Rant - fuel efficiency vs high octane

Rant on...

I seriously get torqued every time someone whines "Oh, I won't buy that, it takes premium".

Seriously, STFU.

Especially when you whine about some car that gets over 30mpg, but you're considering else something that gets in the teens. Um, have you ever done the math on this? Half the fuel economy means you're effectively paying DOUBLE THE PRICE OF GAS, not the measly $0.20/gallon more (A whopping $4/tank, you cheapskate). At $3.00/gallon vs $3.20/gallon, you only have to get a 6.6% increase in fuel economy for the cost to be the exact same. That's 20mpg vs 21.3mpg, and honestly, I've seen better gains from that just from using premium and advancing the timing.

And, as the price of fuel goes up, that needed increase to break even gets smaller, as most gas stations don't increase the price spread. At $4/gallon, you're still only paying $4.20/gallon for premium, which is only a 5% increase.

Just put the damn premium in.

End rant.

Non car-people always amuse me.

I have a friend who wants to buy a fuel-efficent car, that has a hatch and is practical. He is sold on the new Prius, but I tried to steer him towards a new TDI wagon.

His major hangups? The price difference between diesel and gasoline, and his fear of "diesel technology".

Seriously? You are considering a car with a hybrid drivetrain and several hundred pounds of batteries, and a diesel scares you?

You try to help, but just know that your advice falls on deaf ears.

Did you tell him that diesel technology is older?

On the premium thing, there are a few exceptions. The miata comes to mind, the 89-2000's were all regular, the newer ones premium. All other things being equal, I'd go for a regular fuel one.

<<Has a car that requires premium.

I'm a car people and I hate having to buy Premium. If I can buy a similarly performing vehicle that uses regular gas then I save money.

Many of the gas stations rake you over the coals for their premium. Up until just a month or so ago the price differential between regular and premium was .20-.25 a gallon but now it's suddenly .30-.35 a gallon. When that happened my Lexus disappeared.

Well to be fair it wasn't just the gas price jump, and more to do with the fact that it was a souless beast with no character, but that's what made my wife say it was OK to get rid of it.

I know you are complaining more about the math of MPG vs. MPG, but premium gas is on of those emotional things.

Racer1ab wrote: Non car-people always amuse me. I have a friend who wants to buy a fuel-efficent car, that has a hatch and is practical. He is sold on the new Prius, but I tried to steer him towards a new TDI wagon. His major hangups? The price difference between diesel and gasoline, and his fear of "diesel technology". Seriously? You are considering a car with a hybrid drivetrain and several hundred pounds of batteries, and a diesel scares you? You try to help, but just know that your advice falls on deaf ears.

You can lead a man to knowledge, but you can't make him think.

Regular gas has better "bang" which is why old-timer drag racers always tune for 87. Higher octane = higher resistance to explosion = less power potential all things remaining equal.

carguy123 wrote: If I can buy a similarly performing vehicle that uses regular gas then I save money.

True, but what I see most people not putting any thought to is just how little efficiency gain is necessary before premium saves them money.

P71 wrote: Regular gas has better "bang" which is why old-timer drag racers always tune for 87. Higher octane = higher resistance to explosion = less power potential all things remaining equal.

Does resistance to detonation really have anything to do with energy released? (question, not argument)

I mean, diesel is considerably more resistant to detonation, and has a much higher BTU/gallon than gasoline.

I would assume if there was significantly more "bang" to be had from low octane, then the high-compression and advanced timing available through the use of higher octane fuel wouldn't make up for the power difference. But all the high-performance motors I can think of squeeze the holy crap out of their air-fuel, and thus require higher octane-rated fuel.

ReverendDexter wrote: Does resistance to detonation really have anything to do with energy released? (question, not argument)

No. You want the lowest octane that will not pre-detonate.

ReverendDexter as I also said in my post the regular vs. ethyl goes beyond MPG vs. MPG, premium & is also an emotional issue or a hot button.

Of course we all trust those govt mpg ratings on the cars we buy implicitly so we have access to completely accurate information so we should just do the math and come out on top, right? (in case you can't tell, that last comment is just dripping with sarcasm).

My old Explorer Sport Trac with a V6 has a worse govt mileage rating than my new F150. The Sport Trac's rating was 15-20 and the F150 is rated at 15-21. So how come the F150 gets almost 4 mpg les than my Sport Trac did? My math of regular vs. premium would have just been blown out of the water.

So Regular is a fail safe mechanism or equalizer for most people.

But even comparing my S2000 to the F150 regular gas would win out in the math dept. It's only when you add in the fun quotient that premium begins to look good.l

I tried the 87 octane tune in my mustang, checked the mileage over a few tanks and then loaded the 93 octane and rechecked it. The mileage improvement with the 93 octane pays for 50% of the additional cost of 93 plus it has better tip-in acceleration and is slightly smoother on the butt-dyno.

But most people can't solve a basic word problem so they ignore simple math. Their loss

The Wife's Volvo XC90 can take regular but recommends premium. I ran a couple tanks of each, did the math and figured it was a tie on price between regular and premium with the increased MPG I got on premium. As price goes up the $0.20/gal differential doesn't seem to change around here to premium actually gets marginally cheaper. It has better power with premium too, so that's what she runs.

ReverendDexter wrote: Just put the damn premium in.

I just tell people to monitor their fuel economy, then spend the extra $5 per tank and monitor the fuel economy again.

In my Golf, premium fuel started paying for itself aroung the $2/gallon mark. When fuel was $4.50, I was really money ahead.

ReverendDexter wrote: Does resistance to detonation really have anything to do with energy released? (question, not argument)

No. It's more of an artifact of the compounds normally used to raise octane.

Really, the difference in BTU/lb is more than outweighed by the ability to use optimal ignition timing in modern high-compression engines.

Giant Purple Snorklewacker wrote: No. You want the lowest octane that will not pre-detonate.

What is predetonation?

I just throw premium in everything i drive and advance the timing (or crank the boost controller.)

I saw a 4-5mpg increase moving from 87 to 92-93 + timing in the Celica, as well as a performance increase.

Knurled wrote:

Giant Purple Snorklewacker wrote: No. You want the lowest octane that will not pre-detonate.

What is predetonation?

It's the moment before the fuel detonates. Can we all agree that it is called "Pre-ignition detonation"

Knurled wrote: spend the extra $5 per tank and monitor the fuel economy again.

Damn! what are you driving that has a 25 gallon tank?

In a 13 gal tank like my wifes golf buying premium is a $2.60 upgrade made very worth it by the additional power avaliable. Them 1.8T's love the premium.

In my 6 gallon fiat tank I would never notice the extra buck it costs so I advance the timing and go for it.

I get a kick out of folks who will drive 20 miles out of their way to go to a gas station that is .02 cheaper per gallon.

^I think my owners manual says my new-to-me 350Z has a 21 gallon tank.

I think my old '88 300ZX had a 22 gallon tank.

I have ranted about this till I was blue in the face... and people still did not get it. Just like I mentioned getting better miliage 'skip shifting' and people still disbelieved me.

I gave up long ago

John Brown wrote: It's the moment before the fuel detonates. Can we all agree that it is called "Pre-ignition detonation"

Pre-ignition is not detonation, although pre-ignition can cause detonation.

Pre-ignition is hot spots igniting the fuel before the spark, detonation is STILL somewhat incompletely understood, but if you can imagine the correct burn as a smoothly expanding ball of flame,steadily consuming yet compressing further the as-yet-unburnt stuff, detonation is when that as-yet-unburnt is hot enough and high enough pressure to, well, detonate.

Perversely, if you can get the air/fuel to burn REALLY FAST, it can't detonate because end-gases aren't an issue anymore. And if there is no fuel in the end gases, it won't detonate either - so we have direct injection turbo engines with 14 psi on top of 11:1 compression.

Yes, this is one of my huge pet peeves, that and the coolant-restriction myth. Pre-ignition is generally a mechanical problem, detonation is generally a tuning problem.

95 Caprice = 24 US gallons.

Knurled wrote:

Giant Purple Snorklewacker wrote: No. You want the lowest octane that will not pre-detonate.

What is predetonation?

The gas/air mixture burns in the compression chamber before the spark plug fires. Usually when the piston is still moving up (most of the time, a "bad thing" for your pistons/rods/crank bearings ). Some of us old-school guys call it "pinging" or "knocking". Sometimes, if the piston or head surface got hot enough, it would set off the gas/air too early...kind of like how a diesel (heat causes combustion, not spark) works, but since gasoline has a lower flash point, the timing of the burn was off. It was a lot more common in the days of carburetors than it is now, since they were mechanical devices, and couldn't change the mixture based upon operating conditions.

As far as premium gas goes, IIRC..I use it when I can afford it (and I'm lucky enough that I can use it most of the time). Perhaps I'm wrong, but it seems like a modern electronic fuel injection system equipped with a decent knock sensor actually retards the timing to keep the engine from pinging when there's less octane in the fuel. I can't recall ever hearing a modern electronic fuel injection sysem allow a car to "knock", so I tell my non-enthusiast friends that using regular octane fuels won't actually hurt a modern engine, but that using it will reduce the power available. My Corrados seem to really love premium (and even the US version owner's manual reccommend at least a "mid-grade"), but even my 8v Golf seems to be a little more lively when using the stuff (Before E10 was made mandatory here, anyway.. )

Gotta admit, I think a lot of folks my age picked up the habit of using premium in cars that don't really need it during the transition from leaded to unleaded gasoline (please recall this was also during the time most street car engine designs started using electronic fuel injection) because the premium was often the only blend that had the anti-carbon "detergents" in them during that time.

ReverendDexter wrote: Rant on... I seriously get torqued every time someone whines "Oh, I won't buy that, it takes premium". Seriously, STFU. Especially when you whine about some car that gets over 30mpg, but you're considering else something that gets in the teens. Um, have you ever done the math on this? Half the fuel economy means you're effectively paying DOUBLE THE PRICE OF GAS, not the measly $0.20/gallon more (A whopping $4/tank, you cheapskate). At $3.00/gallon vs $3.20/gallon, you only have to get a 6.6% increase in fuel economy for the cost to be the exact same. That's 20mpg vs 21.3mpg, and honestly, I've seen better gains from that just from using premium and advancing the timing. And, as the price of fuel goes up, that needed increase to break even gets smaller, as most gas stations don't increase the price spread. At $4/gallon, you're still only paying $4.20/gallon for premium, which is only a 5% increase. Just put the damn premium in. End rant.

I am just that cheap though.. I am a cheapskate..

Lets do some of math...

at 30 mpg I'm doing a tank a week. So lets use your $4 difference..

Thats 50 working weeks a year so thats $200 a year. I can save.

soo. $200 per year invested @ 8% ( I can get that..) compounded monthly with $200 per year added..

Thats about $6500 in 10 years..

Yes I am that cheap..

Deal with it.

Well, let's see.

My car would get 42mpg when running 92 octane, and about 36mpg on 87 octane.

At 30,000 miles/year, that is about 714 gallons per year on 92, 833 gallons on 87. Premium would have to cost 16-17% more. The break even point if 87 is $2/gallon is premium at $2.33 - any more than that and it doesn't pay to use premium. Locally, there's a 15-cent jump between 87 and 89 and 89 and 92, so $2/gallon is about the break even part.

When it's $3-3.15-3.30, it's cheaper to run premium. When it was 4.50-4.65-4.80, it really hurt to put $70 in fuel in the car but I knew that 87 was DEFINITELY a false economy.

Maybe your car is different. You won't know unless you experiment. My current car gets 20mpg no matter what swill I dump in the tank, so it feeds on 87.

Yes, I miss 42mpg.