4.2.2 MPI 4.2.3 Throttle bodies 4.2.4 Bike carburettors 5 Camshaft profile 5.1 Production 5.2 Tuning & modification 6 Images 7 Footnotes

[edit] 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 Mk I and further refined for the MK II and later 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 1500 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 a new 16v DOHC engine. The engine (using the designation "Torque") featured the bore-spacing, bore and stroke of the 1580 cc 159/160 series engine with a new 16v DOHC cylinder-head. To incorporate the extra oil-return-paths and symmetrical head (as opposed to the asymmetric 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 under the ownership of 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.

[edit] 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 55.5mm 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).

Valid engine bore/stroke combinations

Stroke (mm) Bore (mm)   80 80.5 86 86.4 55.5 1116 1130 1290 1302 Capacity (cc) 63.9 1285 1301 1485 1499 67.4 1355 1372 1566 1581

Key 1st Generation   1st & 2nd Generation   2nd Generation   3rd Generation

Engine specification (by engine-number) Engine Code Bore (mm) Stroke (mm) Capacity (cc) stated Capacity (cc) calculated Production (from - to) BHP Notes 128.A.000 80 55.5 1116 1116 1969 - 1982 55-65 128 128.A1.000 86 55.5 1290 1290  ? - ? 65-75 128 128.AS.000 86 55.5 1290 1290 1972 - 1978 75 X1/9 1300 138.A.000 80 55.5 1116 1116 1978 - 1985 60 - 138.A1.000 86.4 55.5 1301 1302 1978 - 1982 75-65 - 138.A2.000 86.4 63.9 1498 1499 1978 - 1982 75-85 X1/9 1500 138.A3.000 86.4 55.5 1301 1302 1982 75 - 138.A4.000 86.4 63.9 1498 1499 1980 - 1982 75-85 X1/9 1500 138.A5.000 86.4 55.5 1301 1302 1982 - 1985 70 Also 138.AS.000 138.B1.000 80 55.5 1116 1116 1982 - 1985 55-58 Ritmo Mk II 138.B2.000 86.4 55.5 1301 1302 1983 - 1986 68-70 - 138.B3.000 86.4 63.9 1498 1499 1982 - 1985 70-85 - 146.A2.000 80 63.9 1301 1285 1985 - 1989 105 Uno Turbo Mk I (specification A) 146.A2.000 86.4 55.5 1301 1302 1985 - 1989 105 Uno Turbo Mk I (specification B) 146.A8.000 80.5 67.4 1372 1372 1989 112 Uno Turbo Mk II 149.C1.000 86.4 63.9 1498 1499 1989 75 - 159.A2.000 80.5 67.4 1372 1372 1990 - 1993 76 Tipo/Tempra 1.4 (carb & i.e.) 159.A3.000 86.4 67.4 1581 1581 1990 - 1996 75-86 Tipo/Tempra 1.6 (carb & i.e.) 160.A1.000 80.5 67.4 1372 1372 1988 - 1993 72 Tipo/Tempra 1.4 i.e. 160.A2.000 86.4 67.4 1580 1581 1988 - 1993 83 Tipo/Tempra 1.6 (carb & i.e.) 176.A4.000 80.5 67.4 1372 1372 1993 133 Mk I Punto GT (1.4 Turbo) 176.A9.000 86.4 67.4 1581 1581 1994 - 1995 90 Mk I Punto Sporting & Cabrio 176.B6.000 80.5 67.4 1372 1372 1993 133 Mk I Punto GT (1.4 Turbo)

Key 1st Generation   2nd Generation   3rd Generation

[edit] Applications

List of vehicles utilising variations of the Fiat SOHC engine.

[edit] Fiat

• Fiat 128: 1969-1985
• Fiat 127: 1971-1987
• Fiat Regata: 1985-1990
• Fiat Ritmo/Strada: 1978-1988
• Fiat Punto Mk I: 1993-1999
• Fiat Uno: 1983-1995
• Fiat Tipo: 1988-1995
• Fiat Tempra: 1990-1995
• Fiat X1/9: 1972-1989

[edit] Lancia

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

[edit] Yugo

• Yugo 45
• Yugo 55
• Yugo 60
• Yugo 65

[edit] Induction

[edit] 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 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 carburettord models and looks, as well as acting, much like a single-choke carburettor with a single throttle plate but no venturie and a centrally mounted single injector.

[edit] Tuning & modification

[edit] 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^[1].

[edit] 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 flexability.

[edit] 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.

[edit] Bike carburettors

Bike carb's (being a modern and cost-effective alternative to DCOE/DCNFs) have been adapted to work with the 128 derived engines. Bike carb's potentially offer better control over fuelling than DCNF/DCOE but their use on the Fiat SOHC engine is still in its infancy.

[edit] 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 loosing drivability.^[2] 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.

[edit] Production

Standard production camshaft specifications

Profile	Duration (degrees)	Overlap (degrees)	Lift (mm)	Clearance (mm) Inlet/Exhaust	Application
12/52-52/12	244	24	-	-	1.1 L 128
10/54-54/10	244	20	-	-	US Spec 1500 X1/9
24/68-68/24	272	48	9.9	-	128 Rally / 1300 & 1500 (carb) X1/9

[edit] Tuning & modification

Fast-road/race camshaft specifications

Manufacturer	Profile name	Profile	Duration (degrees)	Overlap (degrees)	Lift (mm)	Clearance (mm) Inlet/Exhaust	Application
Kent	FT7	35/67-67/35	282	70	10.33	0.20 / 0.20	NA - fast road
Kent	FT8	52/76-76/52	308	104	10.82	0.20 / 0.20	NA - race
Piper	BP270	33/67-67/33	280	66	9.80	0.20 / 0.20	NA - fast road
Piper	BP285	36/72-72/36	288	72	10.40	0.20 / 0.20	NA - fast road
Piper	BP300	39/71-71/39	290	78	11.18	0.25 / 0.25	NA - race
Piper	BP320	53/83-83/53	316	106	11.18	0.25 / 0.25	NA - race

[edit] Images

[edit] Footnotes

1. ^ To fit DCOE carburettors in an X1/9 a small amount of bodywork must be removed/reshaped
2. ^ The standard European carburettord X1/9 used a camshaft more radical than many after-market "performance" camshafts offered for contemporary engines from other (non Italian) European manufacturers.