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Old 07-10-2011, 08:46 PM
 
Location: Santa Monica, CA
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There were actually people suggesting that damage was being done by running lower octane fuel. I think that's crazy talk but it sounds like we more or less agree it doesn't cause damage.

As far as fuel economy I have my doubts there too. I just can't see how a 1-3% power difference is consistent with any more than a 1-3% difference in fuel economy. For example, Lexus didn't change the fuel economy ratings of the ES350 between 2010 and 2011 despite the change in fuel octane recommendation from 91 to 87. Same thing with my Murano from 2010 to 2011.

I personally think there is no difference in fuel economy under average driving conditions (excluding turbocharged, supercharged or very high compression NA engines.) Anecdotal evidence is very questionable on this point since expectations influence behavior and one or two tanks of gas is not a large enough sample size to be accurate. The only somewhat accurate way to test this would be to alternate 87 and 91 octane gas over a few thousand miles without letting the driver know which type of gas is in the tank.

Last edited by Dunbar42; 07-10-2011 at 09:04 PM..
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Old 07-11-2011, 10:15 AM
 
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There were actually people suggesting that damage was being done by running lower octane fuel. I think that's crazy talk but it sounds like we more or less agree it doesn't cause damage.
The point of it causing damage is valid depending on the driving being done. The car can't compensate for knock until knock has occurred. Depending on the severity and numbers of times it happens, it can and will cause damage to the car. A couple isolated incidents, probably not, but routinely having knock occur, especially if under hard acceleration can cause damage.

Quote:
As far as fuel economy I have my doubts there too. I just can't see how a 1-3% power difference is consistent with any more than a 1-3% difference in fuel economy. For example, Lexus didn't change the fuel economy ratings of the ES350 between 2010 and 2011 despite the change in fuel octane recommendation from 91 to 87. Same thing with my Murano from 2010 to 2011.
The MPG ratings weren't changed because they compensated for the change in octane requirements by dialing back the timing. They gave up a few horsepower to allow the use of lower octane fuel. Timing doesn't directly impact MPG unless it is really out of whack. All they did was change the tune of the engine to allow the lower octane fuel.

This is something anyone could do with any car if they were so inclined. You could take a M3 and dial back the timing and run regular as long as you were willing to sacrifice power. Conversely you could take a car that requires regular, increase the timing and run premium to gain more power.

Quote:
I personally think there is no difference in fuel economy under average driving conditions (excluding turbocharged, supercharged or very high compression NA engines.) Anecdotal evidence is very questionable on this point since expectations influence behavior and one or two tanks of gas is not a large enough sample size to be accurate. The only somewhat accurate way to test this would be to alternate 87 and 91 octane gas over a few thousand miles without letting the driver know which type of gas is in the tank.
There is a fuel economy effect, but it will vary greatly between cars. The first way the car compensates for knock is to richen the fuel mixture to cool the charge and prevent detonation. Driving around under normal loads may be enough to trigger some knock that the car can compensate for by richening the mixture, this will result in lower fuel economy. How much is all dependent on the individal car.

Overall the only way to really tell what the impact is would be to hook up the car to a scan tool and monitor the fuel trim (air:fuel ratio) as well as the timing and the knock sensors. Having done this, I can assure you that it is indeed an issue to run regular on cars that are tuned for premium, especially if it is a more aggressive tune.

The best piece of advice since there are so many variables is to run what is recommended, but know that you can run lower octane gas in an emergency. If you do find yourself having to run lower octane fuel you need to drive carefully and avoid hard acceleration.
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Old 07-11-2011, 04:38 PM
 
Location: Santa Monica, CA
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NJGOAT, before knock sensors became commonplace I would agree damage was possible with undetected detonation. But nowadays where the ECU is receiving hundreds or thousands of readings a second from the knock sensors I don't think it's possible. In fact, I don't know how it's possible for the computer to detect changes in octane (say a tank of 87 replaced by 91) without increasing the timing until knock is detected and then dialing it back slightly. So I don't see how small amounts (I mean very small) of detonation could be harmful if it's part of normal engine operation.

It sounds like you and I agree that octane makes no little to no difference in fuel economy. Other people in this thread were suggesting people stick to premium precisely because it would have a significant impact on fuel economy. I think ignition advance is the primary way that the ECU compensates for knock (not fuel mixture unless under extreme circumstances.) The only way I can see this impacting fuel economy is if the driver pushes the throttle down harder in response to the retarded ignition timing (and thus lower power.) Since we're only talking 1-3% power difference I doubt that most drivers would even notice the retarded timing.

I just read that Kia recommends 87 octane in the 2L turbocharged Sportage and that's in an engine running 17.5psi of boost. So now I'm starting to question how necessary premium is even in the most extreme circumstances. Lord knows a forced induction engine running 17.5psi of boost (over 2X standard sea level pressure) is the torture test for low octane fuels.

Last edited by Dunbar42; 07-11-2011 at 04:53 PM..
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Old 07-12-2011, 10:35 AM
 
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NJGOAT, before knock sensors became commonplace I would agree damage was possible with undetected detonation. But nowadays where the ECU is receiving hundreds or thousands of readings a second from the knock sensors I don't think it's possible. In fact, I don't know how it's possible for the computer to detect changes in octane (say a tank of 87 replaced by 91) without increasing the timing until knock is detected and then dialing it back slightly. So I don't see how small amounts (I mean very small) of detonation could be harmful if it's part of normal engine operation.
Any knock can be harmful and the computer cannot compensate for knock until it is detected. Given, 99.9% of the time it won't damage anything, but prolonged knock can and there is the .1% chance that it damages something each time. Once it has been detected, the ECU makes adjustments, first to fuel trim and then to timing to cancel out the knock. Once it has not detected knock for several cycles, the ECU readjusts back to its original settings and it starts all over again.

The second part of your premise that knock is part of normal operation is completely false. The car comes "tuned" for a specific octane and there are a few factors that go into that:

Let's take a regular engine with a 10:1 compression ratio. This engine will be tuned from the factory to run at a 14-14.7:1 air:fuel ratio. This is on the lean side to increase heat in the combustion chamber to aid the operation of the catalytic converters. This is a very typical setup. The floating variable is the timing or at what point in the compression stroke the mix ignites. The more advanced the timing, the more the mixture is compressed and heated before the charge is lit by the spark plug. The more advanced the timing, the more power you make. However, you need higher octane to resist predetonation.

So, the timing may look like this (timing is in degrees):

22 = 87 octane and 250hp
24 = 91 octane and 260hp
26 = 93 octane and 270hp

These are pretend numbers, but illustrate the point. The car will never run more timing than what is set for in the tune. So, if the car is set at 22 degrees timing to use 87 octane, putting 93 in it won't do anything. Hence, why putting premium in a car that needs regular makes no difference. However, if it is set at 24 degrees to use 91 octane and you put 87 in it, there is a good chance you will develop knock and the car will do one of two things:

1. It will richen the mixture, moving it from 14.x:1 closer to 13:1 or even lower/richer. Adding more fuel cools the mixture. Remember we are compressing the air/fuel mix. The more we compress it, the hotter it gets. Octane is all about resistance to detonation. You are now burning more fuel, hence lowering MPG.

2. Failing that it will pull timing in order to detonate the mix with the spark before it reaches the critical point and detonates on its own.

You won't feel the effect of number 1, but it's happening. Number 2 is generally noticable when it kicks in. I hope that illustrates exactly how it works.

Quote:
It sounds like you and I agree that octane makes no little to no difference in fuel economy. Other people in this thread were suggesting people stick to premium precisely because it would have a significant impact on fuel economy. I think ignition advance is the primary way that the ECU compensates for knock (not fuel mixture unless under extreme circumstances.) The only way I can see this impacting fuel economy is if the driver pushes the throttle down harder in response to the retarded ignition timing (and thus lower power.) Since we're only talking 1-3% power difference I doubt that most drivers would even notice the retarded timing.
You are correct that octane in and of itself has no impact on MPG. It's all about the tune and the design of the engine. In cases where you run lower octane than recommended how much of a difference in power and MPG is all dependent on the individual car. Some are much more pronounced than 1-3%.

Further, as I stated above, timing is the LAST resort, not first. The car assumes you want all the power and does its best to deliver it. The first thing it will do is richen the A:F ratio to compensate, hence lowering your MPG. In most cases of average driving it will never pull timing, just run richer. When you stomp on the gas is the moment that the adding more fuel doesn't work and timing is pulled to compensate. You have the effect correct, but in reverse order.

Quote:
I just read that Kia recommends 87 octane in the 2L turbocharged Sportage and that's in an engine running 17.5psi of boost. So now I'm starting to question how necessary premium is even in the most extreme circumstances. Lord knows a forced induction engine running 17.5psi of boost (over 2X standard sea level pressure) is the torture test for low octane fuels.
What you are missing is that the 2.0T Hyundai/Kia engine is also direct injected. DI, cools the fuel charge, making the mix more resistant to detonation, hence lowering the octane requirement. Most cars use sequential or multi-port injection where the fuel is sprayed into the intake tract or cylinder port. This results in a higher temperature for the mix. DI, atomizes and sprays the fuel directly into the combustion chamber which has the effect of cooling the charge. It also allows for much greater control of fuel delivery letting them do neat things like run the car in ultra lean conditions (promoting fuel economy and lowering emissions).

Using DI gives them more choices. They could keep the 91+ octane requirement and run more boost, timing or higher compression (or a little of each) and make more power. Or, they can dial back the octane requirement and give up some potential power. Hyundai/Kia took the latter route and settled with lower total power, in exchange for not needing premium. If the car came with a requirement for premium, running regular would cause the same issues as in any other car needing premium that you put regular in.
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Old 07-12-2011, 12:35 PM
 
Location: Santa Monica, CA
1,626 posts, read 3,272,553 times
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Quote:
Originally Posted by NJGOAT View Post
Once it has been detected, the ECU makes adjustments, first to fuel trim and then to timing to cancel out the knock. Once it has not detected knock for several cycles, the ECU readjusts back to its original settings and it starts all over again.
Do you have a source to back this up? There's not much info available out there on the net but everything I see indicates knock is corrected through ignition advance not fuel mixture. Here is a quote from Hondata, an aftermarket tuner of ECU's, indicating that fuel mixture is not changed by the knock control system of OEM Honda ECU's.

"Is it correct then that fuel settings are not changed then from knock sensor input?

Hondata - Yes
"

Quote:
The second part of your premise that knock is part of normal operation is completely false. The car comes "tuned" for a specific octane and there are a few factors that go into that
I suspect Lexus didn't change the "tuning" of the ES350 between the 2010 and 2011 model. My hunch tells me Nissan didn't change a thing on my Murano for 2010 to 2011 either. I think there is enough safety margin built into the factory ECU tune to handle the difference between 87 to 91 octane. But since that is proprietary information there is no way to know whether they did or didn't.
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Old 07-12-2011, 01:20 PM
 
14,780 posts, read 35,982,514 times
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Quote:
Do you have a source to back this up? There's not much info available out there on the net but everything I see indicates knock is corrected through ignition advance not fuel mixture. Here is a quote from Hondata, an aftermarket tuner of ECU's, indicating that fuel mixture is not changed by the knock control system of OEM Honda ECU's.

"Is it correct then that fuel settings are not changed then from knock sensor input?

Hondata - Yes "
The only source I have to back it up is my own experience modifying and tuning cars. I will also venture that the various people who are also posting in this thread with the same experience and mechanical background would confirm what I am saying.

There are several sites if you google around about the ways that the ECU controls knock, including adjusting/richening the fuel mixture. Some of it also depends on what kind of knock you are talking about. There is instantaneous knock and knock "sum" tracked as seperate values. The first is clear knock that occurs generally under hard acceleration and timing is pulled to compensate. The other is low level consistent knock that can occur driving at a steady pace. It is the knock sum that is generally controlled via fuel trim.

It kind of works like this...

Instantaneous...KNOCK, KNOCK...pull timing.

Sum...knock,knock,knock,knock,knock,knock...richen mixture.

Quote:
I suspect Lexus didn't change the "tuning" of the ES350 between the 2010 and 2011 model. My hunch tells me Nissan didn't change a thing on my Murano for 2010 to 2011 either. I think there is enough safety margin built into the factory ECU tune to handle the difference between 87 to 91 octane. But since that is proprietary information there is no way to know whether they did or didn't.
Well, they did as evidenced by the change in power between the model years. The 2010 ES350 was rated at 272hp/254tq. The 2011 is rated at 268hp/248tq. Same story with the Murano, the 2010 was rated 265hp/248tq. The 2011 is rated 260hp/240tq. The change is most likely from retarding the timing to allow the use of lower octane fuel as the motors are otherwise identical.

You are correct that factory tunes are conservative, but like I've been saying that is all model dependent. Some are pushed pretty close to the edge where running lower octane fuel is a major issue. Others not so much and the effect is less noticable.

Regardless, of anything I am saying on the technical aspects, the general point remains that this is essentially penny pinching at its finest. There is ZERO benefit other than minor cost savings to running 87 over 91 in a car that requires 91, but there are potential negatives, some of which can be quite severe. Over 100k miles of driving at 23 MPG, you are talking about $869 more in fuel costs to run premium (I assumed premium was at a .20 cent upcharge). Over 5 years, that works out to about $173 dollars a year or $14.50 a month. We are talking chump change when you consider we are applying this logic to $30k+ cars.
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Old 07-12-2011, 01:22 PM
 
Location: Maryland Heights, MO
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VW also utilizes Direct Injection in the VW/Audi 2.0T engine, and running regular fuel in those vehicles can cause some serious issues. Also note, that while nothing changes between VW/Audi power wise, the octane requirement wasn't lowered for the "People's Car" either.

I know people want to 'know" why there's a difference in ratings, and if it's required. What i'd really like to know is what would it take to convince you of it. If the engineerings doing the testing find that the safest fuel to use in the vehicle is 91 or 93 octane, so be it. They did the tests, the engineering, designing, etc...

You can read that the car will pull .91G on the skidpad, but do you hook an accelerameter to the vehicle to confirm this? Do you get up to an honest 60 mph and slam on the brakes to check minimum stopping distance, even though Car and Driver said it could be accomplished in 160 feet? Do you attempt to find the errors in your speedometer, and document them? There are a million things on your vehicle that you could TRY to verify, but i'd really wonder what scientific method you're going to use to do so...or if you'd rather rely on "hearsay" from a bunch of people on the internet. If so...Put in regular gas, it's no biggie...infact, i've used regular ole' 87 octane in my 3000hp funny car...yeh, so it was designed to run on Nitro...but eh, 87 was cheaper...never blew up once!
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Old 07-13-2011, 08:44 PM
 
Location: Santa Monica, CA
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Originally Posted by NJGOAT View Post
Well, they did as evidenced by the change in power between the model years. The 2010 ES350 was rated at 272hp/254tq. The 2011 is rated at 268hp/248tq. Same story with the Murano, the 2010 was rated 265hp/248tq. The 2011 is rated 260hp/240tq. The change is most likely from retarding the timing to allow the use of lower octane fuel as the motors are otherwise identical.
I think we are generally in agreement. I hope you can appreciate my skepticism about the engineer's recommendation at least in these specific vehicles. The 2010 models of each car "required" 91 octane whereas the 2011 models are perfectly OK with 87 octane even though it's likely that no changes were made to the engines or ECU tune. Hard for me to square that against a simple recommendation "just follow the manual."

Quote:
Originally Posted by NJGOAT
I know people want to 'know" why there's a difference in ratings, and if it's required. What i'd really like to know is what would it take to convince you of it. If the engineerings doing the testing find that the safest fuel to use in the vehicle is 91 or 93 octane, so be it. They did the tests, the engineering, designing, etc...
I've pointed out a lot of inconsistencies in these recommendations though. I'm sure the marketing and PR departments at these car companies get to weigh in on this (and other) official recommendations. For me it's not about the money but more the principal. I'm naturally skeptical of conventional wisdom and I have a technical background so things like this interest me. You saw how insulting people were to anybody that might dare to question something in the owner's manual. Even going so far as to suggest somebody isn't worthy of owning a certain type of car unless they are prepared to buy premium gas. That sentiment is bad enough and I've been hearing the same thing for years. But when it's based on an (IMO) shaky foundation I have a hard time keeping quiet.
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Old 07-13-2011, 09:26 PM
 
Location: Chicago
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Old 07-14-2011, 08:10 AM
 
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I think we are generally in agreement. I hope you can appreciate my skepticism about the engineer's recommendation at least in these specific vehicles. The 2010 models of each car "required" 91 octane whereas the 2011 models are perfectly OK with 87 octane even though it's likely that no changes were made to the engines or ECU tune. Hard for me to square that against a simple recommendation "just follow the manual."
If there were no changes made, than why is the rated power output different? Listen, you would have a valid argument if the exact same motor with the exact same power output was in the Camry and Lexus, but the Camry used regular and the Lexus required premium. That isn't the case. I've given you all the technical details possible to show you how it works and why they make the recommendations they do. It's obvious that no amount of explanation will allow you to "square" reality with your perception.

Quote:
I've pointed out a lot of inconsistencies in these recommendations though. I'm sure the marketing and PR departments at these car companies get to weigh in on this (and other) official recommendations. For me it's not about the money but more the principal. I'm naturally skeptical of conventional wisdom and I have a technical background so things like this interest me. You saw how insulting people were to anybody that might dare to question something in the owner's manual. Even going so far as to suggest somebody isn't worthy of owning a certain type of car unless they are prepared to buy premium gas. That sentiment is bad enough and I've been hearing the same thing for years. But when it's based on an (IMO) shaky foundation I have a hard time keeping quiet.
You quoted this as if I said it, but it was another poster, I will respond regardless. I have no issue with people being skeptical and asking a question to pacify themselves and understand. However, when exhausting technical explanations are given as to why it is the way it is, but the points are still ignored, it becomes a bit annoying.
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