Low octane causing anyone else problems?

[tomdeimos]

Today I took a trip to PA and got really bad mpg on a tank of Mobil from my usual station.
It normally gets me reasonable mpg. mpg drop was noticable soon as I filled up.

Arrived in PA with 52.8 mpg, my worst ever for a long trip. Speeds were mostly 65 mph. There was no wind that I could detect. Temperatures mostly low 70s.

I was down to about 1/2 tank so I filled up in PA and used Shell premium fuel. MPG went to the best I've ever had on a trip like this. 59 mpg!

This was what I read at about the CT border! I consider the travel conditions similar down and back up to there.

At that point I got stuck in 10 mph Interstate travel so got off and took back routes home, where the slower travel and lots of stealth boosted my mpg a bit more.

Note my best previous mpg record was 59 with lots of this stealth and slower roads and also mid 80s temps for thinner air but not huge AC loads.

So today conditions were not what I find ideal. But I got my best trip mpg ever coming home. My first time breaking 60 mpg for a big trip. Heres the mpg picture as I arrived home in Lexington. (shows rt 2, 495 and route 225 travel.)

[Tideland Prius]

Like you said, might be bad gas from that station. Premium fuel shouldn't affect the Prius since the engine isn't designed to take advantage of the higher octane fuel.

[ekpolk]

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.

[c4]

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..

[tomdeimos]

I agree the gas is bad. Back in the old days (pre ethanol) I tried out different octanes with no or minor effect.

But today with ethanol in the fuel this problem has happened 3 times and with 3 brands of gasoline. The other two times I boosted octane with a can of booster and also had less control over conditions, so measuring the gain was a problem though I saw improvement.

This time I had as nearly identical before and after conditions as possible. I also used premium gas to bring the octane up because I could not find safe boosters in PA. They seemed to sell just MMT for "off road" use.

[narf]

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.

[tomdeimos]

<div class='quotetop'>QUOTE(narf @ Aug 14 2006, 11:57 AM) [snapback]303003[/snapback]</div>

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.
[/b]

Yes taken the same trip many times either to the same place or beyond further west in PA or south to VA but still same route over the part I went on yesterday. Mostly intestate 84 through CT to NY and PA.

My normal mpg for the trip is typically 55 mpg and might get down toward 53 if it was very slow and hot like over 100 deg so I was using lots of AC. Yesterday in mid 70's at the peak the AC load was very light or turned off.

Lots of elevation changes for the East. up and down, but the end points are not that different. Road goes up and down almost sealevel and up to max of about 1700 feet.

I have taken the trip on windy days and had a few mpg boost with a tail wind going home but wind was very noticable driving and even then I max'd out at maybe 56 mpg. but got 53 going down with strong headwind.

So this trip I did better coming back with no noticable wind than the trips with strong tailwinds. And I was tired so if anything I went home a bit faster at least on the interstate highway portion, plus it was cooler which should normally drop my mpg a bit, since my AC was mostly off both ways.

Note my picture includes my forced off interstate driving due to the traffic jam, so that boost just helped me get a new record. I never broke 60 mpg before on a long highway trip.

The octane effect was the 53 going and the 59 coming back at the point the traffic stalled and I had to take back routes through towns with 30 mph limits between the 50 mph sections, and saw several traffic cops hiding waiting to catch anyone going faster.

So this 6 mpg difference is what I think was due to the octane or other gasoline effects. The other times I've had gasoline problems I've always seen at least a step change of 5 mpg for the 100 miles before fill up to the 100 miles after.

[Tempus]

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

the fuel itself may be formulated with a net higher energy content, [/b]

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.

[sl7vk]

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

Yes taken the same trip many times either to the same place or beyond further west in PA or south to VA but still same route over the part I went on yesterday. Mostly intestate 84 through CT to NY and PA.

My normal mpg for the trip is typically 55 mpg and might get down toward 53 if it was very slow and hot like over 100 deg so I was using lots of AC. Yesterday in mid 70's at the peak the AC load was very light or turned off.

Lots of elevation changes for the East. up and down, but the end points are not that different. Road goes up and down almost sealevel and up to max of about 1700 feet.

I have taken the trip on windy days and had a few mpg boost with a tail wind going home but wind was very noticable driving and even then I max'd out at maybe 56 mpg. but got 53 going down with strong headwind.

So this trip I did better coming back with no noticable wind than the trips with strong tailwinds. And I was tired so if anything I went home a bit faster at least on the interstate highway portion, plus it was cooler which should normally drop my mpg a bit, since my AC was mostly off both ways.

Note my picture includes my forced off interstate driving due to the traffic jam, so that boost just helped me get a new record. I never broke 60 mpg before on a long highway trip.

The octane effect was the 53 going and the 59 coming back at the point the traffic stalled and I had to take back routes through towns with 30 mph limits between the 50 mph sections, and saw several traffic cops hiding waiting to catch anyone going faster.

So this 6 mpg difference is what I think was due to the octane or other gasoline effects. The other times I've had gasoline problems I've always seen at least a step change of 5 mpg for the 100 miles before fill up to the 100 miles after.
[/b]

Sounds to me like this is all air conditioning related.

[smackoww]

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