What is low compression engine? To users living in developed societies by the 1990s, the term is likely unfamiliar. A low compression engine is not a fault, but a deliberate design choice. As a truly global product, Mercedes-Benz vehicles were engineered to traverse every corner of the world — including those less illuminated regions — and their very core, the engine, had to adapt accordingly.
Until the 1990s, high-octane fuel had not yet become widely available across the globe. In many underdeveloped regions, low-octane gasoline was often the only option. However, low-octane fuel lacks the resistance to compression and heat, making it prone to premature ignition under high pressure and temperature within the engine cylinder — a phenomenon known as knocking. This can lead to excessive mechanical stress, degraded lubrication, and even serious engine damage.
Low-octane fuel works in low-compression engines because its lower resistance to auto-ignition (knock) is appropriate for the engine’s design. In low-compression engines, the fuel-air mixture is not compressed as much, generating less heat and pressure, which prevents premature detonation. Low-octane fuels are more volatile and ignite more easily, making them a suitable match for these lower compression ratios, whereas high-octane fuel’s resistance to combustion would be unneeded and could lead to inefficient burning.
For this reason, like many earlier models, the W140 also featured low compression engines during part of its production life. These engines were specifically designed for markets where low-octane and leaded gasoline were still in use.
Under Cover
Equipping a vehicle with a different compression ratio engine is, by all accounts, a significant technical decision. Yet curiously, owners were often never made aware that their cars featured low compression engines. This lack of transparency is one key reason why many people find it difficult to discern differences between various versions of the same model. After all, Mercedes-Benz never actively informed customers that their vehicles were equipped with such engines. To uncover this detail, one needed a certain level of knowledge — either of Mercedes-Benz specifically or automotive engineering in general.
1) Vehicles with low compression ratio engines did not come with exclusive owner’s manuals
The owner’s manuals for these special vehicles were identical to those of the standard models, and all listed performance data corresponded to the regular engine variants — even when the customer base grew significantly. For example, in my home market of China, due to the shortage of premium fuel, all Chinese-spec cars were equipped with low-compression engines from the time the W140 was introduced. Mercedes-Benz began producing dedicated Chinese-language manuals as early as 1993. By 1995 had already seen sales of over 8,000 vehicles, yet despite China being one of the largest markets for low compression engines — with the vast majority of W140s sold there before model year 1996 equipped with such engines. These localized manuals, even with translated content and redesigned covers, still retained the original engine specifications.
Haven’t observed any other market with owner’s manuals that emphasize engine variants, nor is there a larger market for low-compression engines than China. After all, China’s W140 sales volume is fourth. In fact, these owner’s manuals are quite generic, with some versions specifying fuel requirements for both CAT and non-CAT vehicles, allowing a single version to cover all vehicles globally. However, to know which fuel is suitable for a vehicle, owners must first know whether it has CAT, a fact many owners don’t even know because it’s not emphasized anywhere either.
2) Data cards for vehicles with low compression ratio engines were vague and uninformative
For vehicles produced up to October 1992, the data cards included SA Code 473, which denoted a low compression engine. However, no actual figures realted to that compression ratio were provided. Aside from this SA code, neither the data cards stored in Mercedes-Benz’s aftersales systems nor the reference tables in maintenance booklet (2 WARTUNGSHEFT) offered any clue that the vehicle was fitted with a low compression engine. As a result, only those with insider knowledge of SA codes — typically experienced MB technicians or specialists — could decipher this information.
After October 1992, even this small trace was removed: SA Code 473 was eliminated from the data cards, leaving no direct reference to the engine’s compression. From that point forward, the low compression engine became a hidden feature. Its presence could only be inferred indirectly — for example, through SA Code 828, Elimination of Exhaust Emission Control System (CAT). This is because vehicles equipped with low compression engines were also delivered without catalytic converters. In many less-developed regions, leaded gasoline was still in use, often in tandem with low-octane fuel. Leaded fuel severely damages catalytic converters; the lead particles produced during combustion adhere to the catalyst surface, causing what is commonly known as “lead poisoning.” This leads to a rapid decline in catalytic efficiency, and eventually results in blockage and complete failure of the unit.
In short, determining the engine type in vehicles from this period requires considerable experience. Before inspecting the pistons directly, there are only two subtle locations where the compression ratio can be read — and neither is obvious to the casual observer.
1) Label on the ECU module box
Opening the hood, find the ECU module box located in the triangular space between the firewall and the fender. If the vehicle is equipped with a low compression engine, the label on the cover will clearly state “OHNE KAT (NON CATALYST)” along with the symbol ε, denoting the engine’s compression ratio. In contrast, on vehicles with standard engines, the compression ratio (ε) is typically not indicated on this label. However, the compression ratio is not always indicated. For example, on some 120.980s, although the label emphasizes “OHNE KAT”, the compression ratio is not specifically stated.
2) Engine block number stamping
Every engine carries a unique identification number, which is stamped directly onto the engine block. The exact location of this stamp varies depending on the engine model. Unlike the “Table 2” in service booklet or the data card stored in MB’s after-sales systems, this engine number stamp includes one crucial additional detail: the compression ratio (ε) marked after the engine number. This applies to both standard and low compression engines.
However, both of these indicators are relatively obscure — and certainly not obvious to the average owner. For most people, especially with vehicles produced after October 1992, determining the compression ratio becomes nearly impossible without physically checking the ECU module label or locating the engine block stamping. In such cases, only prior technical experience and contextual knowledge offer any clue.
Mysterious Spec
The three petrol engines used in the W140 — the M104, M119, and M120 — were all developed with low compression variants. Compared to standard versions, low compression engines shared the same cylinder heads but employed different pistons. Thus, the change in compression ratio was not achieved through engine management or control systems, but rather through mechanical differences within the engine block itself. Each engine, depending on its cylinder count, had a different compression ratio. In operation, these low compression engines typically demonstrated reduced performance — most notably in slower acceleration and increased fuel consumption.
1) 104.990, 104.994 and 104.944
The low compression variants of the M104 had a compression ratio of 9.2, while the standard versions had a ratio of 10.0. This applies to both the LH-Jetronic-controlled 104.990 and the HFM-SFI, ME-Motronic controlled 104.994, including the 2.8-liter 104.944. MB never officially published the specifications of these particular engines, so any performance differences must be inferred. Even in what appears to be a comprehensive technical table, the low compression variants of the 104.990 and 104.944 are not listed.
A useful point of reference is the G 320 (463.231), introduced in 1994, which was fitted with the HFM-SFI 104.996, featuring the same 9.2 compression ratio as the low compression 104.994. The published specifications were:
• Rated power: 155 kW / 210 DIN hp at 5,500 rpm
• Nominal torque: 300 Nm at 4,500 rpm
This output is 15 kW and 15 Nm lower than that of the 10.0 compression 104.994. Also, in the available tables, the low compression HFM-SFI 104.992 for W124 is listed with 162 kW. Therefore, it is reasonable to estimate that the maximum output of the low compression 104.994 lies somewhere between 155 kW and 162 kW.
2) 119.970 and 119.980
The low compression versions of the M119 engine had compression ratios of 8.8 and 9.1, whereas standard variants featured 10.0 and 11.0. This primarily involved the LH-Jetronic-controlled 119.970 and the ME-Motronic 119.980. To date, no low compression variants have been identified for the 4.2-liter versions. MB has also not released complete official specifications for these low compression M119 engines; in the following available technical table, the output of low compression 119.970 are left blank.
However, it’s possible to infer the performance of the 119.970 from the 119.980 data in the table below. Officially, the 119.980 with compression ratio 9.1 produces 5 kW less than the standard compression ratio 11.0, at 230 kW. Since the 119.980, the compression ratio has been increased from 10.0 to 11.0, so the low compression version has also increased from 8.8 to 9.1. If they were aiming for the same performance, the 119.970 low compression model would likely also produce 230 kW. Compared to the M104, while the M119 has more power, the output loss due to the lower compression ratio is actually less.
Interestingly, these low compression variants are labeled in the tables as “NV” (Niedriges Verhältnis, or “low ratio”), but footnote 1) translates this somewhat as “Fuel-economy.” In reality, achieving comparable performance to the standard versions often requires significantly more throttle input and increased fuel consumption. Based on our long-term experience with such vehicles, fuel consumption is typically at least 15% higher than in standard models. However, when performance is not the primary concern, fuel economy can be seen from another perspective: in markets where only low-quality, low-octane fuel is available, the lower cost of fuel may, in some cases, offset the increase in consumption — offering a form of fuel economy.
3) 120.980
The M120’s low compression model has a compression ratio of 8.0, while the regular version has a compression ratio of 10.0. The compression ratio decreases more and more significantly as the number of cylinders increases. This applies to the LH-Jetronic’s 120.980, but not the ME-Motronic’s 120.982. MB publishes the output of the low-compression 120.980, but only in the table below.
Despite the significant reduction in compression ratio, as explained in Note 1 (Fuel-optimized), the output of the low compression model is only 20 kW less than the standard model before the power reduction, and 10 kW less than the standard model after the power reduction. With the ME-Motronic, this difference seems to have disappeared. This is also the only one of the three gasoline engines that does not reduce the compression ratio.
Potential Pitfalls
First, due to the difficulty in identifying it, the low compression can be easily overlooked. In the commonly used EPC/ISP parts catalogs, especially for cars manufactured after 10/92, even after entering the FIN number of a car with a low compression engine, there’s no indication of the low compression ratio. The corresponding numbers in the parts diagrams don’t filter the right number for low compression model eitehr, the engine numbers on the data card don’t include the compression ratio markings like the engine block does.
For conventional daily use, the biggest concern with a low compression engine is fuel. According to some research: low compression engine cannot effectively use high octane fuel; doing so is a waste of money, offers no benefit, and may lead to reduced power and increased pollution because high octane fuel burns slower and is resistant to pre-ignition, which is unnecessary for a low compression engine. Low octane fuel is the appropriate choice for low compression engines, ensuring optimal combustion and performance. However, today, suitable fuels for low-compression engines are no longer available in most cases. When using high-octane gasoline, low-compression cars do suffer from high fuel consumption and a harsh exhaust odor (with the smell of incomplete combustion).
A further danger lies in extensive repairs involving the engine block. For example, we acquired a 1995 S 500 donor. This car was first registered in Arizona, USA, and equipped with the US version of 119.970 with a compression ratio of 10.0. It was sold as a used car in Hong Kong in March 2006 and subsequently imported into China. When I discovered it in 2022, it was being sold by a repair shop owner. The seller claimed that the car had been sitting idle on the lot for years, and the final move involved an engine replacement, but the car didn’t run, and the owner abandoned it. I presume the repair shop didn’t get paid for the repairs, and that was that.
We’ve tested this. Some ECU versions (such as the crucial E-GAS) are universal, allowing them to be programmed in HHT or similar systems to accommodate different versions. This means that programs for different compression ratios can be switched within the same control unit. However, if the selected version is incompatible with the vehicle’s engine, the vehicle will quickly enter limp mode, effectively rendering it inoperable. Clearly, the repair shop made this mistake and was unable to provide an explanation to the owner, resulting in losses for both the owner and the repair shop. However, MB also has some responsibilities in this case, as identifying a low compression vechile is no easy task.
This survey was based on common sense and didn’t delve into the piston design origins of the low-compression versions of the three different engines. We reached several test engineers about this, but they didn’t provide a definitive answer. As for the applicability of this topic, we may never know exactly how many 140s were equipped with low compression engines, but 10.86% of the W/V/C 140s produced carried the SA code 828, meaning no CAT. Excluding diesel models would likely raise this percentage, so it’s worth keeping this topic in mind.