Octane and Mileage

[Timpo]

Originally Posted by dangonay

There's no reason that lower octane fuel would reduce your mileage. Under normal driving conditions your engine will only be producing a fraction of its maximum output, so you're not going to run into conditions that require higher octane (like knock from advanced timing or heavy load).

Your engine would definitely produce less power at full throttle (or heavy throttle) but should still perform similarly under light throttle.

I doubt anyone could even accurately measure the difference in mileage between going up or down in octane from the recommended fuel your vehicle takes.

How do you know? Are you a car engineer?

I'm sure pretty much everywhere you look on google, they would teel you using lower octane gas will affect your gas mileage.
I'm not an engineer so I have no clue how realiable google articles are, just fyi.

Also it doesn't really matter if he's driving under "normal conditions"...what did you mean by this?

If engine has high enough compression or boost to detonate...I'd say high octane gas is necessary.
I know ECU will adjust ignition timing and stuff...but I don't think ECU can control air getting compressed and air friction igniting fuel in the combustion chamber.
Then again, I am a complete noob so...

[sonick]

Originally Posted by StylinRed

since "regular" is mixed with a higher percentage of additives compared to the other levels of fuel, should that be a consideration to the octane level of fuel you put in? does 86 become an 85 because of the 10% (or whatever it is) mixture of ethanol/etc that they put in?

Actually the opposite; ethanol boosts octane rating.

[dangonay]

Originally Posted by Timpo

How do you know? Are you a car engineer?

I'm sure pretty much everywhere you look on google, they would teel you using lower octane gas will affect your gas mileage.
I'm not an engineer so I have no clue how realiable google articles are, just fyi.

Also it doesn't really matter if he's driving under "normal conditions"...what did you mean by this?

If engine has high enough compression or boost to detonate...I'd say high octane gas is necessary.
I know ECU will adjust ignition timing and stuff...but I don't think ECU can control air getting compressed and air friction igniting fuel in the combustion chamber.
Then again, I am a complete noob so...

Actually I did get my engineering degree recently - you must have missed that post when I came back to RS after my absence.

I've actually built my own ECU from scratch and also built the world's first aftermarket drive-by-wire throttle controller to integrate DBW with non-DBW ECU's. So yes, I do know a bit about how ECU's work.

Your car will never know what octane is in the tank until such time as you require more power (like heavy acceleration) at which time it will detect knock and adjust timing accordingly to compensate. Under normal driving your ECU will not know if you have regular or premium in the tank.

[freakshow]

So you're saying that there would really be no negative effects with regards to mileage or engine health for most modern stock cars to run 87 even if they require 91?

[sonick]

Originally Posted by Timpo

Also it doesn't really matter if he's driving under "normal conditions"...what did you mean by this?

e.g. no merging or overtaking on a highway, no going up steep hills, no living life a quarter mile at a time.

[Doubl3_H]

What about regular for a turbo car?
Posted via RS Mobile

[danny_d19]

Originally Posted by Doubl3_H

What about regular for a turbo car?
Posted via RS Mobile

Just use what is recommended in the manual. My turbo car is tuned for 93/94 octane, so that's what I use.

[dangonay]

Originally Posted by freakshow

So you're saying that there would really be no negative effects with regards to mileage or engine health for most modern stock cars to run 87 even if they require 91?

The negative effects would be a loss in performance. The engine ECU would compensate for lower quality fuel so you're not going to blow up your engine or anything serious like that.

Keep in mind this is for more modern cars. It would be bad to run a low octane gas in an older car without some form of knock control system.

While companies always list the recommended fuel, some actually give ratings for their engine using the recommended and lower octane fuel (for example, the new Coyote engine in the Ford Mustang quotes power/torque outputs on 87 and 91 octane fuel).

You have to remember manufacturers have to allow for all sorts of scenarios from dumb drivers putting in the wrong fuel to people on a road trip at a station that doesn't sell fuel they need. Vehicles will run OK on lower octane fuel - they just perform better when running on the recommended fuel.

[PornMaster]

Few things Ive noticed ever since the switch.
1) Engine response seems faster. b4 I would press the gas down and about half a second then it would rev higher.
2) Hissing sound when car is idling is completely gone
3) Idles at 900rpm now instead of 1100

[dangonay]

^ Something is going on with your car that's not right.

There is no way a change in octane is going to affect your idle speed. And the hissing sound is more likely a cause of the previously higher idle speed drawing more air in the engine.
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[see.lai]

Speaking of idling, when you start up a car, it's not supposed to idle at 1500-2000rpms, right..?

When my car is warm, then it will idle at 1000. :|

[PornMaster]

Originally Posted by dangonay

^ Something is going on with your car that's not right.

There is no way a change in octane is going to affect your idle speed. And the hissing sound is more likely a cause of the previously higher idle speed drawing more air in the engine.
Posted via RS Mobile

Let me rephrase its not a hissing sound but more of a high pitch screeching sound when idling

[dachinesedude]

Originally Posted by PornMaster

Few things Ive noticed ever since the switch.
1) Engine response seems faster. b4 I would press the gas down and about half a second then it would rev higher.
2) Hissing sound when car is idling is completely gone
3) Idles at 900rpm now instead of 1100

a change of octane made that much noticeable difference? LOL

i think its all in your head, placebo, butt dyno, noticing stuff that was prob there before, etc

[Soundy]

Originally Posted by PornMaster

Few things Ive noticed ever since the switch.
1) Engine response seems faster. b4 I would press the gas down and about half a second then it would rev higher.
2) Hissing sound when car is idling is completely gone
3) Idles at 900rpm now instead of 1100

Originally Posted by PornMaster

Let me rephrase its not a hissing sound but more of a high pitch screeching sound when idling

All three symptoms sound like a vacuum leak. Change in octane would not cause a change in the symptoms; any change would have to be coincidental.

[Timpo]

Originally Posted by dangonay

Your car will never know what octane is in the tank until such time as you require more power (like heavy acceleration) at which time it will detect knock and adjust timing accordingly to compensate. Under normal driving your ECU will not know if you have regular or premium in the tank.

ok but you still didn't tell me your definition of "normal driving"

if your definition is this:

Originally Posted by sonick

e.g. no merging or overtaking on a highway, no going up steep hills, no living life a quarter mile at a time.

this is almost unrealistic, so basically as long as you don't rev up more than..let's say 3,000rpm or something, you're ok with low octane gas.

[trd2343]

Originally Posted by dangonay

^ Something is going on with your car that's not right.

There is no way a change in octane is going to affect your idle speed. And the hissing sound is more likely a cause of the previously higher idle speed drawing more air in the engine.
Posted via RS Mobile

I too tried explaining that to my mom, but she said the car (during acceleration) does feel more responsive and has more "ummpph" to it when she stepped on the pedal. She said she gets better mileage too though she hasn't had any hard to data to back that up. I said she must be imaging things.

Is there any possible way why the car might feel more responsive, or is it all in the head?

Only reason why people would put in 94 because it's suppose to contain more cleaning additive than all the other grades?

[Bahhbeehhaaaa]

my famiy car, a 2003 Sienna with 95,XXXkm gets about 350km 80L tank 87 octane cheveron.. soo what'ss up with the poor gas mileage?

[sonick]

Originally Posted by reamemiya

I too tried explaining that to my mom, but she said the car (during acceleration) does feel more responsive and has more "ummpph" to it when she stepped on the pedal. She said she gets better mileage too though she hasn't had any hard to data to back that up. I said she must be imaging things.

Is there any possible way why the car might feel more responsive, or is it all in the head?

Only reason why people would put in 94 because it's suppose to contain more cleaning additive than all the other grades?

The only way would be if the car requires higher octane, there would be a performance difference between filling with 87 and the factory recommended grade octane.
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[GabAlmighty]

My turbo Volvo gets 87 cuz i'm too cheap to put in a higher octane. I'll let you all know when it blows up.

Originally Posted by see.lai

Speaking of idling, when you start up a car, it's not supposed to idle at 1500-2000rpms, right..?

When my car is warm, then it will idle at 1000. :|

Warm up sequence, 2k seems a little high. 1500 sounds about right depending on the car.

My 4runner would do abouty 1500rpm but my volvo only does a little over 1k for warm up i think.

[trd2343]

Originally Posted by sonick

The only way would be if the car requires higher octane, there would be a performance difference between filling with 87 and the factory recommended grade octane.
Posted via RS Mobile

Nope, it's a CR-V, and recommends 87, so I don't know where she's getting the "better response thing from", although I haven't had the chance to actually test it myself.

I did a quick search on google and came across this reply on Yahoo questions.

http://answers.yahoo.com/question/in...4084735AAgGN0T

On some occasions, you may feel an improvement in performance and fuel economy when using premium fuel in an engine designed to run on 87 octane regular fuel. Most times this improvement is slight. Many times, it is a placebo effect. You may even get a slight improvement in fuel economy.

Just to be clear, I'm no expert, nor am I saying whatever this guy is saying is right. I know that using lower octane than recommended is bad, but I've heard numerous time about experiences of increase in performance in using higher octane. I would be incline to believe what this guy said given it's also written cohesively.

[dangonay]

Originally Posted by reamemiya

I too tried explaining that to my mom, but she said the car (during acceleration) does feel more responsive and has more "ummpph" to it when she stepped on the pedal. She said she gets better mileage too though she hasn't had any hard to data to back that up. I said she must be imaging things.

Is there any possible way why the car might feel more responsive, or is it all in the head?

Only reason why people would put in 94 because it's suppose to contain more cleaning additive than all the other grades?

I'd say the performance aspect is probably in her head, even if her car was actually performing better.

I'll try to explain this simply to hopefully get my point across.

Engine ECU's have what are known as "adaptations". Adaptations are what allow engines to perform consistently as operating conditions change (quality of fuel, age of engine, wear of components and so on). In simple terms, adaptations are a set of numbers which tells the ECU how to modify engine parameters like fuel & ignition.

Let's say at a certain RPM and load the ECU has a value of 20 degrees of timing stored. When the engine is running and the ignition is fired at 20 degrees the engine detects knock. So it pulls back the timing to 18 degrees and there's still a knock. It pulls timing back to 16 degrees and the knock goes away.

If this happened once-in-awhile, the ECU would not bother changing the adaptations as it can handle the occasional knock by simply pulling timing. If it's happening regularly, then the ECU might store a value of -4 degrees as an "adaptation" for ignition. The next time the engine reaches the same RPM and load, the ECU gets the 20 degree value from memory and applies the adaptation to it (in this case, -4 degrees) and then fires the ignition at 16 degrees. Knock is eliminated not because the knock sensors detected it and the ECU adjusted timing, but because the timing amount was already corrected by applying an adaptation.

It can get a lot more complicated than this, but this is a general idea how adaptations work.

One key thing to remember about adaptations is it takes time for the ECU to set adaptations. It depends on the car but it can take anywhere from 15 minutes to several hours before the ECU has enough data from various driving conditions to set adaptations. This is why when your mom switches from lower octane to higher octane fuel she likely wouldn't feel an instant increase in power because the ECU would have set its adaptations for the lower quality fuel. Her car will perform better, but it will take some driving before the ECU adapts to the new fuel.


Edit: Just saw the car is a CR-V. In that case, she's just imagining things. If it was a car that required higher octane, then everything I said above applies.

[trd2343]

^I just did a quick edit on my post and added a link. Like I said, by no means believing what this guy says, but I can see where people are coming from when they say that higher octane produces better performance, after reading that.

Wait I just read what you post, so using a higher octane does produce better performance and fuel efficiency?

I'd say the performance aspect is probably in her head, even if her car was actually performing better.

[danny_d19]

Originally Posted by reamemiya

^

Wait I just read what you post, so using a higher octane does produce better performance and fuel efficiency?

It's not a concrete fact for all scenarios, I think this is what you are getting hung up on. Sometimes it does, usually due to the vehicle not using the proper octane it calls for, and therefore isn't running at peak efficiency

[mauricetan]

Interesting thread! Any more opinions or advice?

I was just wondering is it true that the octane rating in the USA is messed up? So if your car requires 89 octane.. you should put premium when you are filling up in the USA?

[bcrdukes]

^
What?

No. They use the AKI (Anti-Knock Index) which is used in Canada as well.