3.6R gas mileage - 87 vs. 91 octane gas

[ChasWG]

I've been doing a simple test over the past several weeks with my 2011 3.6R using both 87 octane gas and 91 octane gas.

 

I bought the 3.6R because it doesn't need the high octane gas like the turbo cars do. But after getting it I kept putting 91 octane gas in the tank mostly out of habit from my days with my Audi A6 2.7T.

 

But the 91 octane gas just makes the engine run a lot better! At least my butt-o-meter says so, but the Average Miles per gallon says differently. I am seeing at least 3 MPG more using the 91 octane over the 87.

 

I believe the engine just runs better on the 91, timing is advanced more and it doesn't take as much gas pedal to get the same performance.

 

This hasn't been the most scientific test as I am not driving on all the same roads. My tests have been a mix of city and highway for both octanes. But after three tanks of 91 octane and 3 tanks of 87 octane and one of a mix of 87 and 91 it is pretty obvious to me that this engine just perfers the higher octane gas.

 

So has anyone else seen anything like this or am I just nuts?

[Peaty]

http://legacygt.com/forums/showthread.php/3-6r-owners-anyone-ever-run-premium-fueliii-144697.html?t=144697

[ChasWG]

OK, I missed that one. But still, the car just "feels" more responsive using the 91 octane over the 87 octane gas. And so far my "tests" are starting to show a difference in milage.

 

Unlike others here that hate that gas mileage gauge in the lower left corner of the instrument panel, I use mine and actually like it. I thought it was odd at first, but now I use it. Kind of a constant reminder that I don't need to bury my right foot into the floor board. But using the 87 octane it seems I need to push a little harder to get the same results in performance. Again, I'm not racing the car or anything, just driving it that same way whether I have 87 or 91 octane in the tank.

[blklgt05]

OK, I missed that one. But still, the car just "feels" more responsive using the 91 octane over the 87 octane gas. And so far my "tests" are starting to show a difference in milage.

 

Unlike others here that hate that gas mileage gauge in the lower left corner of the instrument panel, I use mine and actually like it. I thought it was odd at first, but now I use it. Kind of a constant reminder that I don't need to bury my right foot into the floor board. But using the 87 octane it seems I need to push a little harder to get the same results in performance. Again, I'm not racing the car or anything, just driving it that same way whether I have 87 or 91 octane in the tank.

 

If you really wanna know the difference, the best thing is to put it on a dyno and run the different gas back to back. Running 91, my guess is your car should be slightly richer which in the long run may not be good for the car. But then again I believe our car's timing should adjust with different gas. Either way, I think its more of a placebo/butt effect lol. You are not going to make more power if your car is not tuned. If your car is not knocking, I would just use the regular one. If you want to use premium, you should get the car tuned. With the current gas prices, what you are doing is not worth it IMO. If I can put 87, I would put 87 .

[ehsnils]

For a test to work it needs to be a blind test.

 

But one reason behind why it consumes more may be that if it's more fun the foot gets heavier!

[IcEWoLF]

Go with what your heart desires

[sschmelcke]

the oil companies thank you for your money

[Peaty]

^^^^^

 

What gets me is I read somewhere it only takes a few cents more to make the higher octane. Not sure if that's true or not but I can believe it. I bet they charge more because they can...

[seiyafan]

how to get 3.6R tuned to output more horses?

[sschmelcke]

buy a 3.6 and put a 2.5T badge on it

[ChasWG]

Guys, this was just an observasion on my part. No real science here. No claims of massive HP increase. No Turbo slayer by using higher octane gas. Just a better "feel" to the engine and a slight increase in gas milage. This isn't a life changing realization.

 

Yes, our cars have a modern Engine Control Units (ECU) that will adjust the advance of the timing depending on info from the knock sensors. Higher octane gas is not "richer" it just means that it can burn hotter, it can take more compression before it pre-detonates and the engine can run a more advanced timing and thus produce slightly more HP. That's all. Even if you put 85 octane gas in your turbo charged engine it will still run. It may not perform at it's best, but it will not harm the engine because of the timing control of the ECU and it's knock sensors. The ECU will always try to advance the timing until it starts to receive info from the knock sensors that it needs to back the timing down. And then it starts all over again. It does this all the time, thousands of times a minute. The engine will survive if ran on low octane gas, it just won't deliever as much HP and in the end, to get the same level of performance that you are used to you'll have to push that right pedal down further and use more gas. That's why you use a higher octane gas in performance engines. It was optimized for that level.

 

Of course the 3.6R engine was tuned from the factory to use 87 octane gas. That's not to say that you can't use anything else. And that doesn't mean there is only 256HP in that engine. Have any of you ever driven in the middle of Nowheresville, NE? Yeah, 85 octane is the best you can get there. Not the greatest for my modified 1.8L turbo engine that was properly tuned for 91 octane gas, but the engine survived and the gas mileage dropped. That's all. The engine and the ECU both have a pretty wide range of tolerences so as to be able to perform no matter where you drive the car. All the way from Leadville, CO at 10152' elevation to New Orleans, LA that sits 6' below sea level. That's the point of having a modern ECU, it can deal with all those changes, including whether you use 85 octane or 91 octane gas.

 

Yes, to prove this out a dyno would be the way to do it. But using a production vehicle with a standard gas tank and no fuel line modifications, getting a tank of 87 octane and then a tank of 91 octane and running the car back to back with the exact same atmospheric conditions is a pretty hard thing to do. I'm not tearing up my car for that, so I'll just do my simple, non-scientific fun test.

 

This was just an observation on my part.

[SurlyOldManMN]

The counterpoint is in the details of that "info" from the knock sensors. It doesn't pull timing because it thinks a knock might happen. You rolled the dice and came out a winner, congrats. I wouldn't recommend doing that again.

[TheOneDoubleN]

Guys, this was just an observasion on my part. No real science here. No claims of massive HP increase. No Turbo slayer by using higher octane gas. Just a better "feel" to the engine and a slight increase in gas milage. This isn't a life changing realization.

 

Yes, our cars have a modern Engine Control Units (ECU) that will adjust the advance of the timing depending on info from the knock sensors. Higher octane gas is not "richer" it just means that it can burn hotter, it can take more compression before it pre-detonates and the engine can run a more advanced timing and thus produce slightly more HP. That's all. Even if you put 85 octane gas in your turbo charged engine it will still run. It may not perform at it's best, but it will not harm the engine because of the timing control of the ECU and it's knock sensors. The ECU will always try to advance the timing until it starts to receive info from the knock sensors that it needs to back the timing down. And then it starts all over again. It does this all the time, thousands of times a minute. The engine will survive if ran on low octane gas, it just won't deliever as much HP and in the end, to get the same level of performance that you are used to you'll have to push that right pedal down further and use more gas. That's why you use a higher octane gas in performance engines. It was optimized for that level.

 

Of course the 3.6R engine was tuned from the factory to use 87 octane gas. That's not to say that you can't use anything else. And that doesn't mean there is only 256HP in that engine. Have any of you ever driven in the middle of Nowheresville, NE? Yeah, 85 octane is the best you can get there. Not the greatest for my modified 1.8L turbo engine that was properly tuned for 91 octane gas, but the engine survived and the gas mileage dropped. That's all. The engine and the ECU both have a pretty wide range of tolerences so as to be able to perform no matter where you drive the car. All the way from Leadville, CO at 10152' elevation to New Orleans, LA that sits 6' below sea level. That's the point of having a modern ECU, it can deal with all those changes, including whether you use 85 octane or 91 octane gas.

 

Yes, to prove this out a dyno would be the way to do it. But using a production vehicle with a standard gas tank and no fuel line modifications, getting a tank of 87 octane and then a tank of 91 octane and running the car back to back with the exact same atmospheric conditions is a pretty hard thing to do. I'm not tearing up my car for that, so I'll just do my simple, non-scientific fun test.

 

This was just an observation on my part.

 

You realize you're contradicting yourself in your own post, right? Your ECU is not designed to just randomly advance timing to cater to the current octane you're running. It will definitely pull timing if knock is detected as a reaction, but it isn't going to continually advance the timing until knock is detected in order to optimize the higher octane you've chosen. If I put 93 in my car while running a 91 map with my AP, the power produced is still much lower than when I select the correct, 93 map. The car doesn't sense that I have 93 and thus advance the ignition timing until it knocks. Sorry but that's just not how it works.

[TheOneDoubleN]

Yes, to prove this out a dyno would be the way to do it. But using a production vehicle with a standard gas tank and no fuel line modifications, getting a tank of 87 octane and then a tank of 91 octane and running the car back to back with the exact same atmospheric conditions is a pretty hard thing to do. I'm not tearing up my car for that, so I'll just do my simple, non-scientific fun test.

 

This was just an observation on my part.

 

Plah-seee-bow .

[adamr]

Well, in MN if you are going from 15% ethanol 87 octane regular to 91 octane non oxy (no ethanol), you could see a small increase in gas mileage due to ethanol having less energy per gallon than conventional gasoline.

 

If you go from the winter blend of regular 87 to 91, I would bet 2 mpg gain wouldn't be out of the question in some scenarios.

[ChasWG]

You realize you're contradicting yourself in your own post, right? Your ECU is not designed to just randomly advance timing to cater to the current octane you're running. It will definitely pull timing if knock is detected as a reaction, but it isn't going to continually advance the timing until knock is detected in order to optimize the higher octane you've chosen. If I put 93 in my car while running a 91 map with my AP, the power produced is still much lower than when I select the correct, 93 map. The car doesn't sense that I have 93 and thus advance the ignition timing until it knocks. Sorry but that's just not how it works.

 

I never said that the ECU knows what gas is in the tank and I never said that it advances timing based on any knowlege that it has a higher octane to work with. What I said is that the ECU always trys to advance timing to get the most performance that it can using whatever is in the tank. If it can advance timing a wee bit more because there is no knock signal from the sensors because the engine is burning a higher octane gas, then it does it because that is the protocol of the program. It's an alagorithym, nothing more. Not because it "knows" anything.

 

You are putting words into my mouth. Look again and read it through without jumping to conclusions.

[ChasWG]

Plah-seee-bow .

 

Yes, maybe so, but I admitted that already. Do you actually read what people write or do you just jump into a conversation and think you know what you are talking about? :rolleyes:

[ChasWG]

Well, in MN if you are going from 15% ethanol 87 octane regular to 91 octane non oxy (no ethanol), you could see a small increase in gas mileage due to ethanol having less energy per gallon than conventional gasoline.

 

If you go from the winter blend of regular 87 to 91, I would bet 2 mpg gain wouldn't be out of the question in some scenarios.

 

That could be it too. It is spring time and in Colorado we do have that nasty ethanol in the winter time gas. It gets changed out in the spring. That could simply be why. Great observation.

[SurlyOldManMN]

Your ECU is not designed to just randomly advance timing to cater to the current octane you're running.

 

I never said that the ECU knows what gas is in the tank and I never said that it advances timing based on any knowlege that it has a higher octane to work with. What I said is that the ECU always trys to advance timing to get the most performance that it can using whatever is in the tank

 

That's exactly what he was saying. How would it constantly figure out how much timing it can advance if it doesn't constantly cause knock? That seems like it would be suicidal. As far as I'm aware that simply does not happen.

 

Full disclosure: I'm a tuning n00b and I'm not going to pretend like I know the definitive answer to this. That said, I was under the impression the ECU will only PULL timing when it's trying to save itself. It won't advance timing to squeeze more performance out of whatever is in the tank. Especially on an n/a car where there's little to gain anyway... You know something the rest of us don't?

[ChasWG]

I understand what you are trying to saying, but by your own admission you say you don't fully understand how it works. You are also suffering from the same malidy that OneDoubleN suffers from. You don't read fully. Again, the engine and ECU can not know what octane the gas in the tank is until it starts to burn it. The info from the knock sensors tell the ECU what is happening many thousands of times a minute and the engine makes small changes. I'm not talking about the kind of changes that adding some new software to your turbo ECU can give, but the ECU makes changes based on it's many sensors. IAT, Knock, Radiator temp, coolant temp and on and on. They all have a different effect and outcome on engine performance. And they are all calculated into that majical black box that controls the engine.

 

I too am no tuning noob as you as you say, but I understand a lot more about how our cars really work. More so than you think. I've built cars up from nothing but a rusted body shell and returned them to their former glory. I've built and tuned race engines. I've modified engines (internals and bolt on stuff) to get more HP, more RPMs and better reliability out of them. And I've been successfull at it to. I've built and sold modification parts for performance cars, some of which I still get calls about building "just a few more" parts even though I've retired from doing that.

 

So yeah, maybe I do know something you don't. It's pretty obvious from your statements above.

 

But how can you be so sure of yourself? And why can't the ECU advance the timing? You say it's "suicide" to advance the timing. There you are wrong. Just because a knock happens doesn't mean the end of that engine. It's not going to go off like a grenade. Yeah, over a long period of time, sure. Have you ever done any kind of engine mapping with a quailfied system? I've done a lot of ECU work with one of my former turbo cars. And talking with the experts at APR (Audi Performance & Racing) they confirmed to me what I am telling you here. The ECU does both, advance and retards timing. And before you freak and say, "Well maybe with a Audi or VW engine...", no, Subaru ECUs work pretty much the same way.

 

If what you are saying is true, then the engine can only pull timing and not advance it. So how can it get back to it's default setting or does it just keep pulling timing, forever? Your idea is very wrong. It wasn't that hard to find the huge hole in your logic. The engine can advance it's timing as well as pull it. It does this all the time, thousands of times a minute. It's an protocol buried in among all the other algorithms in the ECU. The ECU uses all it's sensors to continueously adjust the engine. Altitude, Air temp, fuel grade and many other factors contribute to the day to day performance of the engine. So what do you think happens when you drive from Denver Colorado out to McCook Nebraska and you can't always get the recomended fuel octane? Your way of thinking says that you must park the car and wait until someone brings you more 91 or 87 octane gas. These engines and ECUs can deal with all the different changes during that trip from 5280 ft. elevation to some where around 2500 ft elevation and the change in fuel fuel grades. or do you just not drive that far?

 

Also, so you think that just a turbo car has any kind of tuning or ECU control. Just keep thinking that. And what "rest of us" are you refering to? You and one other guy don't get it. That's not an overwhelming group of people. N/A engines have almost as much ECU control going on as turbo charged engines. Yeah, there's no waste gate to control, but the engine has other things it can do to change how it handles it's environment. Obviously you've also never messed around with a big block Chevy engine with a huge carburator and points type ignition. Busted out a timing light and played with the rotation of the distributor and see what that does to engine performance. Take apart one of those distributors and see how timing was advanced back in the day, then you will start to understand the basics of internal combustion engines.

 

I suggest you look into how an internal combustion engine actually works before you respond. You obviously think you know more than you really do.

[nadracer]

I've never had a forced induction car but I've had plenty of N/A cars and reprogrammed the ECU in almost all. Then went on to put more than 50k miles on each before trading them in. Never had a knock and ran 91 to 93 gas.

[ehsnils]

Never had a knock that you heard. And that's what the knock sensor is for, catch the knocking before it's serious and tune down.

[SurlyOldManMN]

... chest thumping purposefully derogatory rant..

 

Gee, thanks for that.

 

 

I'll rephrase the question. It would be nice if you could phrase your answer in a way designed for actual knowledge transfer instead of... whatever it was you going for there.

 

 

 

Why do we even have to tune for anything, ever, if the car is always optimizing based on the same parameters that any protune is going to consider? If there is an envelope that it stays within, what is it? Is it big enough to accommodate various fuel qualities? Are you sure?

 

Nobody ever said the car has an "octane" sensor. My understanding is the purpose of octane is simply to allow one to run better timing. That's the whole point of higher octane isn't it? Am I completely wrong here?

 

How does it detect how far it can advance timing without knocking, without using the knock sensor? How does the knock sensor work if not by detecting a real knock. If it's making these calculations thousands of times per second, how are you not seeing thousands of knocks per second?

 

Once again, I am a noob. I don't think I know as much as you think I think I know. Break it down for me like you would for a four year old.

 

 

You're awesome. A car tuning god. A veritable cornucopia of sage advice rivaling any master technician. I am nothing. No need to cover that again in your answer. TIA

[adamr]

Hey, Stop internet arguing! Its Friday!

 

Beer summit? Surly? I can't even buy any in St. Cloud anymore because its to popular. Boooooo

[SurlyOldManMN]

I'm down. I still think Surly is a little overpriced. I've been rocking Nordeast lately.