Low octane causing anyone else problems?

My vote, assuming that there isn't some other factor (or combo thereof), is also for bad gas. Although the Prius isn't like some of its Toyota siblings that are designed to take advantage of premium (via knock sensors and allowing the ECU to advance spark timing) to increase performance, it does still have a knock sensor, which it used to protect itself from knock. Sensing knock, the 1NZ will retard its spark timing. That should quell the knock, but will reduce output too, and impact mileage. I would theorize that you got some poor fuel, which caused some knock. Unlike in older cars though, you never heard it, since your engine "took care of it" for you before it escalated to where a human could detect it. Now, with some premium fortifying the swill you got earlier, the fuel is less knock prone, and the engine has re-advanced it's timing, giving you back what the earlier retard took away.

 

All these are valid points, but my experience with Shell gas is that I consistently get greater mileage with the premium formula.. Although the Prius engine may not have the compression ratios to take full advantage of the higher octane level, the fuel itself may be formulated with a net higher energy content, which any engine may derive *some* benefit from, regardless of its octane requirement.. I think the old "rule of thumb" that higher octane = less energy in fuel is not necessarily true in all cases anymore..

 

Have you taken this same trip before? Are there any elevetion changes on the trip? I ask because I make a regular trip From Boulder to Downtown Denver, and almost always get about 8 mpg better mileage heading to Denver than returning home. I think it's the combination of the prevailing winds and the minor elevation changes.

 

<div class='quotetop'>QUOTE(c4 @ Aug 14 2006, 10:01 AM) [snapback]302966[/snapback]</div>

That's really not possible.

No Octane boosting additive that I know of has more specific energy than gasoline. 99.99% of them (I won't say 'all' just to be covered), have less.

What is poossible is that the 'mix' of oxygenates varied in the two locations. The US requirements for oxygenate mixes are an insane crazy-quilt. Maybe your initial tank was 10% ethanol, and your second wasn't.

The only possible way that a higher octane gas could give better mileage in a Prius is if you were running crap gas and had some deposit buildup that the detergent in 'quality' gas cleaned.

That's why the Top Tier gas program where manufacturers pledge to put decent detergent packages in all grades is so important.

Plus, high octane gas will damage the emissions control system in the Prius because it does burn less efficiently and dumps unburned gunk into the cats.

You can research all over the place and NO reputable source will ever tell you that you'll get higher mileage with high octane in an engine designed for regular. Just isn't going to happen.

If deposits are your problem, buy a better class of Regular or dump a bottle of Techron in with every oil change.

 

<div class='quotetop'>QUOTE(tomdeimos @ Aug 14 2006, 12:38 PM) [snapback]303028[/snapback]</div>

Sounds to me like this is all air conditioning related.

 

i vote for 10% ethanol in the first tank, and no ethanol on the fill up. that would give you better mpg on the second tank, bc ethanol has less energy than gas

 

but... there is no more energy in high octane gas than there is in low octane gas.

regular use of high octane can cause starting problems- it's documented in the prius tech course manual anyway, and i'm sure they put it in there for a reason.

 

<div class='quotetop'>QUOTE(galaxee @ Aug 14 2006, 07:26 PM) [snapback]303243[/snapback]</div>

Galaxee-
That is good news for me. I don't want to spend more for gas than I am already paying. The lowest price around here is $3.09. And believe me, I am spotting the stations. They range from the 3.09 to 3.22 for regular.

What is the reference to Garfield?

 

<div class='quotetop'>QUOTE(Tempus @ Aug 14 2006, 02:33 PM) [snapback]303149[/snapback]</div>

Sure it is, for several reasons, one of which you alluded to below. First, the BTU content of gasoline products can and does vary. But not in a way that would help much here. Generally, lower octane gasolines have higher BTU content (yes, higher), slightly so, than the higher octane choices. Of course, this doesn't help the OP. Second, the inclusion of ethanol reduces the BTU content of the final fuel product significantly, hence the now almost universal reports of reduced mpg with ethanol "contaminated" gasolines (OK, OK, I know there's a reason for it being there, but I don't like it...).


<div class='quotetop'>QUOTE(Tempus @ Aug 14 2006, 02:33 PM) [snapback]303149[/snapback]</div>

Respectfully, this is incorrect. As is noted in several of the Prius maintenance documents, the 1NZ engine has the ability to retard its ignition timing if the knock sensor pick up bad enough indications of knock. Most often, that would happen as a result of getting a tank of subpar, bad gas. While the retarding of igntion timing will suppress all but the worst case of knock, the engine's fuel efficiency will also be reduced as a result. Thus, the mere presence of bad gas, irrespective of whether it also resulted in some deposits, could significantly reduce mpg. Then, in turn, with the addition of enough premium gas to offset the remaining poor quality gas (average octane in the tank increased), the engine, no longer knocking because of bad fuel, will automatically re-advance spark timing, and regain normal fuel efficiency.


<div class='quotetop'>QUOTE(Tempus @ Aug 14 2006, 02:33 PM) [snapback]303149[/snapback]</div>

I've been through most of the Toyota tech pubs on this car (visit techinfo.toyota.com), and there's no warning that premium gas will harm the emissions system. There are warnings that it's totally not necessary, and not to use it as it may cause starting problems, but emissions, where did that come from? Premium gas may burn more slowly than regular, but not necessarily less efficiently than regular. Finally, the whole point of the catalytic converter system is precisely to burn off remaining unburned fuel. Their entire reason for being is to have "unburned gunk" dumped into them, so that they can convert it into nice, clean, totally burned, no-longer-gunk.

 

<div class='quotetop'>QUOTE(c4 @ Aug 14 2006, 07:01 AM) [snapback]302966[/snapback]</div>

Not possible, as far as I know.

By definition, higher octane gas has *less* energy content, regardless of what the advertisements imply. The products that increase the octane number contain less energy than gasoline. If your vehicle requiers higher octane, then higher octane will allow for higher power output of that engine. Higher octane gas will NOT increase power in an engine that wouldn't knock otherwise.

My vote: Bad gas. But damn. I want some of that bad gas that gives 52mpg at 65!

 

<div class='quotetop'>QUOTE(ekpolk @ Aug 14 2006, 08:44 PM) [snapback]303264[/snapback]</div>

You clearly didn't look hard enough.

There are several references to Emissions Related CELs from high octane fuel. They are directly related to incomplete combustion.

As for the rest of your points.

Yes, in extreme cases anything is possible. But, barring bizzarro gas formulations (for example, some using aromatics like toluene or benzene), or truely 'bad' gasoline (which can, of course, occur in any supposed octane rating), the rest of what I said is perfectly true 99.9% of the time.

 

<div class='quotetop'>QUOTE(Tempus @ Aug 14 2006, 09:14 PM) [snapback]303300[/snapback]</div>

OK, I just went back to the techinfo.Toyota site, and re-ran several searches of service bulletins (a/k/a TSBs), the repair manual, campaign publications, tech training references, and the Technician's References, and did not find any indication that use of premium gas would result in emissions issues or MILs. I did find repeated statements that premium will not provide a mileage benefit (so at least we know where Toyota stands...), and warnings that it may cause starting problems, and one indication that it might cause misfires. Applying some thought to it, I could see where either starting or misfire problems might be read as an emissions issue, but again, no ref to it in the docs I saw. Look, no point in us engaging in mutual skull bashing, if they're there, I certainly could have missed them, and would appreciate you pointing me to where they can be found. Citations? Links?

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<div class='quotetop'>QUOTE(Tempus @ Aug 14 2006, 09:14 PM) [snapback]303300[/snapback]</div>

I respectfully disagree.

First, while the BTU content of various "pure" gasolines does vary, it is only slightly, and generally more BTUs in lower octane products. But now, with the introduction of ethanol blends, the "energy content" or BTUs of fuels is dropping, substantially. Far from bizarre, it's quite possible for someone today to get a tank of ethanol blend one place, and then get some pure gasoline somewhere else. This may well cause confusion as customers see mileage fluctuate, while observing no apparent, obvious cause.

Second, you stated that, “the only possible way that a higher octane gas could give better mileage in a Prius is if you were running crap gas and had some deposit buildup that the detergent in 'quality' gas cleaned.†That’s not correct 99.9% of the time; it’s simply incorrect. The Toyota tech training briefing says this about the knock sensor in the 1NZ: “The Knock Sensor is mounted on the cylinder block and detects detonation or knocking in the engine. The sensor contains a piezoelectric element that generates a voltage when cylinder block vibrations due to knocking deform the sensor. If engine knocking occurs, ignition timing is retarded until the knock is suppressed.†As this occurs, the engine’s efficiency and output will be reduced. If fuel of sufficient, adequate octane, is again fed to the engine, and the knocking goes away, efficiency and output will be restored.

Finally, you also referred to “dump[ing] unburned gunk into the cats.†Unless I’m reading this incorrectly, you’re implying that this is not a good thing. But that’s exactly what the “cats†are there for – to have unburned gunk dumped into them and then process (i.e. “burn†the gunk, namely fuel) so that it does not go straight out into the air.

In the end, I think we'd agree that there is simply no reason to use premium in a Prius, and several reasons not to use it. About the only exception I can think of would be to combat a known tank of bad fuel, which is not as uncommon (in the last two years, I've had two that should have been premium, but clearly weren't, in my previous car which called for 91 or above). If you had, for example, a tank of poor fuel that was really, say 84 instead of 87, and you could feel it causing ping or knock (beyond the engine's ability to adjust), a few gallons of 93 might be enough to raise the avg octane in the tank to a safe level.