| Fiat SOHC engine | |
| Manufacturer: | Fiat Lancia Zastava Automobiles Tofaş |
| Production: | 1969 - 2010 |
| Configuration: | Inline-4 |
| Valvetrain: | SOHC 2 valves x cyl. DOHC 4 valves x cyl. |
| Successor: | Fiat Pratola Serra engine E.torQ engine FIRE Turbojet/Multiair engine |
| Displacement: | NaNcc |
| Power: | NaNhp |
| Block: | Iron |
| Head: | Aluminium |
| Bore: | NaNmm |
| Stroke: | NaNmm |
| Fueltype: | Gasoline |
| Fuelsystem: | Weber carburetor Multi-point fuel injection Single-point injection |
| Coolingsystem: | Water-cooled |
| Turbocharger: | In some versions |
| Aka: | Fiat Torque engine |
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 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 2866cc 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.
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. 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.
The 1301cc version was turbocharged and used in the Fiat Uno Turbo. The 1372cc 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 1581cc 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 1596cc, 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 1839cc version, used in the Linea.
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 NaNmm to produce a displacement of 1116cc. The bore was increased to 86mm (maintaining the 55.5mm stroke) to give a displacement of 1290cc for the 128 Coupe 1300.[1] This engine was later used in the X1/9. The original 1290cc 128/X1/9 engine is the only one of the family to have used the 86mm bore. The second generation used bores of either NaNmm with strokes of NaNmm giving four possible engine capacities.[2] The final generation of SOHC engines standardised on a longer 67.4mm stroke with either an NaNmm bore (giving NaNcc respectively). The 67.4mm stroke with the 86.4mm bore was subsequently used by the early Torque engines, while the later Torque units used an NaNmm bore with a 78.4mm stroke.
| 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 | |
| 78.4 | 1576 | 1596 | 1821 | 1839 | |
| Key | ||
|---|---|---|
| 1st generation | ||
| 1st & 2nd generation | ||
| 2nd generation | ||
| 3rd generation | ||
| 3rd generation & 1st generation Torque | ||
| 2nd generation Torque | ||
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List of vehicles using variations of the Fiat SOHC engine.
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 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.
Since the Fiat SOHC engine series was produced for a long time and has been in a lot of cars it was just matter of time before people realized you can tune them fairly easily
They are oversquared by design, SOHC heads flow very well and 1500/1600sohc engines come with forged rods
The Fiat Tipo/Tempra/Punto 1.6 8v (75-90hp) is commonly used as a base because it's easily accessible and is de-tuned from factory (especially the 75hp versions) leaving tuners with a lot of headroom for horsepower increase
Common modifications are: swapping to a better carburetor, using a higher overlap/lift camshaft, bigger intake/exhaust valves, lowering the head to increase compression and a tuned 4-2-1 exhaust manifold
Most of these modifications are very cheap to do since you can use the parts from Fiat/Zastava parts bin
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 NaNmm, the larger 44-DCNF is deemed too large for the engine. Both 40-DCOE and 45-DCOE are commonly used where space allows.[4]
For a normal street car DMTR carburetor is commonly used in sizes varying from 32/32, 32/34 to 34/34
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.
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, 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.
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.[5] 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.
| 12/52-52/12 | 244 | 24 | - | - | 1.1 L 128 - European Specification | |
| 10/54-54/10 | 244 | 20 | - | - | 1.1 L 128 - North-American Specification | |
| 24/68-68/24 | 272 | 48 | 9.85 | - | 128 Rally | |
| 24/68-68/24 | 272 | 48 | 9.85 | - | 1300 X1/9 - European Specification | |
| 10/54-54/10 | 244 | 20 | - | - | 1300 X1/9 - North-American Specification | |
| 12/52-52/12 | 244 | 24 | - | - | 1300 X1/9 - Australian Specification | |
| 24/68-68/24 | 272 | 48 | 9.9 | - | 1500 X1/9 - European Specification (carb) | |
| 10/54-54/10 | 244 | 20 | - | - | 1500 X1/9 - North-American Specification (FI) | |
| 10/54-54/10 | 244 | 20 | - | - | 1500 X1/9 - Australian Specification (carb) | |
| 9/39-31/1 | 228-212 | 10 | 8 | - | 1.3 L Uno | |
| 0/30-40/10 | 210-230 | 10 | 8 | - | 1.3 L Uno Turbo | |
| 14/36-44/6 | 230 | 20 | 8.8 | - | 1.4 L Uno Turbo | |
| 7/35-37/5 | 222 | 12 | 8.8 | 0.4/0.5 | 1.6 Tipo | |
| - | 244 | - | 9.2 | 0.4/0.5 | 1.1/1.3 "DMB" Zastava - European Specification | |
| - | 256 | - | 9.2 | - | 1.6 90hp i.e. Punto/Tempra/Dedra |
| 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 |