Octane Rating

[Jcanracer]

Actually there is a lot to be said for that level of consistency. For one, your mpg calculations are always consistent if its the same fuel, and if you calculate mpg by fill-up receipts its always best to use the same station if possible.

 

[CX-SV]

Actually there is a lot to be said for that level of consistency. For one, your mpg calculations are always consistent if its the same fuel, and if you calculate mpg by fill-up receipts its always best to use the same station if possible.

Yes, consistency and convenience via Chevron, no cash involved with my gas purchases.

In CA our weights/measure controls and oversight is very good, for accuracy. I use a half dozen stations, Chevron only.

I track my mpg on spreadsheet (by the tank and cummlative) with miles to the tenth and gallons to 3 decimal places. I'm have not reset '"avg mpg" since delivery, I'm finding it to be 3% optomistic.

 

[cucamelsmd15]

Mainly what the engine is capable of doing with it.... true.
But energy potential being the same is incorrect.

The energy potential is determined in the test engines that dictate the octane a particular fuel can display on the pump.
So by definition, the higher octane fuels have higher energy potentials (and potential to knock if taking MON mainly into account).

The AKI Octane value display on US/Canadian pumps is an average of RON and MON (R+M/2).

To get AKI, test engines are used which have extremely variable compression ratios (for the RON value) and variable ignition timing (for the MON value).

Simplifying the subject somewhat, the higher the Octane the higher the activation energy (or energy extracted from the ignition).

Uh, I think you're a little confused on your terminology. You are correct that the higher the octane, the higher the activation energy. However, activation energy does not translate to energy at ignition. For example, alcohols (specifically speaking of methanol and ethanol) have far higher activation energies than gasoline. This is why the "105 octane" commonly assigned to E85 is patently false, because you cant assign octane to neat oxygenates, but I digress. For simplicity sake, Ill compare E85 and gasoline. While the activation energy of E85 is much higher, the potential energy in a gallon of E85 is much lower. A gallon of gasoline has a potential energy of 114,500 BTU, whereas E85 has a potential energy of 81,800 BTU. Note that the RON/MON test theory is not ideal for alcohol based fuels because its extremely resistant to compression based detonation as well as timing based detonation thanks to its evaporative cooling properties. You're also incorrect, test engines do not determine energy potential, they determine activation energy only. This is why the standard for determining RON and MON is isooctane.

 

[Jcanracer]

For simplicity sake, Ill compare E85 and gasoline. While the activation energy of E85 is much higher, the potential energy in a gallon of E85 is much lower. A gallon of gasoline has a potential energy of 114,500 BTU, whereas E85 has a potential energy of 81,800 BTU.

^^^ quoted for truth.

 

[CX-5 Rat]

No kidding, "mind tricks" and placebo effect mostly. Although some are reporting reduced mileage. The main significant and measurable impact will be on wallet for owners of CX-5 engineered to be run on regular gas.

Indeed. That and the infamous butt dyno are so prevalent on pretty much every car forum.

 

[mr.fusion]

For you guys in areas where the cheapest gas is 87, it's an absolute no brainer to use 87. When the 87 costs more than 89/91, then you have to figure out which gives the better value in cost per mile.

I think the bigger issue might be the differences in quality of fuel from one area (or station) to the next.

 

[ta240]

I tried the 91 in mine for about 1500 miles and switched back to 87 and noticed no drop in performance and if anything the mpg reading ticked up .1 so no more 91 for me.

 

[CX-SV]

I tried the 91 in mine for about 1500 miles and switched back to 87 and noticed no drop in performance and if anything the mpg reading ticked up .1 so no more 91 for me.

Thanks for the report ta240, good to hear feedback based on actual driving for 1500 miles (versus speculation, butt dynos, people staring at the mpg meter real-time, etc.).

Conclusion: Most significant impact was the extra $10 in petty cash you paid for gas during that 1500 miles.

 

[nyctravis]

I somehow seem to have lost the average mpg in my display. Gotta pull out the manual to see how to get it back, but does anyone know off the top of their head?

 

[CX-SV]

Is it the info button (if I remember correctly) on left side of steering wheel (up/down arrow) to select avg speed, avg gas mileage, range, etc.

 

[Jcanracer]

Thanks for the report ta240, good to hear feedback based on actual driving for 1500 miles (versus speculation, butt dynos, people staring at the mpg meter real-time, etc.).
Conclusion: Most significant impact was the extra $10 in petty cash you paid for gas during that 1500 miles.

ouch, that bold text was for me I think. I'll admit, I earned that. Funny thing is that I was getting bad mileage with the 93 octane until the last 1/4 tank when I had a chance to hyper-mile it a bit and pulled off 29.5mpg for the tankful. So I ruined my own experiment (bang)

 

[nyctravis]

Is it the info button (if I remember correctly) on right side of steering wheel (up/down arrow) to select avg speed, avg gas mileage, range, etc.

aha! I wondered what that info button was for. Never paid attention to it in the manual as the dealer said it was for tech package only. So I just ignored it and moved on. Thanks

 

[CX-SV]

ouch, that bold text was for me I think. I'll admit, I earned that. Funny thing is that I was getting bad mileage with the 93 octane until the last 1/4 tank when I had a chance to hyper-mile it a bit and pulled off 29.5mpg for the tankful. So I ruined my own experiment (bang)

No way, I wasn't trying to single you out, lol. Gotta admire your sense of humor on this one though. But I hope you didn't have any near crashes while "staring at the mpg meter".

 

[Jcanracer]

LOL no, I am used to occasionally glancing at the dash readout because in my last car, I installed a ScanguageII so I could clear CEL codes and monitor MPG as I was driving.
I hate texting and driving, but monitoring one or two data points somehow doesn't feel as distracting.

Once I hooked up my laptop to my old car to tune the piggyback ECU on the fly, now THAT was damn dangerous. I ended up just recording logs and reviewing them when I pulled over, otherwise I would have crashed for sure!

 

[Thunderchildd]

Good discussion here, but I am going to choose to believe Mazda's engineers:

We also spoke to Mazda engineer Masuhiro Mora, who told us that although the American CX-5 is tuned to run right as rain on 87 octane, there's likely to be at least a modest performance benefit to filling it up with 91 when we eventually test it. "With 91, you should have slightly better torque in theory," he said.

http://blogs.insideline.com/straigh...86-seconds-plus-cameo-by-patrick-dempsey.html

I will be running 91.

 

[CX-SV]

"We also spoke to Mazda engineer Masuhiro Mora, who told us that although the American CX-5 is tuned to run right as rain on 87 octane, there's likely to be at least a modest performance benefit to filling it up with 91 when we eventually test it. "With 91, you should have slightly better torque in theory," he said."


That essentially says nothing, an experiment that never happened, words such as "likely" "modest" and "slightly better torque in theory" blah, blah, blah. And 7 months later, no followup, no test.

I'm going to believe engineers at Mazda that actually finished the job and specified regular gasoline in the Mazda CX-5 owners manual. I will be running 87, until Mr Mora gets off duff (not likely).

 

[Thunderchildd]

"We also spoke to Mazda engineer Masuhiro Mora, who told us that although the American CX-5 is tuned to run right as rain on 87 octane, there's likely to be at least a modest performance benefit to filling it up with 91 when we eventually test it. "With 91, you should have slightly better torque in theory," he said."


That essentially says nothing, an experiment that never happened, words such as "likely" "modest" and "slightly better torque in theory" is the best he could do. And many months later, no followup, no test.

I'm going to believe engineers at Mazda that actually finished the job and specified regular gasoline in the Mazda CX-5 owners manual. I will be running 87, until Mr Mora gets off duff and proves his empty guess.

To each his own. You are correct that there are no controlled scientific tests from which to base a decision. We are left with speculation; Speculation of a Mazda Engineer vs speculation of random forum posters. I choose the former. If his is an "empty guess" then what does that make everyone else's? In the end I choose to spend the (minimal) extra money, as is my prerogative, because I believe there is benefit.

 

[CX-SV]

To each his own. You are correct that there are no controlled scientific tests from which to base a decision. We are left with speculation; Speculation of a Mazda Engineer vs speculation of random forum posters. I choose the former. If his is an "empty guess" then what does that make everyone else's? In the end I choose to spend the (minimal) extra money, as is my prerogative, because I believe there is benefit.

Yes, freedom of choice for sure. At least the Mazda owners manual and related recommendations is not the result of (more) random speculation. Extra gas expense of 5% is petty cash for most of us, but still it's most likely the biggest significant difference resulting from the use of premium.

 

[hotrabbitsoup]

it sure would be nice for mazda or an aftermarket tuner to release somthing that would allow us to program the engine for 14:1 as I understand is possible with the skyactiv engine. i'd be willing to pay a little extra for 91 octane for a little extra performance and efficiency.

 

[EricF]

it sure would be nice for mazda or an aftermarket tuner to release somthing that would allow us to program the engine for 14:1 as I understand is possible with the skyactiv engine. i'd be willing to pay a little extra for 91 octane for a little extra performance and efficiency.

There is no program in the world that will increase the compression ratio.