Saab 9-3: What octane to tank?

There is a line of thought that says the lower octane fuel is more volatile and would cause more violent explosions. Higher octane fuel is more resistant to ignition, causing less violent explosions, but more importantly, fewer uncontrolled explosions that would result in pinking/pinging. All that useless trivia aside, just about any Saab I can think of that has been made in the past 10 years has a knock sensor that adjusts for octane variations. The only difference you should notice with lower octane is reduced performance (because of retarded ignition timing to accommodate the lower octane) and reduced fuel economy (for the same reason). The octane recommendation in the manual provides the spirited performance Saab owners expect and, because the recommendation comes from the factory, is the fuel used to calculate fuel economy during EPA tests in the US.

The only other consideration is that high octane fuels tend to have a better additive package to help ensure that the fuel system and fuel injectors (in particular) remain clean. Not all brands of fuel hold to the same standard, but all must meet a basic minimum in all grades of gasoline/petrol that they sell (in the US. I can't speak to other regions). There are many factors that weigh in on prolonging or reducing lifetime. Given that a knock sensor controls the spark, in my opinion the grade of fuel used comes in rather low in importance on the list.

Walt Kienzle

1991 9000T

"Happy" wrote in news:4223a3ae$ snipped-for-privacy@news.microsoft.com:

Is it a turbo? In my 1997 900SE, I get the most fun with premium (92) also the best mileage at highway speeds. If I'm just going to be driving around town, I'll use midgrade (89). Not as much ooomph when the turbo kicks in but acceptable. Mileage drops though. The first time I ran 87, I thought the turbo had died. It came to life again when I put 92 in it.

Well..... I discovered that UK and US octane are different.

Long story.

Anyway - any Saab will *like* higher octane. The engine management computer is designed to recognise it. Try it and see.

If I want optimum performance I fill the tank with Shell Optimax.

Graham

That's not unsurprising.

Any decent modern car like a Saab adjusts its ECU to the fuel.

Unlike a carburettor car it doesn't even need retuning.

Depending on your driving pattern you may wish to choose for optimum mileage or optimumm performance.

I can only suggest you try it and draw your own conclusions.

Graham

Here is some data that could be useful. WE bought a 2000 9-3 Viggen and brought it from Ohio to Oregon. The previous owner apparently used regular gas. We started filling up with premium. Data:

1st fill up - approx 27mpg 2nd fill up - approx 29mpg 3d fill up - approx 31mpg 4th fill up - approx 33mpg and stayed there (if we stayed under 80mph, of course).

The ECU adjusts to the better gas, but it takes a couple of fill ups.

-Dima

Just a thought: Is it not true that in order for the knock sensor to work it must sense a knock? If so, it follows that running lower octane in a turbo must cause a little more wear and tear than high octane fuel. I suppose it depends on how often the knock sensor lets the ignition and boost advance until another knock is detected.

Another consideration may be carbon deposits. A friend of mine has a Cadillac with signs "Use only premium fuel" all over it. He claims that using premium is a waste and has only used 87 for the last 10,000 miles. He now complains that the car keeps running on roughly after turning off the ignition. Granted, the Cadillac is not a turbo and has a higher compression ratio then turbo engines at atmospheric pressure. Maybe this is not a problem with turbo engines. Does anyone have any data?

Makes sense, but no. No matter where the knock point is, it'll find it and set ignition and boost just short of there.

If he runs mostly city miles, no good long runs to burn the crap out of the engine, it'll build up. I don't know of that being related to octane, more of how the car is driven. People generally will drive a Cadillac differently than a Saab owner will drive a Saab.

Dave Hinz

To elabarate, my understanding is that the knock sensors in most EFI set-ups are able to sense the very early stages of detonation and thereby prevent the damaging effects of pre-ignition.

The SAAB system of sensing using the DI unit is not clear to me, but may offer an even better ability than the acoustic methods normally used.

-Fred W

Yes.

...explains it in more detial than you may care to read. Impressiveengineering, to say the least. Dave Hinz

A few years ago, I started experiencing some rough idle and stalling from my

1992 (classic) 900. I was running 89 octane in it at the time (which is mid-grade around here, and is suggested I think in the owners manual), and someone suggested I try premium (93). I did, and the problem cured itself by the end of the tank. I've continued running 93 octane, and it's been fine since. That was at least 15k miles ago. Of course correlation != causation, necessarily.

This is the 2.1L NA engine.

John

If you have a normally aspirated engine or a low pressure turbo, use the lowest octane gas available. Most NA cars adjust and are designed to use the lowest octane gasoline. Turbos are different since you have a much higher compression, you need detonation resistance. I have a Viggen with a high pressure turbo. I use 92 octane all of the time because without it, the computer will most likely retard boost. On every other car I've had that was non turbo, there was never any benefit to using high octane. Every once in a while I get knocking in a Pontiac and at that point I will put Super in and the problem is gone.