Understanding Gasoline for Pre-War Cars: Octane Ratings and Engine Requisites

When it comes to maintaining pre-war cars from the 1920s and 1930s, a key consideration is ensuring that the appropriate type of fuel is used to support the engine's design and performance. This is particularly important for understanding the role of gasoline octane ratings and engine compression ratios. In this article, we'll explore the nuances of how modern fuels compare to those of the pre-war era and what implications this has for classic car owners.

The Role of Olefins in Fuel Manufacturing

Olefins are a critical component in the manufacture of petrol, and their presence in modern fuels can significantly impact the performance of automotive engines. High levels of olefins in petrol allow for higher compression ratios before detonation (a condition known as engine knock), but they also lead to unburned deposits in the engine. Conversely, lower olefin content in modern fuels reduces the risk of these deposits, making them generally safer for use in modern engines.

For pre-war cars, the gasoline used was often thinly formulated with higher levels of olefins, which allowed for higher compression ratios while potentially leading to more frequent engine knock. This raised an interesting question: is there a specific type of gasoline required to meet the very low octane, thick, and high olefin content of the fuel used in these vehicles?

Modern Fuels and Pre-War Car Compatibility

Modern fuel systems have evolved to disfavor high olefin content, with low or zero ethanol fuels being recommended by many classic car enthusiasts and mechanics. In fact, one of the places I worked had a single petrol pump with 99 octane, ethanol-free fuel. Notably, this station saw a significant number of classic cars, including pre-war models, coming in to refuel. This raises the question of whether these vehicles truly needed the specific fuel formulations from their era or if they could adapt to modern fuels.

It's worth noting that the engines of early 20th-century cars were designed to work with a wide range of fuel qualities. For instance, some early cars could have very low compression ratios, as low as 4:1, with octane ratings as low as 60. However, more common designs like the flat-head engine, with its squish area in the combustion chamber, allowed for higher compression ratios close to 8:1. Harry Ricardo contributed significantly to the development of these designs.

The Impact of Modern Fuels on Pre-War Cars

Given the evolution of fuel technology, many modern gasoline blends are adequate for pre-war cars. In fact, regular pump gas is often more than sufficient for these classic vehicles, and it's beneficial in many ways. Regular pump gas typically has a higher octane rating (often around 87) and a more stable chemical composition, which can help prevent engine knocking and provide smoother performance.

The worst-case scenario if a pre-war car is run on modern fuel, particularly with higher octane, is that the engine might run a bit cooler. However, the engine's own ignition timing is designed to handle a range of octane values, ensuring that performance remains consistent. This means that modern fuels can be used with confidence, without the fear of significant damage to the vehicle's engine.

Common Octane Ratings and Engine Designs

As automobile technology advanced in the mid-20th century, octane ratings increased, and engine designs evolved to incorporate higher compression ratios. For example, pre-war engines generally used octane ratings in the 60 to 80 range, but by the post-war era, the standard had shifted to 87 octane, and some engines could achieve compression ratios up to 10:1. This improvement was often achieved through the addition of tetraethyl-lead to gasoline, a practice that was phased out due to environmental concerns.

It's important for classic car owners to understand the historical context of their vehicles' fuel requirements. While some may argue that sticking to an exact historical fuel formulation is ideal, the reality is that modern fuels can provide adequate performance and reliability for these classic machines. Regular fuel with a higher octane rating is often the best choice for pre-war cars, helping to ensure longevity and smooth operation of the engine.

Conclusion

For pre-war cars, the choice between maintaining historical accuracy with older fuel formulations or using modern, more readily available fuels is a matter of personal preference. While purists might prefer to use the exact fuel from the era, the practicality of higher octane, ethanol-free modern fuels makes them an excellent choice for the vast majority of classic car owners. The key is to ensure that the engine's ignition timing remains optimal, and to understand that the slightly cooler running provided by modern fuels is a minor trade-off for the peace of mind and reliability they offer.