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Interesting 91 Octane article in today's USA Today

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The people whos cars NEED premium and run regular don't need to drive the car anymore. There's a reason it's performing worse, it's called pinging. That'd be like someone who needs 87 octane using 83 octane and saying it's a good idea. You should always use what the manufacturer recommends.

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Most new cars have sensors that will adjust timing to the octane used, so using 87 won't ruin the motor but it will run like crap.

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I can always tell when the octane is lower than advertised. I have a '91 Legacy and SOA says use regular and I do without any problems. I have found however when I go to the "low priced" outlets like Tesoro, Costco, and other off-brands and get regular that I leave the pump and the pinging starts almost instantly. I don't buy anything more than what's needed to run okay; if it runs without pinging I'm happy to go with regular.

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Most new cars have sensors that will adjust timing to the octane used, so using 87 won't ruin the motor but it will run like crap.

If you're talking about a knock sensor(all newgen subarus have at least one) that isn't a good way to use a lower octane than required.

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The hardest thing for people to understand is that premium fuel acutally has LESS energy per gallon in it than regular or mid grade because it has additives in it that make it less likely to ignite.

 

Premium fuel does, however, allow a high compression / high performance engine to run a more aggressive fuel and timing program making more horsepower as an end result.

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I thought it was harder to ignite because it had fewer short hydrocarbons, which is the same reason it has more energy/gallon.

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Aye i do believe greg is right? thats how my science teacher said it. I hear all turbo charged engines shoudl run higher octane because it burns slower causing less wear on the turbo. Also if you run too high an octane for your car itll cause carbon deposits because its burning to slow. I dont know if this is all true or myth or what just my 2 cents.

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Aye i do believe greg is right? thats how my science teacher said it. I hear all turbo charged engines shoudl run higher octane because it burns slower causing less wear on the turbo. Also if you run too high an octane for your car itll cause carbon deposits because its burning to slow. I dont know if this is all true or myth or what just my 2 cents.

You want higher octane with a turbo or super charger because as you build boost you increase your running compression ratio. This running compression ratio can quickly exceed the rating of low octane fuel.

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It almost a waste of time writing about the pros and cons of using a different grade. In retrospect, when you buy a car of $30K or more what's a few $$$$ extra a year for the additional octane? Howerver, that is a personal preference more than anything else. This is one of those topics that is discussed from time to time.

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Higher compression + high timing = need for high octane.

 

I ran a supercharged 5.0 mustang on the street with 87 octane. All I would do is drop my timing down to around 8 or 10 degrees BTC (10 is stock for that car). When I needed the extra power for a track day or a race I would fuel it up with 91 or 92 and raise my timing up to about 16 degrees. I'd have to say the extra timing would add easily 40 to 50 hp espically in the low RPM range. But that's only 10 to 15% of the HP the car was making.

 

If your car will not run without "pinging" on 87 octane then your timing is too high. Every car is different. Every engine is different even when they roll out of the same factory. I've seen stock mustangs that would not allow over 10 degrees BTC, and I've seen ones that would go up to 15 or 16 without pinging.

 

The new subaru motors all seem to be higher compression (over 9.5:1) With compression like this, considerable power gains can be had by running higher octane and more agressive timing. Stock timing seems to be friendly to 87 octane since I have been using the cheapest crud I can find in my 2.5 RS since the day I got it. Never a ping from the engine, nor what felt like silent detonation in my car.

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