why is there the need for premium gas?


Asked by Jan 30, 2010 at 07:21 PM about the 2005 Subaru Outback 2.5 XT Limited Wagon

Question type: General

Looking at purchasing used 60k. want to know more details about gas requirements.

7 Answers

did your car come with a supercharger or turbo?

1 of 1 people found this helpful.

It is a turbo

2 of 2 people found this helpful.

thats why


OK. well thats not necessarily the answer that i was looking for. so i guess i will go and research why turbo engines ONLY run on premium gas. I will look up how it affects the performance and longevity of the engine. If I knew that car nerds on Cargurus like you were were assholes i could have started a thread somewhere else. Is this not a forum to get answers from people that know about cars? thanks.

com down. im not to get technical but a turbo engine has a higher compression ratio then a regular engine. higher compression more hp less dettination on 93 octane more heat and if not maintained right it can suck the engine dry. by the way im not a car nerd. im a delivery driver with 2.5 college to be a mechanic but couldnt get in. and also waich your pees and quus. the editer is waiching.

1 of 1 people found this helpful.

Turbo's force more air into the engine which is then compressed... You vehicle compresses based on teh engine geometry so say you have an 9:1 compression engine. So atmospheric pressure is ~14.7psi so pre-detonation the charge gets to ~132.3psi... Now say you have a turbo throwing another 7pounds of boost then you have ~21.7*9=195.3psi (This is greatly simplify things, but you get the point) the turbo increases the pressure on the charge prior to detonation. Now almost every fuel has a flash point and gas is no different. If the pressure and/or temperature get too high it'll detonate without the spark. Which mean the piston is firing before it should. This is known as predetonation or knock. This can damage an engine very quickly. The different grades of fuel are more resistant to this to varying degrees. Often the fuel grade is known as 'the anti-knock index' The higher the number the less likely you are to get pre-detonation. Contrary to popular belief the higher grade fuels do not contain more energy and will not be the best gas for every car. If you drive a naturally aspirated low compression engine then a lower grade gas may be more appropriate for you. However if there is any doubt about what grade is recommend then go with the higher grade. And always use a grade equal to or greater than the manufacturer recommendation.

8 of 8 people found this helpful.

thank you adam.

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