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A fair question Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 9 Apr 2007 08:20:00
In Reply to: Fuel octane, Annie, Sat, 7 Apr 2007 10:51:09 Members do not see ads below this line. - Help Keep This Site Online - Signup |
A few things to know-
Fuel octane is a measure of how resistant the fuel is to knock in an engine. It is NOT a measure of quality or power. Higher octane fuel contains no more power - in fact, due to the chemistry, higher octane fuel tends to contain a little less energy. But frankly, not enough to matter. So octane is all about knock, or lack of it.
Knock is bad for the engine, especially a turbo engine.
The Automatic Performance Control (APC) in the adjusts the boost and timing to prevent the engine from knocking. If you put in low octane fuel and the engine wants to knock, the APC cuts back some on the boost. The car will have less power, but won't knock. If you put in fuel that has a high enough octane that the engine doesn't knock, you'll get the full boost and full power from the engine.
If you put in a higher octane fuel than the engine needs, you won't get any extra power, economy, or anything else. The engine control doesn't keep adjusting up the boost until the engine knocks - it goes to it's highest setting and stays there. So higher than necessary octane only wastes money.
OK, so what octane fuel should you use? Anybody that says "92" or "87" or any other number is wrong. It depends entirely on you and your car. Period.
First, what does the engine need? The octane rating on the gas cap covers 95% of new engines in most all driving conditions, for full power. You have a 15 year old car. As engines age, their octane need changes. Some engines get a little looser, lose some compression, so they can run at a lower octane and still get full bost. Other engines get tigher, maybe some carbon buildup, a cylinder hot spot or two, and need higher octane to have full boost.
And as engines age, some get faster and some lose power. That's true for all of us, not just cars.
Second, what does the driver need? Do you miss having full boost? Or are you content to drive the car with less than full boost? After all, Saab sold the Low Pressure Turbo which is essentially the turbo engine with the boost cut all the way back to base. Yes, you have an aero. But you aren't 'wasting' the car if you don't drive it to the limit all the time.
So -
It would be good to know what octane fuel your car really needs. Run a full tank of say 91, and see if you notice a loss of power. If not, try a tank of 89. Keep going down until you either notice a loss of power, or hit 87. Note that turbo cars lose power at warm temperatures anyway, and are fastest in cool (like 45 F) weather. Then decide the cost performance trade-off that works for you.
As an example, I had a couple of 88 9000Ts that ran very happily on 87 octane. I got full boost. I had a 86 900T, pretty much the same engine, that needed at least 91 to run at full boost. I have a '97 9000T right now that I run at 87, and get full boost. Yes, that's 3 out of 4 using 87, but I don't say that your car will get full power on 87.
As long as the engine isn't knocking, you aren't damaging it. If you run premium, you are guaranteed to be getting full boost, assuming the car is in good tune. However, you may be spending more money than necessary.
What about gas mileage? Octane rating has no direct effect on gas mileage. It is indirect, but confusing. At full boost, your engine uses more gas than at lower boost. So having more boost means you could use more gas, assuming you're lead-footing it. Of course, if you're lead-footing it, gas mileage isn't that important.
However, if a car is down on power, folks tend to compensate by going to higher RPM and harder acceleration to get where they are going. Not necessarily in a big way, but it's the small changes in driving habits that most impact gas mileage, because they are always there. I've found that cars with undersized engines tend to actually get lower mileage. But if you aren't a very agressive driver, and don't get full boost very often, you won't see a difference in mileage from low to high octane.
I hope this helps.
posted by 192.249....
, Mon, 9 Apr 2007 08:20:00 <-- Viewing This Message
, Sun, 8 Apr 2007 11:50:01, Sun, 8 Apr 2007 12:43:35
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