Tomas

Toyota/Scion recommends fuel with a minimum of 87 octane for the xA and xB. An 87 octane fuel works just fine with the engine and the ECU is designed to handle this low an octane fuel in a high compression engine by retarding the timing to prevent damaging pinging.

With 87 octane fuel in this very hilly part of the country, the ECU retards the timing to prevent pinging, and thereby reduces available power, on a regular basis.

A perfect example is my 1/3 mile driveway, that with my automatic requires 2nd gear with 87 octane while only needing 3rd with 92 octane.

It is not that a fuel with higher than 87 octane creates more HP, but it DOES prevent the drastic retardation of spark timing to prevent pinging with our 10.5:1 compression ratio engines while under load at low RPMs. In other words one does not LOSE power just when it's needed with the higher octane fuel. This is an easily repeatable test at each fill-up.

I doubt the static ignition timing is advanced far enough to take advantage of the higher octane fuel at normal cruise, or even at higher RPMs.

The ECU seems to be programmed to retard from the static setting if it detects incipient pinging, and with small engines like ours, with low torque at low RPM, the point where the ECU starts to retard the timing is exactly where we need all the power the poor little engine can give.

A higher octane fuel allows the ignition timing to remain more advanced for a longer time, giving marginally better low RPM power. This sometimes eliminates the need for the transmission to downshift - the engine can put out enough 'grunt' to do the job in the higher gear.

In my daily driving this is actually a noticeable change. The closest arterial to where I live - which I must use to go ANYWHERE - has a long hill northbound from here. With 92 octane fuel I can maintain the 35MPH speed limit easily while in locked 4th gear. With 87 octane fuel, the transmission needs to shift down to third on that hill to maintain the same speed. The difference is the ECU having to retard the timing to prevent pinging with the 87 octane fuel, thus reducing low RPM power when it is most needed.

Neither fuel will damage the engine in any way, and the vehicle performs as designed, with no problems, when using 87 octane fuel. The difference in MPG is very minor, but the enjoyment is increased with the higher octane fuel not making its systems seem to work quite so hard.

I suspect the only difference one would see on a dyno would be a very slight increase in low-end torque, while under heavy load, with the higher octane fuel.

Anyone else out there notice this effect?

Subcompact Culture

From what I've always heard and read, running higher then the recommended fuel rating will decrease performance and milage because the ignition timing is programmed so with an 87 octane fuel, as much of the fuel will be ignited/burned as possible. With a higher octane rating, not all of the fuel is burned off, thus decreasing power and fuel economy. I can't imagine it's any different with the xB.

Tomas

No, a higher octane rating (1) costs more, and (2) resists detonation better.

There is no loss of burnability or power. A higher octane fuel will not harm an engine or it's performance, a lower octane can cause actual damage and reduction in performance, especially if the octane is lower than the engine is designed to handle.

XBdntusee

I started running 89 octane in my XB and can tell a big differnce. The 87 seems to lag on the bottom and dosnt seem to run as well almost like mising. If I was to race I would definetly spend the $3.30 a gall here in LA for 91 octane. I can barley aford the 89 so I wont.

djct_watt

I used to think that as well, but just look at the ratings for your engine in the manual. Note that all ratings and tests never used 87, but 91, I believe. I think the call it research 91 or something.

So technically, I think the engines were designed and tested using a higher octane, but have been tuned to allow for a wide variance.

I tested this out myself, and found it to be true.

Subcompact Culture

When you say you "tested this out," what was your method? If you showed higher MPG, then I'd spend the extra few dollars to try it out.

However, any "butt dyno" results are often false and, of course, the only way to prove an increased power output would be a dyno.

I'll stick with my saying as of late: "87 For Life!" Plus, if Toyota recommends 87, that's what I'll use. I don't live in a high-altitude environment and I don't have a turbo/supercharger.

Tomas

Did I miss something? I don't see where your "tested this out" refers back to, sorry.

As to any 'butt dyno' I happen to use, I have a reasonable and repeatable test area - the long, smooth 35MPH hill a stone's throw from where I live. I simply attempt to maintain 35MPH on that hill.

If the automatic transmission is forced, consistantly, to shift to a lower gear while using 87 octane, and not when using 92 octane, the engine and transmission control units have plainly communicated that there is a difference.

And Toyota reccomends

"Fuel type: Unleaded gasoline, Octane Rating 87 or higher"

They are clearly stating that 87 octane is the MINIMUM fuel grade to use, not the ONLY fuel grade.

I'll be the first to admit that the effects are subtle, and that the difference in fuel milage is well within the normal variance and error margins. I will also say that there is little, if any, difference in driving at freeway speeds or on flat land - only on hills. What differences I may think are there on flat land could easily be false, which is why I do not reference them AT ALL in this discussion. I depend only on repeatable and readily observable difference in the gear chosen by the vehicle, under essentially identical conditions, repeated a huge number of times with two different fuels.

(Discussions like this are welcome - they help clarify the results, and help to eliminate any false trails. :D )

Subcompact Culture

I was referring to djct_watt's post above mine.

Anyway, now you've got me curious because the manual says "or higher."

Some cars I've had specifically say to use 87 or milage and performance will go down (My dad's Jeep Cherokee, for instance).

Maybe I'll throw midgrade in there and see if there are any benefits. The question is, at $3 a gallon, would they be worth it?

Tomas

Indeed, that is a big question.

In my neck of the woods the difference in cost between 87 and 92 octane Chevron gasoline is about 6%, so for the decision to be made only on cost savings there would need to be a 6% or greater difference in the MPG...

Now that I have changed tire size from stock, I have not run sufficient tanks of fuel in the two grades to prove or disprove any economic advantage of one over the other. (The new tire size, being 3% larger in diameter, increases the instances where the ECU would likely retard timing to prevent pinging. All else being equal, this would increase the number of opportunities for the transmission to remain in a higher gear with the 92 octane, which may increase my 'city' MPG more than it does now. We'll see.)

Prior to this, with the stock tire size, 92 as opposed to 87 octane appeared to 'break even' on cost, so long as the majority of my driving was urban/suburban 40MPH or less (which it usually is).

On longer freeway trips there was essentially no difference in MPG, so the higher priced fuel was not the economic choice.

So much affects this if one looks only at 'which will cost me less' - terrain flatness, mix of city/highway, driving style, tire size, etc., etc. If one also mixes in the 'feel' of the vehicle, its responsiveness, and weights the decision with that, also, one might be willing to spend a bit more per mile. Personal decision.

BrianxB

I like this thread, very well thought out Tom. I can only add my comments from a manual trans perspective.

Ive run 87-89oct in my car most of the time that Ive owned it but I can say that now that it is getting over 20K miles on the odometer that when i put 93 or 94 octane in that it cruises very smooth at 65-80 mph and I notice that I do not downshift as much to pass people.

Match_Box

all i know is,

Using 87 octane:
1. My box pings and i can hear it even cruising at low speeds
2.I just get 27-28mpg

Using 93 octane:
1. Reduced the pinging
2. I have gotten avg 32-33mpg by using 93 octane. Thats a 5-6mpg increase over using 87 oct.

Compared to the USA we have "lower" prices on fuel.
Regular gas, 87-88octane, is around 69 to 71 cents per liter($2.60-$2.68 per gallon)
Premium gas, 93oct, is around 75 to 79 cents per liter($2.83-$2.98 per gallon)

milesm4

I run super (92 octane) the reason being I accidentally bought it when I moved to Vancouver from Portland and had to pump my own gas. I noticed that when cruising at 60-65 on the freeway when I let off the gas the car doesn't "jerk a bit". So since then I run super on purpose, smooth cruisin.

Subcompact Culture

One thing I can't believe is that a new car is pinging! If my car started pinging, I'd bring it to the dealer ASAP!

SaintJ

My understand on the ecu side, is based on the combustion of the fuel. I've said this in some other post about fuel and mpg, the ecu's these days, from what i've seen and not just toyota, uses a series of look up tables/graphs to calculate when in the timing to ignite. after using fuel like 87 octain, the graphs would be adjusted to the spark, and combustion of the fuel, dependent on load and load demand. switching to 92 changes teh graphs and may change the timing by a miniscuel amount as mentioned. i havn't noticed pinging with 87, but i drive relitively level highways

RaginCajun

When you add it up with the supreme being only .20 more per gallon than regular. At most every fill up you will only pay $2.40 more. Thats 12 gallons. Its worth it for me.

LilBlkBox

Everything is very interesting, but what about using......should I say it............an "Octane Booster?"

Putting in 87 octane and an octane booster.

Lets hear it.

Tomas

(1) I trust Chevron knows a bit more about properly mixing a 92 octane fuel.

(2) The difference (locally) between 10 gallons of 87 and 10 gallons of 92 octane gasoline is $2.00, and I suspect that *may* be less expensive that adding an additive, and certainly much surer.

(3) Adding ethanol to gasoline increases it's octane (ethanol is about 110 octane) but does NOT increase one's milage: Gasoline has much higher energy content than ethanol, 124,000 BTU/gallon as opposed to 74,000 BTU/gallon.

The reason I mentioned the last item is many "octane boosters" contain alcohol as one of their main ingredients... (The fact that alcohol only has about half the energy per gallon of gasoline is why adding 10% alcohol can reduce milage nearly 5%...)

Bottom line is I'm not even convinced that adding a pint of mouse milk to the tank will actually make 10 gallons of 87 magically behave like 92-93 octane - and if the gasoline already has 10% ethanol in it, adding more may boost the amount of ethanol above the amount (10%) that Toyota allows to be used in it's engines (potential warranty breaker...).

Grimgrak

Well this is clearly the case for a cheap coolingmist water injection system with a vacuum triggered switch.