Do you use premium gas?

[Groves]

I'm buying a 2009 Odyssey from my in-laws. They're good folks.

I've never ever bothered putting anything but cheap gas in my vehicles.

I'm just checking in to see if that's foolish or as I have thought, not a problem.

Yes, the valve guides would have suffered from unleaded in my '62 Buick, so the lead I did ad back in.

These modern cars, not so sure. What say ye?

[Tombstone RJ]

Well, that's a bummer. I wonder how those additives (octane booster) would work on an engine that is NOT in good running condition, that is, the engine is old, tired and not running as efficiently as it was meant to. How does it work then?

[Donger]

http://www.caranddriver.com/features/regular-or-premium

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 reerun 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.

[ModSocks]

Originally Posted by Radar Chief

Never heard of that one.
But I do know that running 91 octane in an 87 octane motor is sending more unburned fuel down the exhaust and can clog your cat(s). Saw that one for myself.

I'll add that one to the list of things i've heard.

Just more reason to listen to what the owner's manual says.

[ModSocks]

Originally Posted by Donger

http://www.caranddriver.com/features/regular-or-premium

But the article doesn't answer any questions about using alternate octanes over a long period of time and what kind of effects if may have on the longevity of the car, which is the main issue imo.

[Donger]

Originally Posted by Detoxing

But the article doesn't answer any questions about using alternate octanes over a long period of time and what kind of effects if may have on the longevity of the car, which is the main issue imo.

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.

[ModSocks]

Originally Posted by Donger

[I]Our tests confirm that for most cars there is no compelling reason to buy more expensive fuel than the factory recommends,

Right, but it's a 2-day study. You can't possibly reach any conclusions regarding the longevity of the components in the car to see what kind of affect the fuels may have. As Radar noted, running 91 in a 87 octane car prematurely ****ed out his Cat.

And my concern really isn't about 87 Octane cars running on 91, but rather 91 octane tuned cars running on 87.

So really, the article didn't do much to answer the question other than to let us know that 91 Octane doesn't really boost performance in an 87 octane car.....but that should be a bit of common sense. I found that out with my Firebird back when i was 19.

[Donger]

Originally Posted by Detoxing

Right, but it's a 2-day study. You can't possibly reach any conclusions regarding the longevity of the components in the car to see what kind of affect the fuels may have. As Radar noted, running 91 in a 87 octane car prematurely ****ed out his Cat.

And my concern really isn't about 87 Octane cars running on 91, but rather 91 octane tuned cars running on 87.

So really, the article didn't do much to answer the question other than to let us know that 91 Octane doesn't really boost performance in an 87 octane car.....but that should be a bit of common sense. I found that out with my Firebird back when i was 19.

I'm not familiar with any potential damage that running 87 in a motor that "requires" 91 will cause. For me, it's a question of am I willing to spend another $0.30/gallon to not lose 20% performance reduction? My answer is yes.

[Holladay]

Damn Donger. You pissed in my cheerios I have been using octane boost off and on for years.

Welp, learned my lesson. Thanks

[ModSocks]

Originally Posted by Donger

I'm not familiar with any potential damage that running 87 in a motor that "requires" 91 will cause. For me, it's a question of am I willing to spend another $0.30/gallon to not lose 20% performance reduction? My answer is yes.

But as the article noted, the car may start experiencing "knock" and if it does then you'd want to switch back to the appropriate fuel.

When i bought my Turbo Mazda i asked my business partner (A master fabricator/technician) if he thought the 91 Octane was required. His advice was to try it. If the car starts pinging louder than normal, then you need to switch back. Mine did, so i switched back.

My car is also known to have timing chain issues, which results in catastrophic engine failure. I have to wonder if those timing chain failures that people are reporting have anything to do with using improper fuel.

[ModSocks]

For me, Donger, it isn't about the performance of the car, but whether or not improper fuel can cause premature failure of components.

So with that said, I'll play it safe and pony up the extra 10-15 cents per Gallon.

[Donger]

Originally Posted by Detoxing

But as the article noted, the car may start experiencing "knock" and if it does then you'd want to switch back to the appropriate fuel.

When i bought my Turbo Mazda i asked my business partner (A master fabricator/technician) if he thought the 91 Octane was required. His advice was to try it. If the car starts pinging louder than normal, then you need to switch back. Mine did, so i switched back.

My car is also known to have timing chain issues, which results in catastrophic engine failure. I have to wonder if those timing chain failures that people are reporting have anything to do with using improper fuel.

Sorry, I thought that was understood. Yes, if running 87 in a 91 motor leads to pre-ignition, you absolutely want to switch back to 91.

I don't see why running lower octane would cause a timing chain/belt failure. But yes, interference engines are not much fun. I just replaced my belt because it's 14 years old but only 50,000 miles. The tech handed me the belt and just grimaced. No wear at all... But, I knew that if it goes, goodbye engine.

[loochy]

Originally Posted by Detoxing

But as the article noted, the car may start experiencing "knock" and if it does then you'd want to switch back to the appropriate fuel.

When i bought my Turbo Mazda i asked my business partner (A master fabricator/technician) if he thought the 91 Octane was required. His advice was to try it. If the car starts pinging louder than normal, then you need to switch back. Mine did, so i switched back.

My car is also known to have timing chain issues, which results in catastrophic engine failure. I have to wonder if those timing chain failures that people are reporting have anything to do with using improper fuel.

I think most modern cars have knock detectors that dial things back if excessive knock is detected.

[GloryDayz]

Originally Posted by loochy

I think most modern cars have knock detectors that dial things back if excessive knock is detected.

Yeah, my 2002 Silverado has one that's pissing me off! Right after I cleaned the engine (t'was dirty as a power steering line had been leaking for a few months!!!), but ever since I get the knock sensor warning. I've read that it might be from the moisture I introduced while cleaning the engine, but it's been more than a month and I'm getting tired of resetting it.

And the fix seems like it has the potential to break as many things as replacing the sensor would alleviate.

But hey, it's a 2002, 250,000+ miles and I could not be more happy.

[scho63]

I read this article the other day and found it helpful. If I had a vehicle that said "premium required", I wouldn't risk a lesser grade

http://www.edmunds.com/fuel-economy/...g-premium.html

Edmunds has compiled two lists: "premium recommended" and "premium required" for vehicles from the 2010-2015 model years (with some 2016 model-year vehicles). If your vehicle is on the "premium recommended" list, you're OK to try switching to regular unleaded gasoline. If, on the other hand, your car is on the "premium required" list, then you have to run premium fuel. You can confirm the information on these lists by checking your owner's manual.

"premium recommended"
http://static.ed.edmunds-media.com/u...ded.071515.pdf

"premium required"
http://static.ed.edmunds-media.com/u...red.071515.pdf

[Donger]

Originally Posted by Holladay

Damn Donger. You pissed in my cheerios I have been using octane boost off and on for years.

Welp, learned my lesson. Thanks

Sorry.