Why modern gasoline ruins old engines

Many owners of old cars are confident that modern fuel should only benefit the car: higher quality, cleaner composition, better environmental performance. However, in practice, owners of classic VAZ-2101s, old "Volgas," as well as Japanese and German cars from the 1980s–1990s, increasingly encounter the same symptoms. Floating RPMs appear, a smell of gasoline under the hood, leaks, unstable carburetor operation, and accelerated aging of gaskets.

Usually, such problems are attributed to the age of the car. But the reason often lies not only in wear. Over the past decades, gasoline itself has changed so much that old engines and fuel system components simply were not designed for its modern chemical composition.

Modern gasoline was created for different engines

Today, fuel is developed to meet the requirements of environmental standards, catalytic converters, electronic control systems, and modern injection engines. The following have changed:

Old carbureted and early injection engines were designed for a completely different fuel environment. That gasoline was less aggressive, more chemically stable, and interacted differently with fuel system materials.

In addition, modern fuel contains virtually no lead and has a minimal amount of sulfur. The octane number is now increased with ethers and alcohols, particularly MTBE — methyl tertiary-butyl ether.

Why old engines tolerate new fuel worse

One of the main features of modern gasoline is its so-called "dryness." In old engines, this leads to accelerated wear of valve seats.

Gradually, recession begins:

This is especially noticeable in engines designed for old standard fuels.

Oxygen-containing additives destroy old rubber

To ensure more complete fuel combustion, special oxygen-containing components are added to gasoline. They improve environmental performance but at the same time make the fuel chemically more active.

As a result, modern gasoline begins to act significantly more aggressively on:

Microcracks appear on the parts, the rubber dries out and loses elasticity. Over time, leaks and the characteristic smell of gasoline under the hood appear.

Increased volatility creates problems in hot weather

Another important difference of modern fuel is high volatility. For new injection systems, this is an advantage: the mixture forms more efficiently. But for carbureted engines, the situation is different.

In hot weather, gasoline begins to evaporate too actively from the carburetor's float chamber. This causes:

Such problems are especially common in summer.

Detergent additives can harm an old engine

Modern gasolines contain powerful detergent components. In new engines, this really helps maintain system cleanliness. But in old engines, the situation is the opposite.

Over decades of operation, a stable layer of deposits forms inside the engine. When modern additives begin to actively dissolve this plaque, particles of contaminants are carried throughout the system.

As a result:

Some of the dissolved deposits settle where problems were not observed before.

The same octane number does not mean the same fuel

Many believe that modern gasoline fully corresponds to old gasoline if the octane number matches. However, the combustion characteristics of the mixture today are completely different.

Early engines were designed for different fuel components. Their combustion chambers and compression ratios were designed for old standard gasoline — for example, A-76 or early AI-92.

Modern high-octane gasoline:

If modern AI-95 is poured into an engine designed for old AI-93, without adjusting the ignition timing, part of the mixture begins to burn out already at the exhaust.

This leads to several consequences at once:

The reason is that modern additives are designed for the operating modes of new engines with higher temperatures and pressures.

Old materials cannot withstand modern chemistry

One of the most common problems with retro cars remains the destruction of rubber products. Most Soviet and old imported cars used parts made of NBR nitrile rubber, which worked perfectly with previous generations of gasoline.

But modern alcohol and ether additives act much more aggressively on such rubber.

Over time, several processes occur at once:

This is especially pronounced after a long period of car inactivity, for example, in winter in a garage.

As a result, owners face:

Why owners of new cars hardly notice this

Modern cars are already created taking into account the current fuel composition. Manufacturers pre-design for:

For modern engines, such fuel is the norm. But old engines continue to operate according to calculations made decades ago.

The main problem is that gasoline has changed

When owners of classic cars encounter unstable engine operation, the smell of gasoline, leaks, and rapid aging of rubber, they usually blame the age of the car or the quality of the fuel.

But the problem is more often not that the gasoline has gotten worse. It has simply become completely different. And many engines of the past generation were not originally designed for modern environmental standards, additives, and the chemical activity of fuel.

That is why old engines increasingly react to new gasoline as if it is too "harsh" for them.

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