Question from the forum “Behind the wheel” is answered here from time to time as part of a community discussion.
Is it possible to switch to cheaper low octane petrol by fitting shorter spark plugs and thus lowering the compression ratio?
The answer is no. There are several reasons for this. The engine compression will shift by only a few tenths, but the flame front will spread far less reliably when the plugs are effectively suppressed. This means combustion becomes uneven, and the engine loses efficiency rather than gains it.
Additionally, the threads in the cylinder head used by the spark plugs will quickly become coated with soot, making it difficult to return to the recommended plugs without a costly repair. In the past, drivers sometimes tinkered with plug gaps and altered thread lengths in an effort to run on cheaper fuel. They also used shortened plug threads to lower compression in small amounts. Modern engines, however, have very precise tolerances and fuel requirements, and manufacturers supply clear guidelines that should be followed without deviation.
Ultimately the expected savings vanish and a series of problems may arise. It is wise to leave old-fashioned tricks in the past and rely on proven maintenance practices and factory specifications.
Questions or comments can be shared with the community in the forum “Behind the wheel” where members discuss practical automotive topics and share experiences. This conversation is part of a broader discussion on fuel economy and engine health, with insights and examples from real-world driving scenarios.
- Crossovers from other models can be considered for fuel compatibility, but only with explicit approval from the automaker and the engine manufacturer.
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