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I have a 2003 Civic Si and the dealer said to use premium fuel only. The manual says to use regular. Am I wasting money by using the 93 octane, or is it actually better for the vtec engine?

 
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I have owned dozens of cars over the years, both economical and high performance. The manual says to use regular and that is fine, but I have always put a tank of high test through my engines from time to time and can notice a difference in idling and performance. Try filling up with a high octane and detergent gasoline after every four or five tanks.I would also suggest you use Synthetic oil in your engine. Set the date and mileage for 5,000 miles or five months. It is a little more expensive but better for your engine. Plus, you get another 2,000 miles. It does not break down like regular oil.

 
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Actually the dealer is right!The civic si engine in 2003 has a compression ratio of 9.8 to 1.Your gonna need at least 91 octane fuel.Your engine is not going to die from lower octane fuel,it will adapt to the fuel by way of retarding the timing to reduce pre-detonation.This will give it slightly less in the performance department.