octane and mileage

That's not really surprising. The knock detectors in the engine management system will advance the spark as far as they can with no knock. On 91 octane, that is a lot further than on 87. The car will make a bit more power, but fuel mileage goes down. On the turbo cars, it makes a huge difference.

Does this mean higher or lower octane for turbos to get the maximum mpg?

I think it would take several tankfuls to see the 'real' difference between the 2 octanes. The ECU needs a few 'cold' starts to make its adjustments. Though in general, a relatively low compression engine will not be able to take advantage of higher octane unless it is turbo/super charged.

Carl

1 Lucky Texan

dasa wrote:

Is your vehicle with a high-performance or regular engine? The high-perf engines are the ones that require premium gas, and the regular ones are the ones that require regular gas. In general, putting premium gas in a regular engine will not help it one bit, because the engine computer is tuned conservatively. In a high-perf engine, the computer is tuned to make as much power as possible before knock starts with premium gas; if you put regular gas in a high-perf engine, the computer will tune itself even further back because knock starts earlier with regular gas.

If your car has a high-perf engine computer then it might actually get better mileage with regular gas, by tuning itself back.

So which engines are the high perf ones? I usually consider any of the engines producing more than 200HP as the high-perf engines, such as the 2.0L or 2.5L turbo H4's, or the 3.0L H6.

Yousuf Khan

Lower octane will generally cause the engine computer to scale back its performance, therefore it will give you better mpg.

One caution though, you don't want to scale all of the way back to the lowest grade fuel with a turbo, because the computer will be so busy preventing knock that it might actually do some damage to the engine; but it's upto you to experiment. I'd go back to a mid-grade as a first experiment.

Yousuf Khan

the car has non-turbo 2.2, sohc engine used for many us subaru models up to

2001 (mine in particular is 2000 impreza outback sport). i just wanted to see if there will be any difference with higher octane gas. as many of you noted, it didn't do any good. in fact, the mileage went down, and i didn't feel any difference in terms of power. perhaps it may make difference after few tanks, assuming the ecu will actually advance the timing according to the higher octane gas? i doubt the ecu (in my car, in particular) had even been programmed to take advantage of higher octane gas, but who knows... i will stick with regular grade (87). anyways, i am pleased that the engine, as it ages (40k), seems to get mileage better than epa rating. i wonder if synthetic engine oil i have been using actually contributed to this...? again, who knows.

Turbos are forced induction. Consequently, you have to be very careful with them. Generally, you cannot use regular (87 octane) at all. In the case of my car (the WRX STi) it will go into limp mode and damage the engine. The secret to high MPG in a turbo is to use one grade less than recommended and keep the thing off boost. You'll need a boost pressure gauge to tell. That usually means easy accelerations and cruise control.

I can see the knock adjustments being made 'on the fly'(and maybe RPM/idle adjustments) but are those the only adjustments being made?

Carl

1 Lucky Texan

FNO wrote:

It really depends on the car. For most of them, yes. Those are generally the only adjustments the ECU makes. For the turbo cars, it can also adjust the boost controller based on where it detects knock. Its first response is to back off the timing. Its second response is to reduce the pop-off pressure on the boost controller; effectively reducing compression.

In the STi, the car may also adjust the amount of locking on the center differential depending on driver input, the lateral G-forces, and the amount of wheel slip detected.

The pricier you go, the mosre sophisticated the ECU and onboard computers are. The big thing to remember, is that nowadays, they are fast. Consequently, gone are the days of simple ignition maps on the these ECUs. Now, they are adaptive, and they adapt quickly.

For what models/years/engines is this true for?

jw milwaukee

Where did you fill up? I recently did the same trip twice (extending a little further South) and found that the gasoline sold in the valley (oxygenated?) was awful. With gas purchased in San Diego or LA or the urban Bay area I got 28-29mpg, with gas filled in the valley I got 26mpg (Passat 6cyl. 4Motion Wagon [AWD]).

- D.

As a data point, with careful driving (e.g. keep it between 90 - 100 kph, easy on the acceleration) I can get 37 mpg on my 95 Legacy Wagon (Brighton)

2.2l SOHC - tires inflated to ~35 psi (instead of 32/30).

I recently switched to Ethanol mix (10%) and am interested to see what effect that has on milage.

-- Dominic Richens | snipped-for-privacy@alumni.uottawa.ca "If you're not *outraged*, you're not paying attention!"

Canadian (Imperial) gallons, or US?

Rick

US. In Canadian-English that would be 6.4l/100km :-)

-- Dominic Richens | snipped-for-privacy@alumni.uottawa.ca "If you're not *outraged*, you're not paying attention!"

I have been told by Subaru mechanics to use only 87 octane because you don't get better gas milage with higher octane. The higher octane causes the engine to run to rich.