Can anyone explain to me how a motor running with 10.6:1 compression uses 87 octane gas without knocking? All else held equal, ignition timing would need to be retarded, right? In theory, how would that affect performance and mileage?

I get similar results in my echo. My last 2 tanks I gassed up with 89 octane instead of 87. Eventhough it got a lot colder here my mileage went up from about 32 to 37. That is a pretty significant increase. My driving habbits have not changed between the two tanks and it only got colder. I will continue to use 89 octane gas.

My HCH got 52.48 MPG on my 91 Octane test tank, which was my 2nd highest tank ever (except one hwy run of 477 miles). And in the middle of winter.

My current tank is showing 50.3 with 87 Octane.

I still think there might be something to the Octane thing.....not through testing yet, but I'm suspicious !!!

freak lad, do you happen to know the echo's compression ratio?

I misstated the ratio for the HCH by a little - the manual says it's actually 10.8:1. That's pretty dern high. I'm not even close to being a mechanic, but I thought high compression motors needed high octane to run their best. What ratios do the other hybrids use? I have some thoughts as to why Honda would recommend 87, but I'll hold off on speculating until I'm done with my own test.

I'm on my first tank of 91 right now. I put my first 70 miles on it yesterday in the rain and ended up at 51.6 per the computer, 1.3 higher than the end of my last tank of 87. The weather during that tank was generally nice, this is Phoenix after all.

lars-ss, please do keep us posted. I recently purchased a 2008 Prius and the manual says to use "87 or higher." On my second tank (filled up this evening), I went ahead and put in 91 octane and am wondering if there will be any difference. I cannot help but think that if there was a danger in using higher than 87, the manual would have specifically mentioned that. Thanks for the tests that you are running!

My Insight stays in lean burn longer if I use 93 octane

I get more power and don't need to shift down when climbing small hills.

So yes It gives me hihger MPG.

- reading -----

no danger at all. The manual is stating a minimum.

When I was running with E30 gas, which is 'oxygenated' my HCH did run smother. The E85 gas I used to mix with regular is rated about 100 octane.

-Fadi

1991 Mazda Miata to beat Corvettes at the track
1999 Dodge Ram to climb through rocks and brush
2004 Honda Civic Hybrid to make up for the other 2

Hi,

Last spring, I ran a series of tests that included 87 and 93 octane gasoline bought in Huntsville AL and was able to generate this chart:

I tested 87 and 93 octane:

• Shell
• Texaco

What I found is only Shell 87 delivered consistently higher shaft power per gram of fuel. I was able to observe that closer to maximum power, the higher octane fuels seemed to be 'less bad.' The engine was able to tune itself to use the higher octane fuel but never did it equal the 87 octane fuel performance.

Subsequent testing revealed that only Chevron and Exxon 87 had higher than average energy per unit of fuel, similar to Shell 87. BUt these results were only found using the gasoline brands available in Huntsville AL. Other regions are likely to have different options and these brands may not be the best performing ones to use.

Testing the efficiency of different brands is not a trivial problem, especially when whole tanks are used. I'm lucky to have exceptional instrumentation in my 2003 Prius that allows me to read out the ICE shaft power. But one other approach would be to find the tallest hill that can be climbed on cruise control and see if a series of hill climb test, MPG readings could be used to estimate relative fuel efficiency. This would be stronger than trying to compare 'tanks' under different driving and weather conditions.

GOOD LUCK!
Bob Wilson

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

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I put in 91 last tank. and wouldn't you know it, we're experiancing a freakishly warm week so my MPGs is up by about 3. Don't know if it's because the weather, or the fuel. Meh, at least I don't have to bundle up this week...

-Fadi

1991 Mazda Miata to beat Corvettes at the track
1999 Dodge Ram to climb through rocks and brush
2004 Honda Civic Hybrid to make up for the other 2

I was really impressed with your post about the Octane rating! It confirmed some of the myths about higher octane gas. I was curious as to how you obtained the data from the car? You must have some type of computer interface to the car.
>

>
> I have a Hybrid Camry and I would be interested in collecting this data. I spent some time collecting information related to fuel density when I first bought the car last summer. I was measuring the temperature of the gas and found some big differences between stations. I settled in on Phillip 66 that was always 60 degrees (85 Octane) In Colorado we have lower Octane due to our elevation.
> What is the true value of the gasoline? The price is getting high enough to start asking these questions.
>
> I appreciate your help.

Bill