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I just recently got a 2005 Volvo S40 with a turbo engine; I want to take good care of it, so naturally, when the instruction manual recommends 91 octane fuel but says it takes a minimum of 87, I have no problem with buying 91, knowing (or at least believing) that it's better for my engine and improves performance and fuel efficiency compared to just shoving regular unleaded into its tank. I checked online, read a bit about engine knocking and whatnot, and was determined that's what I'd do, and the previous seller also filled it with premium gas so it has been well taken care of thus far.

This morning I stopped at a local gas station to refill it for the first time. My fuel octane choices were 87, 89, or 93. Oh no! I probably should have hopped back in my car and went looking for a "normal" gas station, with premium at 91 octane... But instead I just choice 89 and made a mental note to ask this question.

Should I have chosen 93? Or would that have been "too much"? Is it better to choose a bit higher or a bit lower? Could one be more harmful than the other to my engine?

I'm in the USA if it matters.

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Are you running the car at high speed (150MPH+), or commuting on the local highway (60-70MPH)? If the latter, you are likely wasting money with higher octane fuel anyway. High performance cars (Porsche, Lamborghini, etc) with compression rations of 12:1 and greater may require high octane fuel, but most passenger vehicles do not. –  Tester101 Jul 21 '11 at 16:03
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Indeed it's not required but is recommended. I am only commuting, but I have the T5 engine (inline-5 with turbo - I don't know the compression ratio) and love to accelerate. It could just be me but I think I have noticed a difference today with the 89 octane gas, and though I won't test it personally, some people have pointed out that the reduced fuel efficiency might cost even more money than the few extra dollars for the higher octane fuel. The general consensus seems to be that running lower-than-recommended fuel shouldn't be done long-term. –  Ricket Jul 21 '11 at 21:09
    
I'm skeptical the fuel efficiency would be degraded enough during typical driving conditions, to compensate for the higher fuel cost. (That sounds like something Big-Oil would say). –  Tester101 Jul 21 '11 at 22:08

8 Answers 8

up vote 4 down vote accepted

Being in the midst of summer, I may have erred on the side of caution and gone with the 93 but I'm sure the 89 hasn't been harmful to your vehicle if the manual states you can put 87 into it.

Since you mentioned knocking, it sounds like you have already done some research. Still, to cover it, the higher rated octane fuel is essentially "harder" to ignite and so is called for in either high compression engines or when using forced induction (turbocharger or supercharger).

A lower octane fuel can have the tendency to ignite prematurely which is called knocking or detonation. This can cause many different issues as your pistons and valves will not be at the right positions for the proper engine rotation.

Since your car's manual says it will take 87 but recommends 91, most likely it has a knock sensor that when it detects early detonation it changes characteristics of the engine's operation to prevent further knocking. This will prevent damage to the engine, but at the cost of performance and possibly fuel efficiency.

The reason I said erring on the side of caution with the 93 for summer is that depending on where your engine really falls in its need for 91 for optimal performance, 89 may or may not trip the knock sensor and cause that degraded state. One factor that will impact it on a turbocharged engine is the charged air temperature, which in the summer is already starting at a higher temperature.

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So lower octane fuel ignites under pressure more easily; but is higher octane somehow harder for the spark plugs to ignite or something? Can you put high-octane gas in an engine that calls for lower-octane? I clearly see the disadvantage in putting lower octane fuel in an engine, but what is the disadvantage in using too high octane fuel? –  Ricket Jul 21 '11 at 13:17
    
Correct, essentially as the piston is still moving up and compressing the air/fuel mixture the lower octane has a higher chance of igniting before the parts have traveled to their proper positions and the spark fired. –  ManiacZX Jul 21 '11 at 13:25
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As long as the ignition system is healthy (sparks not corroded, faulty wires, etc) which would hurt you with any octane, it should be able to ignite the higher octanes as a spark is a much stronger ignition source. Some people argue that there are negative effects to using a higher octane than needed, but for the most part, the biggest agreed upon I see is that you are just wasting your money running more then what your car needs to perform. –  ManiacZX Jul 21 '11 at 13:25
    
Okay thanks! I've learned my lesson, I'll top off a few times the next couple days to try and dilute the 89 octane with a higher octane (this time I'll search out 93 octane) and use 91 or better from now on. –  Ricket Jul 21 '11 at 13:34
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I wouldn't worry about it that much, if the car seems normal, it probably isn't triggering. They have to set those guidelines to deal with conditions all year long and all over the world. –  ManiacZX Jul 21 '11 at 13:42

Just to add to what ManiacZX covered in his answer: On modern turbocharged cars, the ignition computer will prevent knocking, which will prevent damage. You will end up with a bit less power, and a bit less fuel mileage.

I have never heard of a car being damaged by using a higher octane fuel than needed - in fact, many gas companies try temp people to do just that with their advertizing when it's not needed.

Next time you need 91 octane, but only have a choice between 89 and 93, fill it with 1/2 of each - this will give you what's needed.

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They don't have what you want? Make it! –  Mark Johnson Apr 12 '12 at 5:19

My wife's car, though designed to run on 99 RON or above can run on lower Octane numbers, but if you do use lower Octane fuel the engine de-tunes itself dramatically, and the process to tune it back up again is difficult to do yourself - so be aware of the particular tuning requirements for your car.

Because of this, my wife's car now runs on 98 RON and has lost around 20 BHP - now down to about 320 BHP, so you can feel the difference - luckily she can cope with it so we are just leaving it requiring lower octane fuel.

My own car will end up with a damaged engine if I try to use anything under 99 RON, so when I go touring round the highlands, where there are few garages with proper fuel, I need to take a can of Toluene with me (it is about 114 RON) to mix in with each tank of fuel.

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"the process to tune it back up again is difficult to do yourself" - what do you mean by this? I thought it compensated accordingly both ways; are you talking about an older engine? –  Ricket Jul 21 '11 at 15:28
    
@Ricket, every car is going to be different, some might recheck every time new fuel is added, some could require an ECU reset by disconnecting the battery and others may require using diagnostic tools, just depends on what the manufacturer felt like doing. –  ManiacZX Jul 21 '11 at 15:51
    
@Ricket - no, typically the highly tuned Subaru engines (think PPP/STi) will detune to protect from damage but require manual work to tune back up again. –  Rory Alsop Jul 21 '11 at 17:12
    
Okay good to know! I'm going to guess/assume that mine is fine, and perhaps I'll ask to make sure when I bring it in for its routine maintenance here in the next couple weeks. –  Ricket Jul 21 '11 at 21:24
    
@RoryAlsop Seriously? I find that hard to believe. Do you have a citation/source on this? –  Mark Johnson Apr 12 '12 at 5:20

I'm not sure whether this is relevant, but the lowest octane rating you can buy in the UK is 95. I recently filled up with 99 octane by accident, and haven't noticed much difference. There is commonality of quite a few models across the pond, so whatever's suitable there should be suitable here, unless they come with a different ECU configuration. It's possible that the definition of octane number is different between here and the USA - anyone able to shed any light on this?

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As is often the case, wikipedia has the answer. en.wikipedia.org/wiki/Gasoline#Octane_rating just appears to be different measurement systems. –  Jessie Sep 19 '11 at 13:57

Your manual states 91 octane fuel (I checked). Filling up with a lower octane fuel will cause knocking and the computer will retard timing to prevent engine damage. You won't hear it, the knock sensor is more sensitive than your ears. Running in this retarded state isn't a normal operating parameter and not recommended. Go with the 93, the .20 more per gallon isn't enough of a savings to go with the 89. The owner's manual states 87 octane is the minimum.

You will not cause engine damage by using an octane several points above the manufacturer's recommendation since this will not cause spark knock and the engine will not then have to retard timing, which are the detrimental operating conditions.

Volvo recommends premium fuel for a reason.

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I have a 2004 VW Passat Turbo. It also calls for 91 octane which I hardle ever see. Like you noticed, most stations have 87,89 and 93 where i live. When we first got the car we always used 93, then about a year in we switched to 89 and did not notice a difference. Then a few years later the VW service shop said 87 is fine, it wont hurt anything. As others have mentioned, it may decrease performance, but other than that, no biggie. So we switched to 87, have never had any knocking, or even any performance issues for that matter.

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You may not have had knocking because your ECU recognized the situation and has adjusted accordingly, making your engine produce less power and probably poorer fuel mileage. The mileage difference may not make up for the difference in fuel cost, but getting less power is no fun :(. –  ManiacZX Jul 21 '11 at 15:54
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Often, the difference in fuel efficiency will make it more expensive to run lower octane fuel. –  qes Jul 21 '11 at 17:46

I'd give it the 93. It's not pretty when a turbo engine starts knocking. You've almost certainly got a knock sensor to detect that and retard timing (reducing power and increasing fuel consumption) to help protect you. However, it's not wise to rely on that. It's a safety device in case you get bad fuel. Continual knocking will eventually kill the sensor. Once the sensor's dead, your car will happily give you full timing advance even on too low an octane. Do that a couple times at full boost and you'll have a very good chance on having to do an engine replacement/rebuild...

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Even if you're ecu changes to help prevent damage to youre engine, you're still causing damage to your head gasket.. This is one of the reasons subaru has had so many head gasket issues.. The manual says 87 or higher recommended... They didn't want Americans to not buy the car if they recommended premium fuel for an awd car.. So if the car says 87 or higher, go higher to prevent major engine repairs in the future..

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Can you explain how this damages the head gasket in the absents of spark knock? –  Larry 18 mins ago

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