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Re: how does octane rating affect performance (86T) Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Thu, 21 Jun 2001 14:13:54
In Reply to: how does octane rating affect performance (86T), matt, Thu, 21 Jun 2001 13:03:13 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Some basics. Sorry if I'm covering something you already know.
Octane rating is the measure of how resistant a fuel is to knocking (pre-detonation). It has nothing to do with the quality or amount of energy in the fuel. The higher the rating, the more resistant the fuel is to knock.
Just a note - octane rating is measured differently in the US than in Europe. The octane number for the same fuel in the US is lower than in Europe. So when you see 93 octane in the US, that translates into about 97 octane 'over there', and 87 US octane is about 91. Same gas, just a different way to measure it. So don't fall for the 'we get better gas over here.'
Higher boost is just like higher compression - an engine is more prone to knock at higher boost.
The APC (Adaptive Performance Control) in the Saab has a knock sensor, and if it detects knock, it dials back your boost. Full boost puts the turbo gauge needle right by the beginning of the red. The APC can dial you all the way back to 'basic boost' - about half-way into the yellow. Saab engineers figure that at base boost, even the lowest octane gas (87 in the US) will not cause knock in most engines. Knock is bad, because it reduces power and damages the engine.
So what? Basically, if you run a lower octane fuel in your car, and it knocks, the APC will dial back the boost, and the car won't go as fast. This is great if you end up with a bad tank of gas, or only 87 or 89 is available, or you feel like saving some $$ today. Running lower octane fuel WILL NOT harm your turbo engine as long as it isn't knocking. You'll just not get as much Whoosh!
OK, your next question is, What Octane Should I Run? Well, if you want to save money, run 87 octane and live with the reduced performance. But that doesn't mean you need to run 93 to get full performance, no matter what anybody else says.
The APC dials back boost when it detects knock. No knock, you'll get full boost. Every engine is different, especially as engines age. Some engines will be very prone to knock, others will be very knock resistant. So what you need to do is try a tank of gas and see. First, run a tank of premium (93 or 94), and see that you get full boost. Find a nice long uphill, and nail the gas in 3rd gear doing about 45 mph. You should see the needle go to right about the red. If you only get half way into the yellow, something needs fixing.
OK, now try a lower octane fuel. Maybe some 89. Run a tank or so. Do you still get full boost? If so, great. Absolutely NO NEED to run higher octane. If that works, try some 87.
Each engine is different. I have had a few Saab turbo engines. Some cars have needed 93 to get full performance. I have had a few that gave me full boost on 87 octane. Great - I save a bucket on gas. Running higher octane wont help. It's not like the car will go even faster if you had 95 or 110. All the octane allows you to do is get the full output of the engine. Now, if you modify the engine to increase boost, fiddle with the timing, that sort of thing, you may need higher octane fuel to take advantage of the changes.
Of course, there are differences. Cars are more prone to knock on a hot day than a cool one. There are seasonal and regional variations in gasoline. Different brands have some minor variation. But basically, find an octane that works for YOUR CAR and be happy.
, Thu, 21 Jun 2001 14:22:18, Thu, 21 Jun 2001 14:13:54 <-- Viewing This Message
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