My car requires 91 but I find it runs just fine on 89 and the fuel economy is the same or better. With 89 I consistently exceed the EPA estimates for city/highway mpg. Out of habit I check the mileage at every fill up.
In truth it depends, the vast majority of cars will get lower MPG and less hp when using lower grade gasoline, but there are a few out there that do get better MPG with lower octane fuel.
The manufacturer will always err on the side of caution, but in some engines rated for 89 or 91, 87 will function almost as well. On high compression, high performance engines,like say a DI Cadillac or Lexus 3.5L V6 making 300+hp, putting 87 in will have much larger consequences than the previous poster's 171hp 2.4L I4. This could be one of the cases where the engineers saw the performance was borderline on 87 and bumped it up to 91 recommended to be safe.
Just from the specifications, you can tell it's not pushing the limits of engine design, 10.4:1 compression while the 3.6L Caddy comes in at 11.3:1 and the Lexus comes in at 11.8:1 so naturally the demands of the Ecotec isn't nearly as high as the 91 octane recommended Direct Injected motors and thus lower octane won't affect it as much.
As for whomever is reading this, don't assume this applies to your car even if ti's similar as this is a highly simplified explanation, we could go down to every little detail and see how it affects gasoline in the combustion chamber since everything from the exhaust, EGR, intake, materials used, etc (some even argue the break in method used will alter fuel burn characteristics from the differences in hone in the cylinder walls acts with the piston rings) affects the way gasoline burns in the engine, this is just a simple way of explaining why Scouzer may get better mpg numbers with lower octane gas. In the end, every engine, even those that seem the same spec wise will treat gasoline differently.
And as always, this will not apply to every engine or even similar engines, the former post about lower octane affecting performance and emissions control reliability still applies to the vast majority of cars. You almost never want to take a definite position since cars are so complex and affected by so many factors, my mentor taught me that you should just say "9 times out of 10" rather than always or definitely when addressing customers about issues.
The manufacturer will always err on the side of caution, but in some engines rated for 89 or 91, 87 will function almost as well. On high compression, high performance engines,like say a DI Cadillac or Lexus 3.5L V6 making 300+hp, putting 87 in will have much larger consequences than the previous poster's 171hp 2.4L I4. This could be one of the cases where the engineers saw the performance was borderline on 87 and bumped it up to 91 recommended to be safe.
Just from the specifications, you can tell it's not pushing the limits of engine design, 10.4:1 compression while the 3.6L Caddy comes in at 11.3:1 and the Lexus comes in at 11.8:1 so naturally the demands of the Ecotec isn't nearly as high as the 91 octane recommended Direct Injected motors and thus lower octane won't affect it as much.
As for whomever is reading this, don't assume this applies to your car even if ti's similar as this is a highly simplified explanation, we could go down to every little detail and see how it affects gasoline in the combustion chamber since everything from the exhaust, EGR, intake, materials used, etc (some even argue the break in method used will alter fuel burn characteristics from the differences in hone in the cylinder walls acts with the piston rings) affects the way gasoline burns in the engine, this is just a simple way of explaining why Scouzer may get better mpg numbers with lower octane gas. In the end, every engine, even those that seem the same spec wise will treat gasoline differently.
And as always, this will not apply to every engine or even similar engines, the former post about lower octane affecting performance and emissions control reliability still applies to the vast majority of cars. You almost never want to take a definite position since cars are so complex and affected by so many factors, my mentor taught me that you should just say "9 times out of 10" rather than always or definitely when addressing customers about issues.
Throckmorton
Lifer
- Aug 23, 2007
- 16,829
- 3
- 0
They aren't consistently useless. The only way it can be wrong is if the manufacturer purposely programmed it to lie. The car's computer KNOWS how much fuel IT is pumping into the engine.
The better fuel economy on the Ecotec Pontiac G5 running 87 octane could be due to variable timing. There may be a horsepower benefit at higher rpm with 91 octane. I can't think of another reason that 91 octane would be *recommended*.
http://en.wikipedia.org/wiki/GM_Family_II_engine#LE5
It's also 10.4:1 compression ratio, which is fairly high, but not quite as high as cars that *require* 91 octane instead of *recommend*. I wonder how it would do on 89 octane.
http://en.wikipedia.org/wiki/GM_Family_II_engine#LE5
It's also 10.4:1 compression ratio, which is fairly high, but not quite as high as cars that *require* 91 octane instead of *recommend*. I wonder how it would do on 89 octane.
I run 87 in my '04 BMW 330i since the compression ratio is only 10.2:1. And no, I don't believe the hogwash that lower octane decreases fuel economy...
Aikouka
Lifer
- Nov 27, 2001
- 30,383
- 912
- 126
I put some 89 in my car yesterday and I don't know what happened, but the car felt completely different. I'm not even talking about acceleration, but the steering felt stiffer 
. I wonder if it has anything to do with the electronic assisted steering.
I didn't really notice any acceleration differences, but I didn't try and floor it or anything.
. I wonder if it has anything to do with the electronic assisted steering.I didn't really notice any acceleration differences, but I didn't try and floor it or anything
.
rh71
No Lifer
- Aug 28, 2001
- 52,844
- 1,049
- 126
Also on the contrast, since it was only like 2cents more, I decided to give 94 a try as opposed to 91. Absolutely no difference in power or fuel efficiency.
Just give it the minimum recommended in your manual. Don't even try to save the couple bucks giving it less than required.
Just give it the minimum recommended in your manual. Don't even try to save the couple bucks giving it less than required.
One time the wife called me all panicked because she accidentally put 87 in my Evo. Besides feeling a bit more sluggish and a tad worse mpg, car seemed fine!
Which is pretty much the case anyway, the only difference is that you get a bit more power with premium.
Any car MY95 or after (start of OBD2) will have very aggressive knock and ping detection. IE you can run any octane you want without damaging the engine.
MY94 or earlier, all bets are off. Ditto if someone has replaced or reprogrammed the factory ECU. Of course, you would presumably know if your car had been hotrodded like that...
MY94 or earlier, all bets are off. Ditto if someone has replaced or reprogrammed the factory ECU. Of course, you would presumably know if your car had been hotrodded like that...
ravana
Platinum Member
- Jul 18, 2002
- 2,149
- 1
- 76
Polygamist :Q
For my 06 R50 MINI Cooper 91 is what is recommended and I usually fill up 93, cuz that's what we get around here.
I did put in 89 once or twice when I first got it, didn't make much of a difference and filling up on certain days at the Shell next to my house wiped out the difference between 89 ad 93 because of the discount on premium gas :thumbsup:
Zenmervolt
Elite member
- Oct 22, 2000
- 24,514
- 34
- 91
If it's not running as well, nor as fast, I can guarantee that your driving style changes to accommodate the reduced power and drivability and it is much more likely that you see the mileage improvement due to the change in your driving style that is necessitated by what you admit is noticeably reduced power.
ZV
Zenmervolt
Elite member
- Oct 22, 2000
- 24,514
- 34
- 91
No, it doesn't. It only knows the duty cycle of the injectors and extrapolates the amount of fuel based upon that. Basic production variances inherent in mass production guarantee that the fuel economy gauge will always be, at best, an approximation.
They also do not continuously monitor consumption, which allows for error based on sampling frequency.
ZV
PlasmaBomb
Lifer
- Nov 19, 2004
- 11,636
- 2
- 81
Throckmorton
Lifer
- Aug 23, 2007
- 16,829
- 3
- 0
Hmmm. Well I have a throttle by wire. Doesn't that mean that it's using the throttle input to determine exactly how much air and fuel to let into the engine?
superstition
Platinum Member
- Feb 2, 2008
- 2,219
- 221
- 101
I've heard about stations giving people gas of one type when it's marked as another, if the station runs out. Maybe some of us have filled up with "premium" only to get regular? I've heard stories of the opposite happening?that when a station runs of of regular sometimes it will use premium. Obviously this isn't good business practice, but all anyone can do is trust that the gas they're receiving is the correct octane. Also, how much ethanol can be added to improve octane? Ethanol will lower mileage.
Zenmervolt
Elite member
- Oct 22, 2000
- 24,514
- 34
- 91
Nope. Throttle-by-wire is essentially just a potentiometer connected to the accelerator that transmits an electrical signal to a servo activating the throttle butterfly. 50% accelerator opens the throttle butterfly 50% (there may be a non-linear relationship for part of the travel to smooth out throttle inputs, but that's functionally no different than an elliptical throttle cam in a conventional mechanical throttle and it's not important to this issue). The metering for fuel is still done exactly the same as it would be for a conventional throttle as even cars with a conventional throttle still employ a Throttle Position Sensor that gathers the same information used by a throttle-by-wire to open the throttle using the servo.
The EFI system still uses a MAF or MAP or similar device to measure airflow and still uses the injector duty cycle information from the ECU signal to extrapolate the amount of fuel used. It's precise enough, and more precise than a carb, but it's still not going to ever be an exact measurement because there's typically a percent or two variation among a group of injectors even when freshly rebuilt or brand new, and those differences increase with time.
ZV
Zenmervolt
Elite member
- Oct 22, 2000
- 24,514
- 34
- 91
It is illegal for a station to supply an octane lower than that listed on the pump. They can supply higher octane however.
On a side note, I'd know the difference essentially as soon as the low-octane fuel hit my engine. Both of my cars are turbocharged and would run like shit on regular if I opened up the throttle.
ZV
superstition
Platinum Member
- Feb 2, 2008
- 2,219
- 221
- 101
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