ksp@ulysses.UUCP (Krishna Prasad) (07/24/85)
Thanks to everyone who replied to my questions about which gas I should use for my new Dodge Daytona. The general opinion was that I should use the lowest octane gas on which my car runs without knocking. A couple of responses gave detailed explanations of what compression was and and what knocking meant. I also did some research on my own. My conclusions were as follows: In general, knocking is more likely to occur in high compression, high performance engines. I looked up a book giving engine specifications and found that the engines which you would normally think of as high performance as indeed high performance. Examples are Audi, BMW, Saab, and the VW Golf GLI. Almost all Ford and GM cars tend to be very low compression and have very low HP/engine volume ratios. Chrysler and Japanese cars generaly fall in between. I derived a thumb rule that cars with compression ratios of above 9.25 were definitely high performance and needed premium unleaded. Cars with ratios of under 9.0 would definitely be satisfied with regular 87 octane gas. In between use your judgement. This thumb rule is of course, a simplification. I remember from a undergraduate Internal Combustion engines class that such factors as the material used to make the cylinders, and the shape of the cavity can affect the proclivity to knock. My engine has a compression ratio of 9.0 and I haven't heard it knock yet. However, after all the careful and scientific analysis, and considering other factors not related to engine dynamics, such as the fact that this is my first car, I am not married and I don't currently have a girlfriend to lavish attention on, and I make reasonable money, I decided, "Oh, what the hell, I am going to use Premium Unleaded". So much for science. Thanks to all those who replied. Krishna.