Premium gas vs. regular: What's really better for your car? (CBC Marketplace)

[Zenmervolt]

Some cars have had knock sensors since the 1980s - most however, most have not.

Your post implied strongly that no meaningful portion of vehicles manufactured before 2000 had knock sensors. That is wrong by at least a decade. Knock sensors were standard fare in vehicles with engines designed for premium fuel since about 1990 and were reasonably common in such engines as early as the mid-1980s.

For your other points - you can argue semantics with yourself to your heart's content - it really doesn't affect me.

It's more about questioning the knowledge of someone who could make the mistake of confusing "predetonation" for a legitimate term of art.

ZV

 

[Brian Stirling]

My wife's vehicle is supercharged.. doesn't require premium..
Plenty of power with standard 87 octane with zero alcohol.

Supercharged and turbocharged engines tend to have lower compression ratio and thus need lower octane. Many modern engines are direct injection and have high compression ratios. My 2012 Ford Focus with 2.0L engine has a 12:1 compression ratio and normally that would require high octane, but with all the sensors and adjustments a modern engine can make including timing of both the ignition and fuel delivery the engine can run just fine on 87 octane.

BUT, running higher octane permits the engine to be operated with greater optimization for power and economy so running higher octane, even though not needed, can result in both higher power and mpg. The power and mpg benefit isn't enough to offset the higher fuel cost, however, so the cheapest solution is to use 87 octane -- usually.


87 octane E0 will provide less power than 87 octane E10. But, E0 will provide better mpg.


Brian

 

[Blanky]

This matter was solved years ago. Everything in this thread has been said quite literally a hundred thousand times online before. If your manual says premium go premium, else go regular.

 

[Zenmervolt]

Supercharged and turbocharged engines tend to have lower compression ratio and thus need lower octane.

You are confusing static compression ratio with dynamic compression ratio and, in so doing, have gotten it completely arse-backwards.

While there are some forced induction vehicles that are tuned for 87 octane or that can pull back the boost and retard the ignition timing sufficiently to compensate for lower octane, the vast majority of forced induction designs require premium fuel to make full power.

This is because the true driver for octane requirements is cylinder pressures (among many other smaller ancillary issues, but I'm simplifying for convenience), not static compression ratio. Static compression ratio is simply a convenient rough approximation of cylinder pressures when dealing with naturally aspirated engines. You cannot use static compression ratio alone when dealing with a forced induction engine.

To give you an example, let's take a hypothetical vehicle with a low 8:1 static compression ratio and a relatively modest 7.5 PSI of boost. Air entering the cylinder is already compressed at an effective compression ratio of about 1.4:1 (due to the combined gas law, the relationship of compression ratio to pressure ratio is non-linear, so while the pressure ratio is effectively 1.5:1, the compression ratio is slightly lower). When that pre-existing compression ratio is combined with the engine's static compression ratio of 8:1 you end up with an overall effective compression ratio of a bit over 11:1.

87 octane E0 will provide less power than 87 octane E10.

No. 87 octane E0 and 87 octane E10 will produce the same power. To compensate for the higher octane of ethanol, the refinery simply uses lower octane base fuel to get back to an overall 87 octane rating.

It does bear noting, however, that the stickers on the pump are the minimum octane rating; a station could sell 93 octane from a pump labeled 87 octane with no legal consequences. So if, instead of reducing the octane rating of the base stock they simply added 10% ethanol to 87 octane regular gasoline and sold the resulting mixture as "87 octane E10", then you would get (very slightly) more power from the E10 (assuming, of course, that the ECU had the appropriate maps for it) because the E10 would actually be 88.2 octane despite the label on the pump.

But, E0 will provide better mpg.

Yes, due to the lower energy density of ethanol.

ZV

 

[Shlong]

All this bickering back and forth over saving $2/tank on a $30-40,000 car. :/

Not all cases are equal. $2.09 for 87, $2.59 for 93, $0.50 per gallon difference x 18 gallons for fillup = $9. Average 5 fill ups per month ($45 a month difference).

 

[SparkyJJO]

Point still is if you can afford a $40,000 car then you can afford the gas for it. If you don't want to pay for the gas it requires, don't buy that car and get something else.

 

[tortillasoup]

Point still is if you can afford a $40,000 car then you can afford the gas for it. If you don't want to pay for the gas it requires, don't buy that car and get something else.

and the point you're missing is that some cars recommend premium and others require it. The point is to separate which ones recommend and which ones require it. Knowing your vehicle's limitations and what you can and cannot get away with are useful and important to anyone who cares about their vehicle.

 

[SparkyJJO]

I'm missing nothing. If you care about your vehicle then why try to "get away" with things in the first place? Whining that your $40k car calls for premium is a bit stupid.

 

[hans007]

i just put whatever the car says it needs?

i've got a current generation bmw. turbo 4 . the sticker on the fuel door says 89. 89 goes in it.

i've had friends say , why dont you put 91? and i'm like... the sticker says 89 why would you put 91 in . i wouldn't put 87 either. i've had the car for 30k miles and i've never had a problem with it, and have driven it 125mph. most reliable car i've ever owned.

so yeah uh, the guys who designed your car, they told you what to use, just use that.

 

[iamwiz82]

how small is your tank? Up here Premium is about 12 cent more expensive than regular PER LITRE. My tank is 75L :awe:

Well my truck takes 89 and is a 32 gallon tank. I rarely see the point in quibbling over a few cents even in this case. I spend more on my ISP at home.

 

[Phanuel]

i just put whatever the car says it needs?

i've got a current generation bmw. turbo 4 . the sticker on the fuel door says 89. 89 goes in it.

i've had friends say , why dont you put 91? and i'm like... the sticker says 89 why would you put 91 in . i wouldn't put 87 either. i've had the car for 30k miles and i've never had a problem with it, and have driven it 125mph. most reliable car i've ever owned.

so yeah uh, the guys who designed your car, they told you what to use, just use that.

Because you're actually losing engine performance due to it cutting boost and pulling timing by running 89 instead of 91. Congrats on not understanding modern engines?

 

[ultimatebob]

That Edmunds "Premium Required" list is crap. I can tell you with authority that my MINI with the 1.6L I4 engine runs fine on mid-grade (89 octane) gas.

The car has a 110,000 miles on it, with no engine problems. It runs "rough" on 87, though, so I stick with 89 most of the time.

 

[Railgun]

That Edmunds "Premium Required" list is crap. I can tell you with authority that my MINI with the 1.6L I4 engine runs fine on mid-grade (89 octane) gas.

The car has a 110,000 miles on it, with no engine problems. It runs "rough" on 87, though, so I stick with 89 most of the time.

Recommended gasoline grades
The manufacturer of your MINI recommends:

AKI 91.

John Cooper Works AKI 93

Minimum grade
The manufacturer of your MINI recommends AKI 89.

In short, RTFM.

/thread

 

[Ackmed]

Point still is if you can afford a $40,000 car then you can afford the gas for it. If you don't want to pay for the gas it requires, don't buy that car and get something else.

What if the car is 5 years old, and is no where near 40k anymore? It still has the same requirements.

Point is they're fucking us by charging so much between grades. It was 10 cents FOREVER, what logical reason is there for it to have gone up so much? None that I have seen.

 

[LTC8K6]

My Hemi is 89 Recommended, 87 acceptable.

I put 89 in it, unless I am at Sam's club, where they only have 87 and 91, then I use 87.

 

[SparkyJJO]

What if the car is 5 years old, and is no where near 40k anymore? It still has the same requirements.

Point is they're fucking us by charging so much between grades. It was 10 cents FOREVER, what logical reason is there for it to have gone up so much? None that I have seen.

*shrug* then don't buy it if you don't like the gas it calls for.

You do know there is something called inflation and cost increases right? Stuff costs more to make now than it used to in general just because the dollar isn't worth as much as it used to be. Just inflation alone makes 10 cents in 2005 worth nearly 13 cents in 2015, and that doesn't take into consideration the effect that it has had on the cost of every step of the process.

Also, remember, every time you want to get mad at an oil company for gas prices, blame the fed first. The government makes far more money in taxes off every gallon of gas than the oil companies make. Oil companies profit 7-ish cents a gallon. Just the sheer volume of oil/gas though makes that a ton of money. Government rakes in 48 cents or so per gallon. Think about it.

Oil profit margin is about 6.5% which is pretty darn low. Volume makes up for it.

 

[KMc]

Some data on the price gap discussion: