[strat81]

So after my thread a week or two regarding the lack of 91+ octane in Nebraska, I figured I'd feed my Linear some 89 here in New York to see what would happen. It seemed to run fine with no apparent knocking, stalling, rough idling, etc. However, I noticed that on the NJ Turnpike, when accelerating at the on-ramp from say 35 to 75+ mph my boost gauge would not poke near the red and I didn't have the usual spry acceleration I'm accustomed to. It felt like the car weighed another 800 pounds or something. I thought it was perhaps an ECU problem so when I got to my destination, I turned the car off for 30 seconds then fired it up again and took it back out, with the same results. I've since refilled with 93 octane (my usual choice) and full boost seems to be back for good. FYI, the weather was on the mild side, around 60-65F degrees, but humidity was high and it was overcast.

 

[sbl]

That is about what I would expect. With the compression high, there will be more knocking, and it will back down the boost.

I've run 89 octane (compared to 93) for the last couple of tanks and have not noticed a driveability difference, but I do not typically accelerate hard. What I find interesting is that my average MPG has gone up, from 25.5 to 27.5! Pretty much the same driving.

 

[ctrlz]

I've run 89 octane (compared to 93) for the last couple of tanks and have not noticed a driveability difference, but I do not typically accelerate hard. What I find interesting is that my average MPG has gone up, from 25.5 to 27.5! Pretty much the same driving.

I've gone from 92 to 89 and also jumped +3 mpg in mixed driving. There has been no noticeable performance change.

Sbl, you and I have the 2.0T, which recommends 92 minimum octane. Strat has the 2.0t, which I think recommends 90. You guys aren't really comparing apples to apples.

Strat really isn't deviating too far from the recommended grade. Maybe under continuos hard driving it would knock and the ECM would retard the timing/decrease boost. But I doubt his predicament is due to fuel grade. I suspect it has more to due with differences in gasoline formulation between the Northeast and the Midwest. Off the top of my head I think you will find that one of the major differences is ethanol content. There may also be MTBE. Both are oxygenates, I believe. I would bet the ECM has some adjusting to do when the alcohol content of fuels changes significantly.

 

[NocBlue]

That is about what I would expect. With the compression high, there will be more knocking, and it will back down the boost.

I've run 89 octane (compared to 93) for the last couple of tanks and have not noticed a driveability difference, but I do not typically accelerate hard. What I find interesting is that my average MPG has gone up, from 25.5 to 27.5! Pretty much the same driving.

Hey! I noticed that same change... Why would a lower ocatane result in a better mpg?!?!? I did not notice any drivability issues either... What gives?

 

[Automaggot]

I use 89 octane in my Linear and it boosts all the way to the red every time. Nothing changes when I switch to 93 octane either.

 

[ctrlz]

Hey! I noticed that same change... Why would a lower ocatane result in a better mpg?!?!? I did not notice any drivability issues either... What gives?

Well, I think the recommended octane is to satisfy conditions at high load, WOT, and maximum boost-- when pre-combustion pressure is very high. If the fuel is too volatile (i.e. lower octane than required), the mixture could spontaneously ignite while being compressed (pre-iginition, a.k.a. "knock"). The high load WOT situation rarely happens in everyday driving. But the "recommended octane" has to account for these extreme conditions.

The engine may just be more efficient running the lower grade for everyday driving. As long as you don't get into the extreme load situations, you may never get knock, and the ECM keeps calling for normal boost and timing advance. The mixture may be burning a bit faster and hotter than it would with premium fuel, and this translates into better economy. The engine design tolerates a certain maximum spark advance, and the combination of mid-grade fuel and the ECM maps for premium may get you closer to this theoretical maximum than actually running premium (which burns slower). So using mid-grade is a way of fooling the engine into thinking you've turned the distributor a few degrees (if it actually HAD a distributor).

Take all the above with a few grains of salt, please!

 

[saab9-3LA]

...But can anyone tell me why higher octane fuels are available in some states and not others?

I recall having 87, 89, 91 and 93 back in Massachusetts (though some consumer groups often said that 93 wasn't necessary for most cars), whereas here in California, we only have up to 91. I suppose this is just a difference in state regulations, taxes, emissions controls...? My mechanic also mentioned, and I do now recall myself, that some European countries have octanes of up to 97.

I'd like to know what effect this has on performance (and what Saab specifically recommends since my dealer was frustratingly vague about it when I first bought my car)...and also on maintenance. Any answers? Just curious...

 

[ctrlz]

My mechanic also mentioned, and I do now recall myself, that some European countries have octanes of up to 97.

I'd like to know what effect this has on performance (and what Saab specifically recommends since my dealer was frustratingly vague about it when I first bought my car)...and also on maintenance. Any answers? Just curious...

I think for the US market Saab recommends 92 for the Arc/Aero and 90 for the Linear.

There is a lot of variation within fuel. The "octane" number is really the AKI (Anti-knock index). The number was originally intended to give an idea of the composition of the gasoline fraction. Pure n-heptane is 0 octane. Pure iso-octane (2,2,4-trimethyl pentane) is 100 octane. Modern fuel additives can boost octane to over 100.

The difference between the US and European systems is the European number is the octane determined solely on chemical composition of the fuel. The US number is the average of two methods, which is why you always see "(R+M)/2 method" stamped on US gas pumps. That's "research" octane and "motor" octane. Research octane is same as the European number. The Bureau of Standards and fuel manufacturers in the U.S. have special engines with variable compression ratios they use to test fuels. This gives the "motor" octane. The number on the gas pump is the average of the two.

There's a lot more that can be said. Because of how these numbers are calculated there can be diffeences between brands. Modern cars all prevent knocking, so you can't use that as any kind of guide.

Deviating from the recommended grade by 1 or 2 will probably go unnoticed in 99.9% of cases. I'm currently running 89 in my Vector (92 recommended minimum). I think the power is equivalent to the 92. The fuel economy is certainly better. It just may be that where I'm buying fuel the 89 is closer to the optimum grade for my car, and the 93 is overkill. That may not be the case in other states.

 

[Vector-SS]

I have noticed a difference in the pull switching from 91 to 94. I feel like the boost comes in much stronger

 

[mbodo]

unfortunatley, only having used the "butt-dyno", I can only tell a modest amount of difference when I have racing gas mixed with 91 octane on the standard tune.

As ctrz eluded to, the timing will adjust to compensate for different octane ratings in the fuel. Having just walked back from the fuel door on my Arc it states "Premium Recommended" (93 AON) and "Minimum 87" .. this is in stark contrast to my porsche, which states "93 Minimum" (where the heck am I to find that in California?!?)

One can reasonably expect the Engine Management routines of both vehicles to continue to advance the timing until a knock is detected, and then retard off until optimum spark advance is achieved.

In both cases, the factory will follow up with "Our timing will adjust to accomodate your crummy gas" (in so many words).

Of course, some of the aftermarket tuning negates this compassion for lower fuel grades.
YMMV ...

 

[Aero_adam]

Power output is directly related to fuel consumption.
Less fuel consumption = less power..

I would suggest that by using a lower grade fuel, the engines are developing less power (through the retading of the ignition and anti knock control strategies) and therefore have improved gas mileage figures. How many people would notice a 5% drop in power?

 

[ctrlz]

Of course, some of the aftermarket tuning negates this compassion for lower fuel grades.
YMMV ...

Yes, it seems some aftermarket software does not offer adequate compensation for lower fuel grades. BSR, in particular, now apparently includes a fuel grade warning with their products.

Having read about ECM reprogramming in other vehicles it seems that a typical strategy to improve performance is to expand the fuel maps. If, for example, you are going to allow higher boost pressure for a given load/rpm, then there has to be injector data for that situation. If you just cut the boost, you don't need the fuel data. If you think of the ECM as having a finite amount of memory, you can imagine why reprogrammers might dump the low octane tables to make room for expanded high octane tables.

Not sure this is how it exactly works in the 93SS. Just a guess.

 

[strat81]

I wouldn't expect aftermarket manufacturers to support low octane situations. Regardless of what the manual says, I'd bet that most modern mass-produced cars (keyword here being MODERN) will run on 87 octane. They may not run well, or feel powerful, but they'll run. Manufacturers would probably do it to limit their costs as far as warranty coverage. Imagine: I just bought a new Aero 2.0T and feed it 87 all the time. Assuming my knock sensor works, I'd probably be ok most of the time. But, let's assume that my knock sensor works intermittently. So, sometimes I get heavy detonation and other times not. One day, my motor starts running like complete butt and I put some 93 in there but it's too late. I drag it to the dealer and they can't explain it and end up rebuilding the motor.

Manufacturers know some people are cheap and/or stupid. They will never ever use high test, even if it's a performance machine. Heck, even my mother's Altima 3.5SL calls for high octane for best performance. But, even the most caring owners will get stuck once in a while. As I mentioned in my original thread, 90+ octane is very hard to find out in Nebraska. I'm sure if I lived there I could find a station with it, but would I go nuts looking for it if I was passing through? Nope.

So, if aftermarket guys only have a finite amount of space to use for fuel maps, it's logical that they'd only use high-octane maps. But, if I did PPC my car, I'd make sure to carry a bottle or two of octane booster with me, just in case.