Fuel economy

Yeah, I was thinking of the ECU issue too, but in my previous car, the

I don't know what the compression ratio is with the 3.6L H-6, but I'd assume at least 9:1, which is usually pretty much within the range for regular gas.

Well, as I said previously in this thread, this particular Soob has a weight disadvantage even over *other* Soobs, including the latest Outbacks. Outbacks weigh around 3300lbs, while the Tribecas weigh around

BTW, I did run though my previous batch of fuel, and so I refilled with premium yesterday, I'll report if there is any difference after it gets near the end. Interesting thing was that at this particular gas station, which must've been a fluke, they had ran out of regular gas, so they told customers to fill-up with premium instead -- for the exact same price! The premium was being sold for less than even the mid-grade! I couldn't believe how well the timing of this experiment worked out. :)

Yousuf Khan

Unless your tank was really empty, which is unlikely even if the gauge pointed to E, you don't have a tank full of 93 octane. You have a mix of 93 and whatever was left in the tank. Never mind the ECU, you won't be running on something close to 93 octane until at least the second or third tank.

If half and half 87 and 93 makes 91, even 3/4 93 will be better than

Not necessarilly true. The knock sensor retards the timing enough to eliminate detonation - which gives both best economy and best performance for that fuel in that engine.

Running the maximum possible advance without detonation gives the highest mean effective pressure in the cyl - meaning you get the most power out of every gram of fuel - which is good for fuel economy as well as power. If you try to use all the power, the spark retards to give more torque without detonation. Just think about the vacuum advance on an old distributor. High vacuum - meaning low engine load, advanced the timing for fuel economy. Low vacuum, meaning high engine load, retarded the timing for more power.

But if you retard the timing too much you loose both power and economy.

One standard driving cycle will usually have all settings updated. Anti-knock timing settings should be almost instant - as the ECU retards the timing ON DEMAND by sensing detonation and does a "retard to stop detonation - add a bit of advance - does it detonate? if so retard, if not advance - ad infenitum"

high atmospheric pressure, high air density, or high combustion chamber temperature can all affect octane sensitivity of an engine. An engine that runs fine on regular in colorado springs may need premium to run well in tallahassee, particularly on a hot humid day.

It doesn't. It makes 90: (87 + 93)/2 = 90

(87 + (3 x 93)/4 = 91.5 octane

According to cars.findthebest.com the Tribeca has a 16.9 gallon tank. If he ran the tank down until the needle pointed at E and the warning light came on he would still have at least two gallons of 87 in the tank.

((2 x 87) + (14.9 x 93))/16.9 = 92.3 octane

I'll concede that's pretty close to 93. Anyway, my point is made that if he just makes a routine fill-up with the higher octane he won't be giving it a fair test.

We run them as fleet workhorses on jobs. At one time JDM represented best purchase value for our purposes and we have had a few. To get best economic returns on jobs, we have done fuel consumption tests on a variety of Subarus over several years covering millions of miles.

On those 2L JDM single/twin turbos fitted with factory Lines ECU rated for 89+ on average we gained about 1-2L/100k by switching from 91 to 96 (typically 10-11L/100k) although we did run one single turbo permanently on 91 and it returned 12L/100k over its life but it did get thrashed by the boys. JDM twin-turbo Subs fitted with alternative Prova ECU gave best economy on 98 at 8.8 l/100k. Not all centers have 98 so vehicles were restricted to those runs where available which meant less hill trips which may have kept the consumption figures lower. Non-turbo 2.5L local editions ran on 91 as advertised but performance was unacceptable so we moved them to 96 where they average about

We are also running 2012 Ford 6cyl 4L (8.2L/100k) and older v8 5L (10.5L/100k) on 96. The Fords are bigger, heavier, more powerful but decidedly more economical on the long hauls despite constant higher speed running and that's why we have them. But of course they can't get off wet grass despite all the traction stuff so we need the Subarus for the local rural work esp when the rains and frosts come. We have others like diesel SUV etc but each has its purpose.

I know, but close enough. I did wait till the gas light came on in both cases.

Yousuf Khan

well, 98 is about $1.29(US) / liter around here and 95 is around 88 cents/liter. guess which one I use ;-)

thanks for sharing

Well here's the final update on this question. As you may recall, I was going to attempt to find the mileage differences between different grades of gasoline. My first experiment was upgrading to mid-grade from regular: no difference. Now my final experiment was to upgrade to premium: no difference, again! In fact, premium fuel was a little worse than mid-grade or regular, but that's easily attributable to random traffic conditions which are out of my control during these experiments. Though I'd say I experienced my normal traffic conditions during each testing period, I don't know that for sure, so the variability is understandable. The average fuel economy was around 16.5 L/100km in mixed driving, which is like 14.25 mpg.

Yousuf Khan

like 14.25 mpg.

Thanks for report.

Just came back from trip. Outback did 24.7 mpg over 400 mile trip. This is highway, much if it through steep grades. I was a bit disappointed. Avalanche did 17.8 over same stretch. 2001 cavalier 2.2 did it 34 mpg.

It's easy to get used to the cavalier mpg. But, it's gone, totaled. The replacement is a 98 olds 88 3.8 , which can get better milage than the outback, but untested on my route.

Outback did much if the trip around 3000 rpm near 70 mph. A little too much rpm to expect great milage. My 93 Dakota v6 was a gas hog. With just a bit over the outback hp it was always downshifting over the same route. I didn't measure it, but figure close to 20 mpg estimated over my route.

Greg

My "Outback" (2.5l manual 1997 Legacy Lancaster Grand Wagon (what a mouthfu= l .. same as this one

most cars.

I hope I can get better outback milage after plug and wire change. Also my vibration problem.

Greg