JamesKim

The compression ratio for my 2011 Hyundai Tucson engine (does not have direct injection) is 10.5:1

The compression ratio for my 2011 Mercedes C class is 10.7:1

Hardly any difference? The Tucson recommends regular and the C class requires Premium, but I must be missing something because the compression ratio seems awfully close.

Why do I need premium gas on my C class, but not my Tucson? Can someone enlighten me? Can't I just use regular? Or Medium to just play it safe?

Also, I live in Peg city, where the temperature roams somewhere between -10 to -40 Celsius in the winter (14 Fahrenheit to -40 Fahrenheit). Since the cylinder temperature would be much cooler, would it be acceptable to just use regular in the winter?

I wouldn't raise this issue if Premium gas wasn't so expensive in Canada, but I pay around 13 cents per liter more per premium than regular (around 49 cents more per gallon). It is a complete robbery compared to the states. I'd love to just use regular if I can get away with it.

edit: Also, why is it that gas companies in Canada can get away with such robberies? Isn't the difference only Octane?

MBRedux

REGULAR or PREMIUM / From Car&Driver (See Bold and underlined paragraphs.) That should answer your question. Bottom line... your Hyundai's engine has been detuned from the factory, where the timing has most likely been permanently retarded, where the Mercedes engines aren't.
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There's no shortage of opinions on who is to blame for gas-price gouging. One thing that's certain is drivers tend to economize at the pump during extreme price rises-they buy cheaper, lower-octane gas.

In the old preelectronic days, cars would protest such parsimony by pinging like a pachinko parlor, but most modern cars don't complain audibly, so maybe they don't mind. Or do they? And conversely, is there any benefit to be had by springing for the expensive stuff when you're feeling flush?

To find out, we ordered a fleet of test cars-some calibrated to run on regular, others that require premium-and tested them at the track and on a dynamometer.

But before we go into the results, let's go to combustion school. When a spark plug fires, it does not cause an instantaneous explosion of the entire cylinder's charge of fuel and air. The spark actually lights off a small kernel of air-and-fuel mixture near the plug. From there, a flame front expands in every direction, gradually igniting the rest of the air and fuel. This takes some time, as much as 60 degrees of crankshaft rotation.

Meanwhile, the air-and-fuel mixture that the flame front has not yet reached is experiencing huge increases in pressure and temperature. If any part of this air-and-fuel mixture gets heated and squeezed enough, it will explode spontaneously, even before the flame front ignites. This self-ignition is called detonation, or the dreaded "knock."

Now for the chemistry lesson: Oil is a hydrocarbon fuel, meaning the individual molecules contain carbon and hydrogen atoms chained together. Modern gasoline is blended according to various recipes, the active ingredients for which include about 200 different hydrocarbons, each with a spine of between 4 and 12 carbon atoms. One of them, isooctane, consists of 8 carbon and 18 hydrogen atoms (C8H18) and is exceptionally resistant to exploding spontaneously when exposed to the heat and pressure found inside a typical combustion chamber. Another, n-heptane (C7H16) is highly susceptible to such self-ignition.

These two compounds are therefore used to rate the knock resistance of all gasoline blends. A gasoline recipe that resists knock the way a mixture of 87-percent isooctane and 13-percent n-heptane would is rated at 87. Racing fuels with octane ratings over 100 resist self-ignition even better than pure isooctane. The octane ratings for regular-grade fuel range from 85 to 87, midgrades are rated 88 to 90, and 91 and higher is premium.

Mind you, premium fuel does not necessarily pack more energy content than does regular. Rather, it allows more aggressive engine designs and calibrations that can extract more power from each gallon of gasoline.

An engine's tendency to knock is influenced most by its compression ratio, although combustion-chamber design also has a large effect. A higher ratio extracts more power during the expansion stroke, but it also creates higher cylinder pressures and temperatures, which tend to induce knock. In supercharged engines boost pressure behaves the same way. That's why the highest-performance engines require higher-octane fuel.

If you feed such an engine a fuel with insufficient octane, it will knock. Since it is impossible, for now, to change an engine's compression ratio, the only solution is to retard the ignition timing (or reduce boost pressure). Conversely, in some engines designed for regular fuel, you can advance the timing if you burn premium, but whether this will yield additional power varies from engine to engine.

Knock sensors are used in virtually all new GM, Ford, European, and Japanese cars, and most DaimlerChrysler vehicles built today. According to Gottfried Schiller, director of powertrain engineering at Bosch, these block-mounted sensors-one or two of them on most engines and about the size of a quarter-work like tiny seismometers that measure vibration patterns throughout the block to identify knock in any cylinder. Relying on these sensors, the engine controller can keep each cylinder's spark timing advanced right to the hairy edge of knock, providing peak efficiency on any fuel and preventing the damage that knock can do to an engine. But, noted Schiller, only a few vehicles calibrated for regular fuel can advance timing beyond their nominal ideal setting when burning premium.
Older or less sophisticated cars with mechanical distributors do not have the same latitude for timing adjustment as distributorless systems do and therefore may not always be able to correct for insufficient octane or additional octane.

We should note that even cars designed to run on regular fuel might require higher octane as they age. Carbon buildup inside the cylinder can create hot spots that can initiate knock. So can malfunctioning exhaust-gas-recirculation systems that raise cylinder temperatures. Hot temperatures and exceptionally low humidity can increase an engine's octane requirements as well. High altitude reduces the demand for octane.

Got all that? Good. Let's meet the test cars and ponder the results. At the lower-tech end of the scale was a regular-gas-burning 5.9-liter Dodge Ram V-8. This all-iron pushrod engine has a mechanical distributor and no knock sensors, so the computer has no idea what grade of fuel it's burning. A Honda Accord V-6 with VTEC variable valve timing represented the mainstream-family-sedan class, and a 4.6-liter V-8 Mustang stood in as an up-to-date big-torquer. Both of those were designed to run on regular unleaded. Our premium-grade cars included the hard-charging 333-hp, 3.2-liter BMW M3 straight-six boasting individual throttle by wire for each cylinder and enough computing power to run Apollos 11 through 13. A Saab 9-5 gave us a highly pressurized 2.3-liter turbo. For the sake of repeatable track testing, all but the M3 were equipped with automatic transmissions.

We ran all vehicles on both grades of fuel, at a drag strip near our offices and on a Mustang eddy-current dynamometer that was offered to us by the engine-tuning pros at Automotive Performance Engineering in nearby Clinton Township, Michigan. On arrival, all fuel tanks were drained and filled with 87-octane Mobil regular fuel and driven for two days before track and dyno testing. The tanks were drained again and filled with 91-octane Mobil premium and again driven for two days to allow time for the engine controllers to acclimate to the fuel type and tested again. All dyno and track results were weather-corrected.

Our low-tech Ram managed to eke out a few extra dyno ponies on premium fuel, but at the track its performance was virtually identical. The Mustang's knock sensors and EEC-V computer found 2 hp more on the dyno and shaved a more impressive 0.3 second off its quarter-mile time at the track. The Accord took a tiny step backward in power (minus 2.6 percent) and performance (minus 1.5 percent) on premium fuel, a phenomenon for which none of the experts we consulted could offer an explanation except to posit that the results may fall within normal test-to-test variability. This, of course, may also be the case for the gains of similar magnitude realized by the Ram and Mustang.

The results were more dramatic with the test cars that require premium fuel. The turbocharged Saab's sophisticated Trionic engine-control system dialed the power back 9.8 percent on regular gas, and performance dropped 10.1 percent at the track. Burning regular in our BMW M3 diminished track performance by 6.6 percent, but neither the BMW nor the Saab suffered any drivability problems while burning regular unleaded fuel. Unfortunately, the M3's sophisticated electronics made it impossible to test the car on the dyno (see caption at top).

Our tests confirm that for most cars there is no compelling reason to buy more expensive fuel than the factory recommends, as any performance gain realized will surely be far less than the percentage hike in price. Cheapskates burning regular in cars designed to run on premium fuel can expect to trim performance by about the same percent they save at the pump. If the car is sufficiently new and sophisticated, it may not suffer any ill effects, but all such skinflints should be ready to switch back to premium at the first sign of knock or other drivability woes. And finally, if a car calibrated for regular fuel begins to knock on anything less than premium or midgrade, owners should invest in a tuneup, emissions-control-system repair, or detergent additives to solve, rather than bandage, the root problem. Class dismissed.

kevink2

Sorry, too much to read now MB, I'll read it later.

IMHO, the basic reason the Benz needs higher octane vs the Hyundi is the slighly higher static (ie non-existent on the road) CR, and more important, a much higher dynamic compression ratio, that includes the effects of the cam timing.

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JamesKim

Thx.

But, isn't it false advertisement for Hyundai to claim the vehicle gets 10.5:1 compression ratio if it won't?

I didn't understand any of that, but I'm going to assume that I have to put premium. Oh well, it was what I expected when I first bought it.

Still pissed off about the premium gas to regular difference in Canada though. 13 Cents per liter seems so dumb when the neighbors down in the state barely pays 5-6 cent more.

MBRedux

No, it's not false advertising, because there's nothing "false" about it. Retarding the timing does not reduce the compression ratio. The Hyundai has the advertised C/Ratio, but they decided to retard the ignition timing (thus giving the car the capability to use a lower octane fuel) at the cost of lower HP output.

But all cars these days have this capability built-in... it's called a "knock sensor". If you use regular gas in your Mercedes, it will pre-detonate (knock), and the engines ECU will automatically retard the timing to compensate. It will also reduce your engine's HP/Torque, fuel mileage and make it run a little hotter. This is okay but only when you have no choice. MB recommends the proper grade fuel for their engines to achieve the best overall performance and trouble free motoring.

JamesKim

At least I'm actually getting better than the listed mileage of 20 MPG on my car, getting around 22-23 City. Thx.

Domm

Could be driving habits. If you don't need all the horsepower, then no one is stopping you from using regular over premium. At least you educated yourself on the subject first

MBRedux

I wouldn't use anything less than 91 R+M/2 pump Octane. Remember we're talking about 91 minimum on the Ron+Mon/2 average method.

Quoted from the C&D article above: "Cheapskates burning regular in cars designed to run on premium fuel can expect to trim performance by about the same percent they save at the pump. If the car is sufficiently new and sophisticated, it may not suffer any ill effects, but all such skinflints should be ready to switch back to premium at the first sign of knock or other drivability woes."

JamesKim

That doesn't apply to Canadians, because we actually pay significantly more for premium gas, so we would save money. I'll just use premium though, it seems like it's the way to go. Maybe I'll try using regular once or twice in the winter, just to test it out.

kevink2

But it's a less useful term for determining performance, vs "dynamic compression ratio"

Just asking ... how do you know the hyundi uses more timing retard?

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Glyn M Ruck

OP ~ All you really need to know is that using regular fuel will keep the timing of your engine retarded as the knock sensor detects the onset of knock. This will adversely effect engine efficiency & consumption.

MBRedux

This^

MBRedux

Because it's simply the easiest solution to maintain a high compression ratio engine (which the Hyundai has) but still be capable of running it on lower octane gasolines.

kevink2

logical assumption.

tasho3

I used mid grade once, and the MPG was noticeably worse. so cheaper gas, less power, terrible MPG IMO

KlinksGarage

You guys are lucky you dont live in New Zealand... OUr GOvernment has taxed the hell out of our Fuel. Its about 70% tax now. so a Litre costs about $2.15 or $1.80 USD.

BauerR

Which engine do you have?
The 3.0L V6 has a compression ratio of 11.3:1.

Glyn M Ruck

Join the happy club. Same in SA.

TexFinn

That's actually still fairly cheap. Last time I was in Finland a couple of months ago, the price was around $2.22 per liter. Even diesel I filled in my rental was around $2.00 per liter. USD, that is.

And I'm pretty sure they haven't gone down too much since then, if at all.

Oh..., and what comes to the subject of this thread, my wife has now had her car for just over a year and since she has filled the tank probably at least 90% of the time, that car has seen pretty much nothing but regular.

Yeah..., I know... Don't get me started on that. I have tried, but since she will not listen, I have given up, that's all. And no, she is not doing that because she wants to save 50-60 cents per tank of gas.

kevink2

Hey, our government is just warming up on tax increases !

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kevink2

One thing that was suggested, that I would not try is just running 87 octane (US low octane), and running on the knock sensors. I guess if you live in a moderate climate in a flat state/country, and you could drive with light loads without being a danger to other drivers, it might be worth a try. I'd suggest doing data logging to make sure you are not knocking.

The ecu has a conservative reaction to knock, and may even increase the retard increment (vs the 1st knock reaction) if a second knock is detected.

It may be worth the effort to get a tuner to detune the ecu to judiciously add retard to the timing maps. If you save 50 cents/gal, there may be a lot of interest.

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dmm_c250

Not sure about other countries, in Canada the premium grade has less ethanol. 87 is up to 10%. Technically, ethanol has a lower energy density (creates less energy when burning) which results on lower mpg.
Another + for using premium.

Larry3Jswft

Here in Seattle Premium is 91 octane and can be 10% inkyhol, too!

Fung63

imo you buy the car that fits your lifestyle, usage and wallet. no point in getting a car that requires premium and you cant afford to pump the gas into it. As the article and subsequent posts suggests, the performance of your car is adversely affected in direct proportion of $ saved. this will also negatively impact your experience driving said car.

not worth it

Hey I drive a C63 and know how tough the gas is on the wallet but that's ok with me because every km i spend in it, is pure bliss.

Big Daddy

in Australia the minimum available is normally 90 RON. or about $1.30 a litre 95 RON is the premium grade
The local BP pump is supplying 98 RON available for $1.55 a litre for the petrol heads that have cars that require this fuel