Fuel Octane Ratings, 2002 9-3 [Archive] - SaabCentral Forums

: Fuel Octane Ratings, 2002 9-3


sirapm
3rd November 2008, 10:24 AM
I drive a 2002 Saab 9-3 SE 2.0T with 60,000 miles on it.
I have owned the car since it had 25,000 miles on it and I have always used regular 87 Octane fuel.
I am thinking about switching to premium 93 Octane fuel.
2 questions:
1) Is it bad to use 87 octane?
2) Is it bad to switch to 93 octane now?

99Saabnut
3rd November 2008, 10:34 AM
Your car's engine control module is designed to adjust to various octane ratings from 87 up. Will 87 hurt it? No; not at all. It will not run as well on 87 as it will on 91 or 93 though; or at least mine did not.

I found it less expensive to use 93 octane than it was to use 87 octane, because my car runs so much more efficiently on 93. It also has a bit more power on 93 as the engine management can advance timing more with the increased knock protection. (I didn't get that backwards did I?)

Switching at any time will not hurt your car; however you will not see the full benefits/drawbacks immediately. It will take more than 1 tank full (Likely 2 or 3) to completely adjust to the changes, so stay with it for about a week before you decide.

Darren900
3rd November 2008, 10:38 AM
Saab recommends to use above 90 AON but here in western PA, I can usually only choose 87, 89, and 93 so I go with 89. Higher octane fuel has a high energy content (heating value) and your car will get better gas mileage with higher octane so it's not as expensive as you might think.

bkrell
3rd November 2008, 02:28 PM
Back in the spring, when gas was going thru the roof, I did the calculations and found for the cost vs MPG I was getting, it was pretty much a wash with using premium having a slight advantage.

But performance-wise, go with the premium gas. Your engine runs much more efficiently and effortlessly.

Diosnoche
3rd November 2008, 06:26 PM
My opinion, Turbo engines should be using highest you can get, but for N/A engines, like mine, go with regular 87

watkins
3rd November 2008, 07:22 PM
Heres the deal. It doesnt cost much more to use high test fuel. There is absolutely no reason not to buy it. The engine runs better, which is protection from problems down the road. Its also more efficient, which actually makes buying 91 or 93 octane less expensive in the long run.

SaabHat93
3rd November 2008, 07:28 PM
I never saw much of a difference performance or efficiency wise when I switched to 91 for a few months. Wasn't worth the extra money to me. I drive conservatively as it is, and the higher octane really didn't do a whole lot, so I guess it also depends on your driving style and patterns as well.

Red99
3rd November 2008, 10:22 PM
Octane rating is nothing more than a measure of the autoignition resistance of gasoline. PERIOD. Octane rating has ABSOLUTELY NO bearing on the "energy content" or (heating value). PERIOD.

Octane rating (or knock resistance) is important in high compression or forced induction engines (turbo charged). The higher the compression ratio or the more boost requires higher octane fuel. Most modern engines will adjust to almost any commercially available octane rating. Using a lower than "recommended" octane rating will only guarantee that you will be unable to reach the normally expected PEAK power output.

1) Using 89 octane will guarantee you sub-standard peformance but should not hurt an otherwise well running vehicle.
2) Switching octane ratings will not hurt the car at all. It will adapt within a few minutes of driving.

On a stock 9-3 I would recommend using a 89 octane. With my stage 3, I only use 93 octane (116,000 miles of hard trouble free driving and a lifetime average of 27.29 MPG) Why spend big money for more performance (ECU, exhaust, intercooler, etc) and then reduce the performane to save 20 cents per gallon.

Your Saab will reduce boost and/or ignition timing advance if the octane is not high enough (if knock is detected). Higher boost does not equate to "more efficient". There should be absolutely no change in MPG. In fact, with lower octane, there will be lower boost, with lower boost, there is less fuel used (ECU controlled), less fuel=higher MPG. But if you really wanted better MPG than you wouldn't have a turbocharger.

And remember, Google is your friend: http://www.absoluteastronomy.com/topics/Octane_rating

musky
3rd November 2008, 10:40 PM
Mine pings like crazy on 89 (is something wrong?). Runs normal on 93+.

Cheers M.

Diosnoche
4th November 2008, 06:14 AM
My engine is naturally aspirated, no turbo on it hence why I use regular 87 (91 RON).

But curious, all the europeans on here, is 95 RON (91 octane) the minimum mostly available in Europe?

Owen423
4th November 2008, 07:58 AM
http://www.faqs.org/faqs/autos/gasoline-faq/part1/

All you ever needed to know about petrol...

To achieve higher RON, you need branched or longer chain distilates - both of which have higher calorific values, but thier proportion of the 'cut' of the fuel varies, so there is no general rule of increase in one increases the other.

The higher RON enables much higher advance timings to be used, to gain greater power from a given fuel load - irrespective of the type of induction system in use on the vehicle, dependant on the sophistication of the engine management.

Switching to a higher RON fuel may throw a CEL initially as the fuel map (and ignition curve) will result in higher than expected hydrocarbons for a short while hitting the sensor. This will go away.

The result will be a smoother running engine, as the flame front advance is smoother, and more predictable (ie more in line with the engine designers 'idealised' design model). This will reduce engine noise. (my 6 cylinder BMW is so quiet on 'premium' fuels that it is hard to tell if the engine is running, from outside!).

The range of boost is managed, so that it may be possible to use higher boost earlier with a premium fuel - this can result in higher MPG, as the least efficient utilisation of fuel is during acceleration - so more power - period accelerating is shorter, longer cruise, overall fuel use is marginally down.

I get around 1 MPG more on premium fuels with the same driving style (odometer/fillup measuring). The prime benefit is a better detergent base (although in some countries this isnt the case) that keeps the fueling system working well. I run around 1 in 6 tanks on premium (old habits), and find that the fuel use is more consistant (no dipping prior to a service), and have not had a problem with sticky injectors etc - but I do run a dose of cleaner through every 10k or so.

Just my perspective



Owen

Darren900
4th November 2008, 10:08 AM
Octane rating is nothing more than a measure of the autoignition resistance of gasoline. PERIOD. Octane rating has ABSOLUTELY NO bearing on the "energy content" or (heating value). PERIOD.

You are correct. My mistake.

Your Saab will reduce boost and/or ignition timing advance if the octane is not high enough (if knock is detected). Higher boost does not equate to "more efficient". There should be absolutely no change in MPG. In fact, with lower octane, there will be lower boost, with lower boost, there is less fuel used (ECU controlled), less fuel=higher MPG. But if you really wanted better MPG than you wouldn't have a turbocharger.

This is not true. Using the designed octane rating for the engine will guarantee proper performance of the engine. If you use lower octane than design, efficiency and power output will decrease because the ignition is delayed and less fuel is burned. When these engines are boosting, they are running at their maximum design efficiency (granted, they are using a whole lot of gas, but they are outputting a whole lot of power). In general, a turbo charged engine is more efficient than a N/A one since the engine itself can be smaller and auxiliary fuel costs are lower on average.

Saabohème
4th November 2008, 02:59 PM
Car & Driver, November, 2001, testing regular unleaded and premium fuels in several cars, including a turbocharged 9-5, and addressing the following questions:

Is premium fuel worth the premium price? Can you hurt a high-octane car by running it on the cheaper stuff?

BY FRANK MARKUS



There's no shortage of opinions on who is to blame for gas-price gouging. One thing that's certain is drivers tend to economize at the pump during extreme price rises-they buy cheaper, lower-octane gas.

In the old preelectronic days, cars would protest such parsimony by pinging like a pachinko parlor, but most modern cars don't complain audibly, so maybe they don't mind. Or do they? And conversely, is there any benefit to be had by springing for the expensive stuff when you're feeling flush?

To find out, we ordered a fleet of test cars-some calibrated to run on regular, others that require premium-and tested them at the track and on a dynamometer.



But before we go into the results, let's go to combustion school. When a spark plug fires, it does not cause an instantaneous explosion of the entire cylinder's charge of fuel and air. The spark actually lights off a small kernel of air-and-fuel mixture near the plug. From there, a flame front expands in every direction, gradually igniting the rest of the air and fuel. This takes some time, as much as 60 degrees of crankshaft rotation.

Meanwhile, the air-and-fuel mixture that the flame front has not yet reached is experiencing huge increases in pressure and temperature. If any part of this air-and-fuel mixture gets heated and squeezed enough, it will explode spontaneously, even before the flame front ignites. This self-ignition is called detonation, or the dreaded "knock."

Now for the chemistry lesson: Oil is a hydrocarbon fuel, meaning the individual molecules contain carbon and hydrogen atoms chained together. Modern gasoline is blended according to various recipes, the active ingredients for which include

about 200 different hydrocarbons, each with a spine of between 4 and 12 carbon atoms. One of them, isooctane, consists of 8 carbon and 18 hydrogen atoms (C8H18) and is exceptionally resistant to exploding spontaneously when exposed to the heat and pressure found inside a typical combustion chamber. Another, n-heptane (C7H16) is highly susceptible to such self-ignition.

These two compounds are therefore used to rate the knock resistance of all gasoline blends. A gasoline recipe that resists knock the way a mixture of 87-percent isooctane and 13-percent n-heptane would is rated at 87. Racing fuels with octane ratings over 100 resist self-ignition even better than pure isooctane. The octane ratings for regular-grade fuel range from 85 to 87, midgrades are rated 88 to 90, and 91 and higher is premium.

Mind you, premium fuel does not necessarily pack more energy content than does regular. Rather, it allows more aggressive engine designs and calibrations that can extract more power from each gallon of gasoline.

An engine's tendency to knock is influenced most by its compression ratio, although combustion-chamber design also has a large effect. A higher ratio extracts more power during the expansion stroke, but it also creates higher cylinder pressures and temperatures, which tend to induce knock. In supercharged engines boost pressure behaves the same way. That's why the highest-performance engines require higher-octane fuel.

If you feed such an engine a fuel with insufficient octane, it will knock. Since it is impossible, for now, to change an engine's compression ratio, the only solution is to retard the ignition timing (or reduce boost pressure). Conversely, in some engines designed for regular fuel, you can advance the timing if you burn premium, but whether this will yield additional power varies from engine to engine.

Knock sensors are used in virtually all new GM, Ford, European, and Japanese cars, and most DaimlerChrysler vehicles built today. According to Gottfried Schiller, director of powertrain engineering at Bosch, these block-mounted sensors-one or two of them on most engines and about the size of a quarter-work like tiny seismometers that measure vibration patterns throughout the block to identify knock in any cylinder. Relying on these sensors, the engine controller can keep each cylinder's spark timing advanced right to the hairy edge of knock, providing peak efficiency on any fuel and preventing the damage that knock can do to an engine. But, noted Schiller, only a few vehicles calibrated for regular fuel can advance timing beyond their nominal ideal setting when burning premium.

Older or less sophisticated cars with mechanical distributors do not have the same latitude for timing adjustment as distributorless systems do and therefore may not always be able to correct for insufficient octane or additional octane.

We should note that even cars designed to run on regular fuel might require higher octane as they age. Carbon buildup inside the cylinder can create hot spots that can initiate knock. So can malfunctioning exhaust-gas-recirculation systems that raise cylinder temperatures. Hot temperatures and exceptionally low humidity can increase an engine's octane requirements as well. High altitude reduces the demand for octane.

Got all that? Good. Let's meet the test cars and ponder the results. At the lower-tech end of the scale was a regular-gas-burning 5.9-liter Dodge Ram V-8. This all-iron pushrod engine has a mechanical distributor and no knock sensors, so the computer has no idea what grade of fuel it's burning. A Honda Accord V-6 with VTEC variable valve timing represented the mainstream-family-sedan class, and a 4.6-liter V-8 Mustang stood in as an up-to-date big-torquer. Both of those were designed to run on regular unleaded. Our premium-grade cars included the hard-charging 333-hp, 3.2-liter BMW M3 straight-six boasting individual throttle by wire for each cylinder and enough computing power to run Apollos 11 through 13. A Saab 9-5 gave us a highly pressurized 2.3-liter turbo. For the sake of repeatable track testing, all but the M3 were equipped with automatic transmissions.

We ran all vehicles on both grades of fuel, at a drag strip near our offices and on a Mustang eddy-current dynamometer that was offered to us by the engine-tuning pros at Automotive Performance Engineering in nearby Clinton Township, Michigan. On arrival, all fuel tanks were drained and filled with 87-octane Mobil regular fuel and driven for two days before track and dyno testing. The tanks were drained again and filled with 91-octane Mobil premium and again driven for two days to allow time for the engine controllers to acclimate to the fuel type and tested again. All dyno and track results were weather-corrected.

Our low-tech Ram managed to eke out a few extra dyno ponies on premium fuel, but at the track its performance was virtually identical. The Mustang's knock sensors and EEC-V computer found 2 hp more on the dyno and shaved a more impressive 0.3 second off its quarter-mile time at the track. The Accord took a tiny step backward in power (minus 2.6 percent) and performance (minus 1.5 percent) on premium fuel, a phenomenon for which none of the experts we consulted could offer an explanation except to posit that the results may fall within normal test-to-test variability. This, of course, may also be the case for the gains of similar magnitude realized by the Ram and Mustang.

The results were more dramatic with the test cars that require premium fuel. The turbocharged Saab's sophisticated Trionic engine-control system dialed the power back 9.8 percent on regular gas, and performance dropped 10.1 percent at the track. Burning regular in our BMW M3 diminished track performance by 6.6 percent, but neither the BMW nor the Saab suffered any drivability problems while burning regular unleaded fuel. Unfortunately, the M3's sophisticated electronics made it impossible to test the car on the dyno (see caption at top).

Our tests confirm that for most cars there is no compelling reason to buy more expensive fuel than the factory recommends, as any performance gain realized will surely be far less than the percentage hike in price. Cheapskates burning regular in cars designed to run on premium fuel can expect to trim performance by about the same percent they save at the pump. If the car is sufficiently new and sophisticated, it may not suffer any ill effects, but all such skinflints should be ready to switch back to premium at the first sign of knock or other drivability woes. And finally, if a car calibrated for regular fuel begins to knock on anything less than premium or midgrade, owners should invest in a tuneup, emissions-control-system repair, or detergent additives to solve, rather than bandage, the root problem. Class dismissed.

9-3Pilot
4th November 2008, 08:08 PM
*bookmarks this page*

Red99
4th November 2008, 11:08 PM
You are correct. My mistake.



This is not true. Using the designed octane rating for the engine will guarantee proper performance of the engine. If you use lower octane than design, efficiency and power output will decrease because the ignition is delayed and less fuel is burned. When these engines are boosting, they are running at their maximum design efficiency (granted, they are using a whole lot of gas, but they are outputting a whole lot of power). In general, a turbo charged engine is more efficient than a N/A one since the engine itself can be smaller and auxiliary fuel costs are lower on average.

You are correct. If you could shrink the displacement of your engine on demand and maintain boost at its theoretical peak you would be right. But you can't.... Your displacement and compression ratio are fixed.
Forced induction engines almost always have volumetric efficiency great than 100% by definition. But that doesn't mean you get better MPG by having more boost available by using higher octane fuel. That is called false logic, by definition.
I spent months data logging the effects of octane on timing advance when my 9-3 was on a MBC. The long turn result is simply this: 89 octane and 93 octane both had the exact same amount of ignition advance at a given boost level (I have huge excel graphs documenting this). My deduction is that at a max boost of 15 psi (MBC controlled, no ECU input), there was no advantage to the 93 octane fuel over the 89 octane. Is this true for every car... No, but it was for mine. But, now that I am hitting 21 psi daily, 93 octane is critical.
But you will get no fundamental argument out of me. I am a big believer in turbo engines. Audi A3, Saab 9-3, and a Volvo 850 wagon all grace my garage. A great compromise between power and economy. My point is simply that less than recommended (optimal) octane will reduce power. Higher than necessary octane will NOT increase power. Octane change will have no measurable effect on MPG.

Great thread!

Owen423
5th November 2008, 05:58 AM
The long turn result is simply this: 89 octane and 93 octane both had the exact same amount of ignition advance at a given boost level (I have huge excel graphs documenting this)

Very interesting. This begs two questions:

1/ was the advance unable to move further?
2/ are gas stations 'gouging' through 'rebadging' regular as premium?

I've joined the open source ECU project to look at stuff like this. Fascinating.


Owen

Darren900
5th November 2008, 10:08 AM
From WIS:

Knock control
The Saab Trionic does not have a conventional knock sensor. Instead the ignition discharge module analyzes the ionization
currents for all the cylinders and sends signals to pin 44 on the Trionic ECM. This function is adaptive with respect to upsetting
fuel additives.
From the combustion signals the engine control module knows which cylinder has fired and if at the same time the knocking
signal is above a definite level, the control module records knocking on that cylinder. The ignition timing is then retarded 1.5 5
on that cylinder.
In the event of persistent knocking, the timing will be retarded additionally but not more than a maximum of 12 5 . If ignition
timing is retarded by more than a certain amount on all cylinders, the quantity of fuel injected will be slightly increased.
If knocking occurs when intake manifold pressure is higher than about 140 kPa, knock control will take place in a different way,
as follows: first the fuel injection and ignition matrices are changed and if this does not help then boost pressure will be reduced.
The aim is to achieve good performance even in the event of knocking.
In the event of a break in the lead connected to pin 44 of the electronic control module, basic boost pressure will be obtained
and ignition timing retarded by 12 when the engine load is so high that knocking could occur.

It seems that knock control is totally different during boost than other times. The WIS is not very specific.

Red99: I wasn't saying you could achieve higher fuel economy by running higher octane rating fuel. I was saying that running lower octane rating fuel will decrease fuel economy since ignition timing is greatly retarded. In most engines, a later spark will cause more incomplete combustion. I know it's probably not quite that simple which is why I would love to see some of your results, and to hear a little bit more about your experiments. Perhaps you could do a little right up?? Someone once said, "The worst thing a scientist can do is hide his results."

99Saabnut
5th November 2008, 10:20 AM
Well; since Saabs do not require "High Performance" fuel according to published information/Saab literature; the standard for their performance numbers is 87. So running higher octane than that will result in better mileage and performance figures...

I would guess something else in Red's system was inhibiting timing so that there was no change. I did not do such a thorough examination of my car; but I did do a very thorough comparison between 87 and 93 octane fuels and mpg; and my car solidly and consistently turns in about 4 mpg higher on 93 than on 87.

this is regardless of how I drive; weather conditions or any other contributing factor...

Saabohème
5th November 2008, 12:37 PM
It makes plenty of sense when you read between the lines Darren. When boost is high enough to elevate manifold pressure above the threshold of 140 kpa, the Saab engineers made the reasonable assumption that any knock detected at that point and beyond is probably boost induced, so they programmed the ECU to add fueling. If progressively additional fueling does not abate the knock signal, the ECU has been programmed to begin reducing boost until safe conditions are again present. This is good. You don't want the fueling to get so pig rich that you begin fouling plugs and doing other undesirable things, so tweeking another parameter in addition to the fueling ultimately gives more, and more flexible control.

Darren900
5th November 2008, 01:24 PM
Ok, that makes sense. It seems that if, under boost, the ECU corrects knocking by running richer, one would also expect lower fuel economy. So, assuming that running lower octane fuel than recommended causes knocking, it also causes lower fuel economy. Am I correct?

Owen423
6th November 2008, 07:23 AM
Yes.


Owen