Column Shift

Frank L. asks: My Volkswa­gen Jetta has a turbocharged en­gine and the manufacturer rec­ommends 91 octane.

I used to think that this re­quirement was related to the turbo, but lately I’ve noticed that some manufacturers recom­mend higher octane for non-tur­bocharged engines, too. Am I going to damage my engine if I use a lower octane than the manufacturer suggests, or will I just see lower perfor­mance?

Is there any advantage to using higher octane versus the manufacturer’s recommenda­tion? That engine is a gem and is found in vehicles ranging from mainstream cars like the Jetta to rockets like the souped-up Audi TT.

The simple answer is that the engine is so smart that you can feed it “regular” unleaded and it will be O.K., according to Dar­ren Daley at Daley’s Service in Norfolk Center. Darren tells me that during all his years as a me­chanic he has not seen any car brand have trouble using regular unleaded — except BMW. I asked him about the other premium car brands like Acura and Infiniti and he stuck to his statement. Let’s look more closely at this issue, starting with why the oc­tane rating matters. The octane rating of gasoline is a rating of its resistance to detonation in the engine. Detonation is an im­proper, too-rapid burning of the fuel. The results of detonation are now mainly part of old-car lore, and are known as “knock­ing and pinging.” Back when people used to think of gas as inexpensive, fuel manufacturers needed a way to try to differentiate their products from their rivals. They started to offer gasoline blends that were a little bit different from the con­ventional blend. Some engine tuners could make use of the difference to adjust an engine in a way that would produce just a smidgen more power per drop of fuel consumed. From this evolved the current state of affairs in the United States, where you are of­fered many different octane lev­els at the pump. In modern cars of almost every make and model the en­gine

contains what is called a “knock sensor.” This is really just a microphone. It listens for the sounds that the engine starts to make if the fuel is not being properly burned in the engine. When it detects the sound it sends information to the car’s computer. The computer then looks at the exact location of the pistons in their stroke and it can tell how the car should adjust the timing to correct the situation.

“Timing” is a word that en­compasses a few different things, but can be simplified to mean the time that the spark is ap­plied during the car’s compres­sion stroke in the engine. The car will automatically adjust the timing of the spark (and some other things) so that the fuel is burned to its best effect and the car generates as much power as possible from that specific fuel it is being fed. Old cars had none of this. The timing could only be adjusted manually and mechanically by a technician when the car was not being driven. So you could only have one setting. If the fuel did not well match the setting there was knocking and pinging. One interesting fact many folks don’t know is that the re­fineries don’t actually make all those different octane blends (85, 87, 89, 91, 93). They just make a high and a low and those two are blended to make up all the other ones.

How do I know this? Back in the 1990s I worked at a company that made parts for some of the older blending systems. Newer gas pumps can actually do this blending “on the fly” and deliver whatever you select at the pump. Some gas stations offer up to fivedifferent octane levels at the pump. By only refiningtwo blends they can just send out two trucks to each gas station, instead of three to five. As a car nut and a person passionate about all things pow­er and-energy-related, I think having more than one octane rating for gasoline is crazy and wasteful. All of the premium au­tomakers

have engine designs so powerful that the few percentage points of extra horsepower pos­sible from 93 octane gas versus “regular” 87 octane are not no­ticeable in normal driving. Base model cars with the less power­ful

engines are all rated to use regular, 87 octane gas. That is ironic, since in those cars a few more horsepower might be gen­uinely useful, so the logic of this is all backwards. Frank’s situation is even more frustrating, in that Volkswagen suggests 91 octane, which is not usually sold in New England. We typically get to choose from 87, 89, and 93. Why don’t the auto­makers

get together and just pick a middle rating and make all the cars operate properly using that one? Why doesn’t our govern­ment, which seems to meddle in everything else related to auto­mobiles and fuel, mandate one octane number? I can tell you why, but then I would be writing a political ar­ticle, not an automotive column. One thing that the car makers started to experiment with re­cently

is to market some of their fancy cars as “Able to use regular gas.” They became aware that some frugal folks did not want to buy cars that recommended premium fuel because they real­ize

there is no practical benefit to the more expensive gas. I hope this trend sticks. When you shop for your next car, ask the dealer to ex­plain to you — and show you in the owner’s manual — what the manufacturer’s policy is regard­ing

using regular fuel. In almost all cases the dealer will be able to show you something stating that regular is O.K. (even if the manufacturer says say premium is recommended).

Finally, a quick math chal­lenge for you: Let’s say you burn a full tank of gas each week. What is the annual difference in price between 87 octane and 93? The answer is “about $150.” If you have a question or comment, please email me at john@columnshift.com.