Thursday, September 27, 2007

Fuel Grade and the SAAB 9-5, an Observation

The owners manual for my base 2.3L SAAB 9-5 recommends 87-93 AON (Average Octane Number). It says that 90 AON is recommended for optimal power (93 AON for aero).

But what is the “average” octane number?
Good question. The octane ratings in my area are (R+M)/2. That is the RON and MON added together and then divided by two (giving the average). As opposed to RON ratings I’m told are normally seen in Europe. Search Wikipedia for “octane rating” if you want more details about RON and MON. It’s enough to know that the numbers on the pump in the United States are “AON” even if they aren’t labeled as such.

So SAAB is saying that the non-aero 2.3L 9-5 can safely run on anything from regular to premium and that the mid-grade/premium will give you the best performance.

I’m an armchair physicist, so I have ran multiple tanks of all three grades available where I live (87,89,91)  and here is what I’ve found. My car prefers premium and runs fine on regular.

What isn’t so clear is the mid-grade. Mid-grade around my neck of the woods often has 10% ethanol and my 9-5 doesn’t seem to like it. I haven’t ran enough non-ethanol mid-grade to have a final opinion, but I do know that once I get down to about a quarter of a tank of 10% ethanol mid-grade my check engine light comes on.

Tightening the gas cap doesn’t turn the light off but filling the tank back to full does. There is never a code to pull when I plug into the OBD-II. It’s the phantom CEL. I don't understand it either. So I tend to avoid it or just make sure to fill up at half a tank.

Premium gives me the best MPG. The ethanol blend mid-grade, not surprisingly, brings down my MPG but gives me what feels like the same performance as premium. Regular fuel lowers the MPG a little and my butt-dyno can feel a performance drop.

So what do I do? I mainly run premium, sometimes mid-grade (making sure to refill at half a tank) but rarely do I fill up with regular.

I occasionally fill with 10% ethanol mid-grade in the winter to capture any water that might find its way into my fuel system. It’s cheaper than a bottle of gas-line antifreeze and think about how much more there is of it in the tank. 1 gallon of ethanol in every 10 gallons of fuel.

“I drive a ______, should I use premium?” AKA “But my SAAB runs fine on regular.”
The higher the octane rating, the harder it is for the fuel to ignite. High octane isn’t helpful in most engines, but high performance and supercharged or turbo engines (like the SAAB 9-5) have to do something to keep the gas from going off too soon. That is where mid-grade and premium fuel come in. Higher octane is needed to end knocking or pinging in some engines. But remember that it only maintains performance. It does not give you gains in performance.

What if you fill up with regular in a modern high performance car?
The car will use the knock sensor to detect premature firing and, if found, will “retard” the timing to stop the knocking at the cost of performance.

What about octane boosters?
Have you ever really looked at the wording on the bottle? They are rarely needed. I guess if it ends the knock in your engine, great, but you aren’t getting much for your five dollars when you look closely at the label. They raise octane by “points”, not by whole numbers. A point is one tenth. So “6 points” is really “0.6”. A tank of 87 octane with a bottle of the average octane booster is now a tank of 87.6 octane. Not that impressive. But like I said,  if bottled octane booster is the only cure for your car’s pinging, go for it.

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