# Why do some cars require premium fuel?

**URL:** <https://boards.straightdope.com/t/why-do-some-cars-require-premium-fuel/311864>\
**Category:** Factual Questions\
**Created:** [July 10, 2005, 6:58pm UTC](https://boards.straightdope.com/t/why-do-some-cars-require-premium-fuel/311864 "2005-07-10T18:58:59Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![dalej42](https://avatars.discourse-cdn.com/v4/letter/d/67e7ee/32.png) [@dalej42](https://boards.straightdope.com/u/dalej42)\
**Post date:** [July 10, 2005, 6:58pm UTC](https://boards.straightdope.com/t/why-do-some-cars-require-premium-fuel/311864/1 "2005-07-10T18:58:59Z")

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I know we have discussed that most cars don’t need the higher octane fuel that gas stations sell. However, why do some cars require this type of gasoline? I’m considering purchasing a 202 Volkswagen Passat which needs higher octane fuel.

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**Author:** ![danceswithcats](https://avatars.discourse-cdn.com/v4/letter/d/9e8a1a/32.png) [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)\
**Post date:** [July 10, 2005, 7:04pm UTC](https://boards.straightdope.com/t/why-do-some-cars-require-premium-fuel/311864/2 "2005-07-10T19:04:50Z")

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The short answer is higher-performance, higher-compression ratio engines need increased octane fuel.

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**Author:** ![Nunavut\_Boy](https://avatars.discourse-cdn.com/v4/letter/n/e95f7d/32.png) [@Nunavut\_Boy](https://boards.straightdope.com/u/Nunavut_Boy)\
**Post date:** [July 10, 2005, 7:31pm UTC](https://boards.straightdope.com/t/why-do-some-cars-require-premium-fuel/311864/3 "2005-07-10T19:31:15Z")

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Higher performance engines compress their fuel/air charge more than a ‘normal’ engine. This results in more power. The problem with this is detonation. Basically what happens if you compress a low-octane fuel/air charge too much (or at too high a temp) it can spontaneously combust before the piston is at top dead centre. Imagine the enormous stress that is caused to an engine when the power is being generated when the piston is still trying to compress the charge. High octane fuels burn slower and at higher temps helping to prevent autoignition thus preventing detonation.

At least, I’m pretty sure this is how it works!

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**Author:** ![Nunavut\_Boy](https://avatars.discourse-cdn.com/v4/letter/n/e95f7d/32.png) [@Nunavut\_Boy](https://boards.straightdope.com/u/Nunavut_Boy)\
**Post date:** [July 10, 2005, 7:32pm UTC](https://boards.straightdope.com/t/why-do-some-cars-require-premium-fuel/311864/4 "2005-07-10T19:32:17Z")

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Also, [this](http://www.faqs.org/faqs/autos/gasoline-faq/part4/) is pretty interesting.