Premium vs Regular: The octane thread

[jcaputo]

-------------The backstory---------------

I know the owner's manual recommends premium only. But being the skeptic that I am, I decided to test it to see what difference it really makes. With rising gas costs and all, I wanted to find out which was cheaper in the long run, regular (87) or premium (93). I find it difficult to imagine that subaru would sell a car that can't run on 87, even though it does run better on 93.

What if you're stuck in the middle of nowhere, run out of gas, and the only thing available is 87? Well somewhere along the lines subaru made a decision to tune the car for 93 (or 91?), and put safeguards in place that retard timing if it detonates, which the ECU assumes is due to crappy gas.

Call me cheap, but I wanted to find out if it was cheaper to use bad gas, let it detonate and retard timing. I wanted to see what effect it had on fuel mileage.

-------------The test-----------------

I decided to run 87 for three tanks of gas, and then run 93 for three tanks of gas. I was concerned about the ECU learning period interfering with test results, so I built in a buffer tank of gas between the two. I.e., I actually have results for 7 tanks, but I'm throwing out number 4 because it was the "relearn tank" going from 87 to 93. I had been running 87 prior to starting the test, so there was no need to use a buffer tank initially.

The test had some (subjective) rules. I planned to drive in a similar manner on each tank, with a similar amount of spirited driving. Obviously things like traffic were beyond my control. I also planned to calculate mileage in the same way each time. When I filled up, I divide the number of miles on the trip odometer by the number of gallons of gas I put in. When pumping, as soon as the nozzle stopped pumping the first time, I stopped, i.e. I wasn't topping it off.

---------------The Data-------------------------
Data.bmp

----------------The Results----------------------

Ok, I'll admit it. I'm an engineer. This is way beyond the amount of analysis you would need to do to get results from this test, but here goes.

A simple average of the mileage shows that there is a difference in using 87 and 93. The mean of the mileage for 87 was 25.57, and the mean for 93 was 27.80. I achieved 8.7% better mileage using 93, so in order for it to be cheaper, 93 would have to be less than 8.7% more than expensive than 87. Just for kicks, let's say 87 is $3.50 today, as long as 93 is less than $3.80 it is cheaper to use 93.

From what I've seen during the test, 93 seems to be on average about $0.25 more per gallon and 87. This means that as long as regular is more than $2.87 per gallon ($0.25/8.7%), it will be cheaper to use 93.

Just for kicks I did a statistical analysis of the data, to determine if they are different. The test I used is called a 2-sample t-test. This test is used to determine if two sets of data come from the same population (i.e., that they have the same mean and variance).

The results of this test are shown below.
Stat_Data.jpg

This test just tells us that we can say with 99% confidence that there is a different in the two sets of data, i.e. that they have a different mean and/or variance. Not particularly useful in an obvious case like this, but interesting anyway.

-----------------The Conclusion-------------------

After the test I can confidently say that it is cheaper to use 93 in a stock 05 5-speed wrx driven by me in the traffic conditions I see. What does this mean for you? I'm not sure. But it probably holds true.

How much cheaper are we talking here? Let's use the $3.50 and $3.75 numbers for 87 and 93, and the means of the fuel mileage above. Let's assume you drive 15,000 miles per year. Fuel cost for 87 would be $2053, fuel cost for 93 would be $2023. The average driver would save a whopping $30 per year.

There are, of course, added benefits of 93. The car runs better. I had problems starting my car in the rain/cold using 87, which I have never had using 93. (I actually took it to the dealer to see if they could diagnose the problem, and they couldn't). There is more power due to proper timing using 93, though the butt dyno doesn't really register it. I also feel better giving the car 93, since that's what I know it wants. And I get a smug sense of satisfaction knowing that I am screwing the oil companies out of $30 per year.

[Trainrex]

Ray's long lost brother!

[chrisp]

holy crap dude! I would have just done what the manual said and not worried about the extra 2 DOLLARS it costs to use the more expensive gas and better gas.

[20WRX04]

Nice write up.

[jcaputo]

Ray's long lost brother!

Wow, quite a compliment. Thanks!

I hope that when newbies and potential owners ask whether or not they have to use premium, we can point them to this thread.

holy crap dude! I would have just done what the manual said and not worried about the extra 2 DOLLARS it costs to use the more expensive gas and better gas.

The manual also says to observe the speed limit and avoid racing or race-like activities.

Plus, now you know that it's cheaper, not an extra 2 dollars.

Nice write up.

Thanks!

[RayfieldsWRX]

Wow, quite a compliment. Thanks!

Probably not how he meant it.

Great experiment. Nice box plot. Want to come work for me in a quality department?



Hint: No, you don't.

[Timdog1650]

Very cool experiment. I like your thought process. I also like how the data reinforced what I have long suspected: When the ecu is running its most aggressive/optimized map, your fuel economy will benefit. When the ECU is in damage control mode from running poor gas, fuel economy suffers.

Also, not that your experimental method wasn't good, but I wish there was a much larger sample set of data here to back this hypothesis up with various levels of modification, climate, altitude, driving style, etc.

Good stuff.

~Tim

[06STImulus]

since i cant find 93 i just use 91... because when i think about it i'm only paying like a $1-2 per gallon when i fill up anyways... when i had my prelude it also only recommended 91 and honestly i could tell the difference when i filled it with 87... so in conclusion... 91 ftw lol

[jcaputo]

Probably not how he meant it.

Great experiment. Nice box plot. Want to come work for me in a quality department?



Hint: No, you don't.

That's how I took it anyway. C'mon Ray, you're a respected member of the community, being compared to you is a good thing.

HAHA, quality isn't really my thing. I had enough of it going through lean six sigma training.

Very cool experiment. I like your thought process. I also like how the data reinforced what I have long suspected: When the ecu is running its most aggressive/optimized map, your fuel economy will benefit. When the ECU is in damage control mode from running poor gas, fuel economy suffers.

Also, not that your experimental method wasn't good, but I wish there was a much larger sample set of data here to back this hypothesis up with various levels of modification, climate, altitude, driving style, etc.

Good stuff.

~Tim

Thanks Tim. I also suspected that fuel economy would benefit, but I was quite surprised to see just how much it went up. Nearly a 10% jump switching from 87 to 93 was not what I expected to see.

I don't have the capacity to collect a whole lot more data, but anyone who'd like to send me data on their car, I'd be happy to do the analysis. Though I can't imagine too many owners of tuned cars are going to be willing to throw 87 in the tank just to see what happens.

[jcaputo]

since i cant find 93 i just use 91... because when i think about it i'm only paying like a $1-2 per gallon when i fill up anyways... when i had my prelude it also only recommended 91 and honestly i could tell the difference when i filled it with 87... so in conclusion... 91 ftw lol

I'd be interested to see if there is as big of a difference between using 87 and 91 as I saw using 87 and 93. Care to be a guinea pig?

[wrx wagone]

Nice work!

[Speedette]

Very interesting write up.

But I had a couple quick questions:
What gas were you running in your car prior to the first tank of 87 that was in there prior to the start of the test?

Did you fill up at the same gas station each time, and did you go at the same time period (i.e. morning, noon, night) each time for the length of the test?

Variables
Learned car behavior prior to testing
Time of fill up (i.e. gas is denser in the early mornings)
Gas station (Gas stations vary on content in their gas)
Other variables congruent with this thread: http://www.clubwrx.net/forums/off-to...-gas-read.html


In conclusion, there could be many other factors affecting the study that if you chose a different set of variables you could have entirely different results showing that 87 gas is the better bang for your dollar. However, I'll stick with my 93 because it gives me peace of mind.

Ray's long lost brother!

Exactly what I was thinking.

[jcaputo]

Nice work!

Thanks!

Very interesting write up.

But I had a couple quick questions:
What gas were you running in your car prior to the first tank of 87 that was in there prior to the start of the test?

Did you fill up at the same gas station each time, and did you go at the same time period (i.e. morning, noon, night) each time for the length of the test?

Variables
Learned car behavior prior to testing
Time of fill up (i.e. gas is denser in the early mornings)
Gas station (Gas stations vary on content in their gas)
Other variables congruent with this thread: http://www.clubwrx.net/forums/off-to...-gas-read.html


In conclusion, there could be many other factors affecting the study that if you chose a different set of variables you could have entirely different results showing that 87 gas is the better bang for your dollar. However, I'll stick with my 93 because it gives me peace of mind.

Thanks. I was running 87 prior to the start of the test. So you could say that the test included 2 "learner tanks", one 87 before the 87 tests, and one 93 before the 93 tests.

I did not fill up at the same station, pump, or time for the length of the test. I did think about it, but it would have really been a pain to plan. As far as the "more for your money" pumping in the early morning theory, I agree that you will get more gas for your money, however the change is even less significant than the money you'll save according to my study. See Here.

I wanted the study to be indicative of a typical driver, while following some good experimental practices. Typical drivers don't fill up at the same station, same pump, same time of day all the time. They get gas when they need it from wherever is closer/cheaper. So that was what I did.

[Speedette]

I did not fill up at the same station, pump, or time for the length of the test. I did think about it, but it would have really been a pain to plan. As far as the "more for your money" pumping in the early morning theory, I agree that you will get more gas for your money, however the change is even less significant than the money you'll save according to my study. See Here.

Interesting find. Should link that in the other thread I posted to see what kind of feedback the OP would illicit. Though frankly, I don't know the credentials of anyone on the internet and as far as I know, everyone is wrong and I'm always right.

I wanted the study to be indicative of a typical driver, while following some good experimental practices. Typical drivers don't fill up at the same station, same pump, same time of day all the time. They get gas when they need it from wherever is closer/cheaper. So that was what I did.

That's a very good point. Though I believe a typical driver has a favorite/familiar/local gas station that they go to all the time (people like routine), the day and time that they fill up will vary and the variables that could effect your gas content are then negligible. Overall it was a really good study and hopefully people will continue to fill their Subarus (or other performance vehicles) with 93 octane.

Side note: do you think you'd see the same increase when using 93 octane versus 96-100 octane (race gas)?

[Micah]

Side note: do you think you'd see the same increase when using 93 octane versus 96-100 octane (race gas)?

If the ECU isn't tuned for it, I don't think there would be much benefit other than a better resistance to detonation from the higher octane.