# What is real gas and why is it more expensive?
**URL:** https://boards.straightdope.com/t/what-is-real-gas-and-why-is-it-more-expensive/683445
**Category:** Factual Questions
**Created:** [March 11, 2014, 3:41pm UTC](https://boards.straightdope.com/t/what-is-real-gas-and-why-is-it-more-expensive/683445 "2014-03-11T15:41:40Z")
**Posts on this page:** 4
**Page:** 2
### Author:  [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)
#### Post date: [March 11, 2014, 6:06pm UTC](https://boards.straightdope.com/t/what-is-real-gas-and-why-is-it-more-expensive/683445/21 "2014-03-11T18:06:18Z")
> [@epbrown01](#):
>
> The Pure Gas app shows stations in IL, just none within 50 miles of Chicago.
I believe that E10 is mandated in the Chicago metropolitan area (and pure gas isn’t legal for sale), in the name of reducing air pollution. IIRC, the refineries used to use MTBE for this purpose, but were forced to switched away from it about 10 years ago due to health concerns around MTBE. And, this being a state where an awful lot of corn is grown, mandating ethanol in the Chicago area likely went over very well downstate.
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### Author:  [@Arcite](https://boards.straightdope.com/u/Arcite)
#### Post date: [March 11, 2014, 8:48pm UTC](https://boards.straightdope.com/t/what-is-real-gas-and-why-is-it-more-expensive/683445/22 "2014-03-11T20:48:09Z")
> [@Johnny\_L.A](#):
>
> I have noticed that some ‘economy’ cars like the BMW MINI and, I think, the Smart, require higher octane fuel. I haven’t shopped for a car in quite a while. When I did last Autumn, it was for a direct replacement for the car I crashed, which was a 2005 model. I’ll accept that a higher proportion of cars now have smaller, higher-compression engines (turbocharged or not). But I’d bet that the majority are still normally-aspirated.
Perhaps I misunderstood you. I took your use of the word “nowadays” to mean that a higher percentage of cars used to require octane fuel than now do. I’d also bet that the majority of cars are still normally aspirated, but I think that a more substantial minority than in the past require high-octane fuel.
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### Author:  [@epbrown01](https://boards.straightdope.com/u/epbrown01)
#### Post date: [March 11, 2014, 9:49pm UTC](https://boards.straightdope.com/t/what-is-real-gas-and-why-is-it-more-expensive/683445/23 "2014-03-11T21:49:33Z")
> [@Johnny\_L.A](#):
>
> I have noticed that some ‘economy’ cars like the BMW MINI and, I think, the Smart, require higher octane fuel. I haven’t shopped for a car in quite a while. When I did last Autumn, it was for a direct replacement for the car I crashed, which was a 2005 model. I’ll accept that a higher proportion of cars now have smaller, higher-compression engines (turbocharged or not). But I’d bet that the majority are still normally-aspirated.
Octane requirements are related to an engine’s compression ratio and yep, “economy” cars are coming with high compression engines(10.0:1 and up) nowadays. The real surprise is the turbos hi-comp are as well, because early on turbos ran lower compression than NA; the first 911 turbos ran 6.5:1 (compared to 8.5:1 for the regular 911)!
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### Author:  [@Amateur\_Barbarian](https://boards.straightdope.com/u/Amateur_Barbarian)
#### Post date: [March 11, 2014, 10:12pm UTC](https://boards.straightdope.com/t/what-is-real-gas-and-why-is-it-more-expensive/683445/24 "2014-03-11T22:12:22Z")
> [@epbrown01](#):
>
> Octane requirements are related to an engine’s compression ratio and yep, “economy” cars are coming with high compression engines(10.0:1 and up) nowadays. The real surprise is the turbos hi-comp are as well, because early on turbos ran lower compression than NA; the first 911 turbos ran 6.5:1 (compared to 8.5:1 for the regular 911)!
Yes, turbos, especially high-performance ones, often have a relatively low compression ratio, because that gives more control using variable boost. The effective CR on some engines can be double the mechanical one or more. This is another case where sensitive knock sensors are used to run the engine at the maximum boost and timing the fuel will permit. Running the same engine on 87 octane and 95 octane can mean a 200HP difference.
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