better mileage with higher octane?

Depends on the ECU. Some can be "chipped" for more advance.Some OBD2 boxes can be "reflashed" Some that still have a distributor can have the timing physically advanced. On some you can adjust the cam sensor to provide more advance. On most, it's pretty well "cast in stone"

If you turbocharge with enorgh mean pressure to need the extra octane.Your mileage will improve.

With a performance chip set.......sold at any rod shop. And while you are there pick-up your turbocharger kit....and if you want to use premium switch to bi-turbo.

but only if the computer calls for more timing with higher octane. If your computer calls for a max of 35 degrees and you can reach that on regular gas, 91 won't do anything.

That said, I have a Trans Am that will ping if you put in anything lower than 91. Manual says so, and I accidentially put in a tank of 87 by accident. Stopped 50 miles later and bought a shot of octane boost and then filled up again at 1/2 tank to stop the pinging.

Ray

It may.

As others have stated, octane in itself has nothing to do with fuel mileage. But increased octance may allow for more spark advance and more power.

I had a '91 Caprice that I typically ran on 87 octane, but did run on

91/92 octane depending on price. I would typically get 2-3 mpg better(28 vs 25) on the higher octane fuel. I no longer have the car, but the price difference between grades has grown to the point that it probably wouldn't make sense any more anyways.

Try it and find out. But it is unlikely that it will make enough difference to acount for 30c of price difference.

JW

Saab's Trionic ECUs will deliberately advance the timing until the engine *does* pink intentionally so. And much much more besides. They even use spark plug ionisation levels to determine mixture richness ( on a per cylinder basis ) !

They're probably the most advanced around. Many cars have clueless ECUs by comparison. So it does depend hugely on the car.

Graham

No ECU ? What year is that ?

Graham

Very true. I used to work for Imperial Oil and they said to use the highest octane you can use before there was no improvement in fuel mileage. It takes repeated mileage tests averaged out to determine the best fuel required.

My 1971 Datsun 240Z had to run on "supreme" 91+ octane fuel to maximize power and fuel mileage. Funny to think that 13 years ago I used to cringe at the price I was paying for it which was less than the regular priced today.

I once owned a beater 1976 Toyota Celica GT hatchback who's carbs I leaned out for maximum fuel mileage and I found it ran best on 87 octane fuel. Thank god as I was a student at that time and needed cheap transportation. Until I got my new Civic, this car gave me the best highway mileage.

I had a 1994 Pontiac Sunbird (w/EFI electronic fuel injection) that recommends minimum 87 octane fuel. But I found thru repeated tests that I got slightly better fuel mileage using 89 octane. Anything higher provided no gain and was a drain on expenses.

My current car, a 2006 Honda Civic recommends a minimum of 87 octane as well. But I almost run it exclusively on E5 89 octane (5% ethanol blend) and I have found virtually no difference in mileage. I pay about 5 cents more per litre but that turns out to be about $10 more per month in fuel expense, a small price i'm willing to pay for reduced GHG emissions.

"Mike Romain" wrote

It may possibly help keep your engine cleaner too.

Graham

With the gearing on the '75 Celica GT hatch (longer legs than the '76) I got 55MPG at 70+mph if I could hold steady speed. (Waterloo to Peterborough Ontario at 2AM Saturday morning, 1979 0r 1980)That trip was closer to 75MPH than 70 - and at 55 I never got more than 45. 5th gear, right on the cam (4200rpm) seemed to be where it was happiest

>

A decent ECU will do that instantaneously 'on the fly'.

Graham

In any case and slight increase in mpg will be more than offset by the additional cost. Just my opinion but I am on the side of 'no increase'. I have seen this thing discussed and written about many times. The "experts" all say it is just a waste of money.

Harry K

You may not get any increase in mpg but decent cars will see an improvement in performance.

I had one car in fact that allowed to me select the 'ideal' octane as either 95 or 98 ( RON ).

Graham

Say, whatever happened to the water injection systems that they used in the world war II fighter planes? They could advance the timing, reduce the ping, get better performance and economy. So, how come these brilliant automotive engineers aren't using this technology?

What water injection systems ?

There were ethanol and methanol injection sytems ( improve the octane rating ) but water ?

Graham