Fuel Grades vs. MPG

[Benzadmiral]

Originally Posted By: Kestas
BuickGN is correct. You'll only see a difference if your car has a knock sensor. If your car has no knock sensor, and is specified for 87 octane, you're wasting your money.

Does your car have a knock sensor? What fuel is specified in yor vehicle?

Well, I'd have thought my 2003 was new enough to have a knock sensor. But here's this from the owner's manual:

"Gasoline Octane

"If your vehicle has the 3800 V6 engine (VIN code K), use regular unleaded gasoline with a posted octane of 87 or higher. If the octane is less than 87, you may get a heavy knocking noise when you drive. If this occurs, use a gasoline rated at 87 octane or higher as soon as possible. Otherwise, you might damage your engine. A little pinging noise when you accelerate or drive uphill is considered normal. This does not indicate a problem exists or that a higher-octane fuel is necessary. If you are using 87 octane or higher-octane fuel and you hear heavy knocking, your engine needs service."

??? If it has a knock sensor, wouldn't the manual say "no damage will be done, but you will lose power and fuel economy"?

 

[Kestas]

I agree that the manual would likely have been worded differently if the vehicle had a knock sensor.

 

[oilyriser]

Does the trip computer measure fuel flow, or air flow?

 

[LTVibe]

Originally Posted By: Benzadmiral
"Gasoline Octane

"If your vehicle has the 3800 V6 engine (VIN code K), use regular unleaded gasoline with a posted octane of 87 or higher. If the octane is less than 87, you may get a heavy knocking noise when you drive. If this occurs, use a gasoline rated at 87 octane or higher as soon as possible. Otherwise, you might damage your engine. A little pinging noise when you accelerate or drive uphill is considered normal. This does not indicate a problem exists or that a higher-octane fuel is necessary. If you are using 87 octane or higher-octane fuel and you hear heavy knocking, your engine needs service."


My Vibe has a knock sensor and the owner's manual says word for word exactly the same as yours except for the first line: "If your vehicle has the 1.8L L4 (VIN Code 8), use regular unleaded gasoline with a posted octane of 87 or higher...."

Your '03 Park Avenue does have the sensor: http://www.autopartswarehouse.com/detail...05QQSIKS45.html

 

[sciphi]

The 3800 Series II definitely has a knock sensor. My ScanGauge is set to display knock retard as one of the gauges.

 

[Vikas]

If your engine tries to advanced timing every second, then you will hearing the ping all the time.

I would love to see your timing advance graph during a typical drive when you know it is retarding the timing. I am assuming your scanner has that ability.

It is also possible that ECU has two maps, one for high octane and another for regular octane. ECU might be programmed not go below the regular octane map. If the car still pings and the computer has retarded the timing as much as it could, then that is it.

- Vikas

 

[Benzadmiral]

Originally Posted By: sciphi
The 3800 Series II definitely has a knock sensor. My ScanGauge is set to display knock retard as one of the gauges.

Ah-ha! So does this mean it might be worth my while to try a higher octane?

 

[blackdiamond]

It appears that my Sequoia also has a knock sensor:

http://www.trademotion.com/partlocator/index.cfm?action=getJointLocator&siteid=214001

I haven't mentioned that I've had one tank of gas where the engine seemed to develop a bit of a ping. Since it was an isolated thing it must have been the tank of fuel.

 

[ARB1977]

I did a comparison between 89 and 93. The 89 got me 1 more mpg as were 93 was the same with 87. After this tank of 93 im switching back to 89.

 

[element_42]

Originally Posted By: Vikas
If your engine tries to advanced timing every second, then you will hearing the ping all the time.

I would love to see your timing advance graph during a typical drive when you know it is retarding the timing. I am assuming your scanner has that ability.

It is also possible that ECU has two maps, one for high octane and another for regular octane. ECU might be programmed not go below the regular octane map. If the car still pings and the computer has retarded the timing as much as it could, then that is it.

- Vikas


Since the early 90's some ECUs will try to get as much timing out as possible by dynamically testing for knock throughout the RPM range. The old style knock programs heard knock and then pulled a certain amount of timing off the whole RPM range indefinately until restart. The dynamic type notes where in the RPM band, and past what load level the knock occurs and pulls back only enough degrees timing for that section till knock stops. This is a good idea if your engine/fuel combo knocks at 2500rpm 70% throttle but not at 4500 full throttle, dynamic tuning will allow full timing at 4500 WOT and wherever knock ceases before that. My car does this, and past 5500rpm it gives full timing anyway.

 

[BuickGN]

Originally Posted By: Vikas
If your engine tries to advanced timing every second, then you will hearing the ping all the time.

I would love to see your timing advance graph during a typical drive when you know it is retarding the timing. I am assuming your scanner has that ability.

It is also possible that ECU has two maps, one for high octane and another for regular octane. ECU might be programmed not go below the regular octane map. If the car still pings and the computer has retarded the timing as much as it could, then that is it.

- Vikas


That's how the TL is. People that have the pinging probelm have it pretty much continuously. I've seen 18 degrees being pulled at partial throttle, low rpm, heavy throttle.

It's better to have the correct tune rather than rely on the knock sensor to pull timing. Once the pinging starts you have to pull more timing to stop it than if it never occurred. Obviouisly my car can't pull enough timing to stop the pinging. IMO, the knock sensor should be there for engine safety, not used as a tuning tool.

I forgot to mention, the GN, stock, pulls timing in 3 degree increments and puts it back in within a second in 1 degree increments. Popular aftermarket chips would change this and have it pull less initially or put it all back in at once. I'm assuming that's how many cars of that era did it.

 

[Benzadmiral]

Well, I'm bored, and mid-grade is only 7% more, so I filled up with the 89 yesterday. If I get 7.5 to 8% better mpg over the next tank or three (something like 23.5 mpg overall), it'll be worthwhile.

 

[blackdiamond]

My wife filled up yesterday completing the first tank of premium grade fuel. I didn't get home last night in time to check the mileage so will update later today if things go as expected.

I may have inadvertantly reduced the chance of a fair comparison since my wife reported that she got the trip computer to 18.5 beating my 18.4. I think she's actually trying to drive smoother since she noticed that I get better mileage almost always.

 

[BuickGN]

I find it funny that the males usually get the best mileage in the shared vehicle even though we are portrayed as the aggressive drivers. I regularly beat the girlfriend by 5mpg or more in her car. Being competitive like she is, that number is dropping.

 

[swalve]

Just to reiterate BuickGN's comments, it really depends on the particular engine. The engineers quoted above who make fun of people for running higher octane might be correct about the engines they worked on. But some engines have the engine management capabilities to take advantage of higher octane than is required to make it run right. I have a Grand Prix supercharged that says it requires 91. But it runs better and gets better mileage when I feed it 93.

I also had a car with a Ford Duratec 2.5L. It only required 87, but it liked 93. It was never worth it in a dollars and cents calculation, but it was in a "my car runs great" sense.

(Also, the automaker engineers aren't necessarily the last word in engine perfection- I look at real world results. Last year when gas got really high, I looked at a bunch of articles and forums about e85. It was interesting to compare the results of people with factory flex fuel vehicles versus non-flex ones. The flex-fuel vehicles never got nearly as good results as the non-flex ones when fed e85. I'm not sure why that was, but it was illustrative of the fact that engine designers aren't necessarily perfect.)

Also, just as a minor quibble, the octane rating of gas isn't really a quality grade, where one fuel is of a higher guality than another. The fuel just has different burn characteristics. (Or at least, it shouldn't be- I imagine some refiners and stations use worse fuels in the lower octane ratings because they can get away with it. But they shouldn't.)

 

[swalve]

Originally Posted By: BuickGN
I find it funny that the males usually get the best mileage in the shared vehicle even though we are portrayed as the aggressive drivers. I regularly beat the girlfriend by 5mpg or more in her car. Being competitive like she is, that number is dropping.


It probably works out to about even if you correct for differences in "caring". If one person cares about getting good MPG and the other doesn't, it will show. I'm sure there are just as many ladies out there who get great mileage in their white convertible Camaros, but when their dirtbag boyfriends take the car and beat it to within an inch of its life, their MPG is a lot lower.

The only real difference, to me, is that females aren't nearly as likely to be on message boards about cars...

(I also just heard a radio report about drunk driving- females are getting nabbed at an increasing rate in the last few years, where the rate for males is actually going down slightly. I don't know what that says about the male/female struggle. But it's interesting.)

 

[labman]

Years ago most cars got their best millage at about 45. The newer, more aerodynamic ones. Thus, around town, the faster driver may get better millage. Also the one that doesn't dawdle at lights and stop signs may get better millage. The longer you sit after the green, the more gas you waste.

 

[BuickGN]

Originally Posted By: labman
Years ago most cars got their best millage at about 45. The newer, more aerodynamic ones. Thus, around town, the faster driver may get better millage. Also the one that doesn't dawdle at lights and stop signs may get better millage. The longer you sit after the green, the more gas you waste.


True but around here, it's impossible to get better than 15mpg with pure city driving. My monthly average speed is 17mph around here. In between lights I usually hit 55-65mph. This shows how much time I spend at redlights and the traffic isn't even bad. It's not like I have to sit through the same light multiple times due to traffic being backed up. I catch 98% of the lights, even the ones that are spaced 400' apart near the shopping centers. I always have to wait for an entire cycle which may include two turn lanes going one at a time. I've started timing the idle times at lights and I'm running an average of nearly 2 minutes. I hate to say that I've now started to look both ways and go through them when there's no traffic around, treating them as stop signs. I can't afford not to. I'm starting to see more and more people running redlights too.

 

[labman]

With the way so many people set at green lights and yak on their phones, more people may go through on the red. I am really sick of stopping for green light due to dolts that don't get moving.

 

[blackdiamond]

I would argue that women GENERALLY don't pay as much attention to their driving (men have one track minds) and as a result are less smooth which yields less mpg. Another physiological fact is that women GENERALLY have poorer depth perception making it more difficult for them to adjust to speed changes. They also have faster reactions times to even things out. Also, many of the ladies that ARE interested in cars tend to drive agressively so they either drive fast or don't care.

I actually use the trip computer as a form of entertainment when I drive, my wife NEVER does this.