Fuel Cap says 91. Gas pumps say 87-89-93 so what is best?

[Kram71]

Do I mix 89 and 93?

[khammack]

The octane number is a measure of how well the gas will keep knock from happening. Knock is not good for an engine, so I always use gas with at least the required octane. So, in your case, I would put 93 in my car.

[Mr Bingley]

93

[Kram71]

Thanks guys. I put 93 this morning. I did not think knocking was a major concern for these cars or most modern cars but what the heck.

[PbFut]

In part Octane is a rating on how slow and hot the gas will burn. Higher the number the slower the burn rate. This allows time for the higher compression piston to get pushed back down the cylinder. If the air fuel burns too fast you are basically smacking the top of the piston with out of control explosion. This gives you that ping sound. Lower octane burns faster and hotter. So if the motor can handle the lower octane, it is better performance wise. But to protect the engine, higher within reason, is always recommended. Like almost everything else in car performance, it is a balance.

[JRowe]

These engines have knock sensors built in to retard the ignition if knock is detected. It will not harm your engine to use a lower octane as the engine management will adjust, but it could result in slightly decreased performance.

[zellamay]

While we're on this topic. If one puts 87 in the car, won't the car compensate to avoid knock? I guess this would mean less power, but still driveable.

[JRowe]

While we're on this topic. If one puts 87 in the car, won't the car compensate to avoid knock? I guess this would mean less power, but still driveable.

yes. Engine management will compensate. 20+ years ago when I was in college, I worked part time as a mechanic at an independent BMW shop. I still keep in touch with the owner regularly, and I've discussed this topic with him numerous times. He also regularly gets this question from clients. His standard response is as stated above, the engine management will compensate, so you do not risk damaging your engine. Then, he also challenges anyone to be able to detect the difference from one tank to another of gas with varying octanes.

So, I'm on BMW number 7 for me, and I've used varying octanes in all of them. I've never been able to notice a difference. Perhaps if I had an aftermarket tune or where doing regular performance driving I would, but my cars have been daily drivers. I've not noticed any difference. Maybe some can depending on the car and the gas in your local market, but for me in KS, there's no meaningful difference.

[BimmerBreaker]

I've explained the concept of octane multiple times very in-depth so won't do so again here but all octanes of gas have the same energy density. Higher octane fuel does not contain more energy. Higher octane fuel is ONLY harder to combust than lower octanes. That's it. As PfFut mentioned, this means the engine can compress these fuels more. More compression = greater power. There are other things like burn rates that affect it too but the critical piece is how compressible it is. Higher octane fuels can be compressed further before igniting, which means simply they are harder to combust. For this reason, it is often times more dangerous to run high octane in a low octane engine, vs. running low octane in a high octane engine. Low octane in a high octane engine will combust easily, so easily it may knock. Knock detectors will detect that and pull timing to compensate. High octane in a low octane engine though, the engine does not physically have the means to compress the fuel to where the fuel was designed to be compressed to. It will ignite it later, which in these types of engines is often a double edged sword, because later ignition also means less time for the fuel to fully combust. In extreme circumstances, a low octane engine will guzzle high octane gas because it is not fully combusting it.

As such, I have often advised people on long road trips to run a lower octane. On road trips you arent usually flooring it, so knock isn't a concern. Running a lower octane fuel will allow easier and more complete combustion and will often see (slightly) better mpg.

yes. Engine management will compensate. 20+ years ago when I was in college, I worked part time as a mechanic at an independent BMW shop. I still keep in touch with the owner regularly, and I've discussed this topic with him numerous times. He also regularly gets this question from clients. His standard response is as stated above, the engine management will compensate, so you do not risk damaging your engine. Then, he also challenges anyone to be able to detect the difference from one tank to another of gas with varying octanes.

So, I'm on BMW number 7 for me, and I've used varying octanes in all of them. I've never been able to notice a difference. Perhaps if I had an aftermarket tune or where doing regular performance driving I would, but my cars have been daily drivers. I've not noticed any difference. Maybe some can depending on the car and the gas in your local market, but for me in KS, there's no meaningful difference.

You are only likely to notice the difference when flooring it near redline, even then, you may not always notice. Because there may not always be a difference. It will pull timing depending on multiple factors, but fact is - there isn't an octane sensor. The car will behave normally, unless it has a reason not to via the knock sensors detecting knock. So unless the knock sensors are triggered, the timing will remain advanced. So if you normally drive and cruise around, particularly in an automatic, shifting at 3 or 4000 or whatever, it wont ever have to pull timing and it will behave exactly the same with any octane.

[Tigershark48]

Based on all the advice, it's your choice. Flip a coin and fill up. I went through exactly the same feedback on my BMW motorcycle gas options. If it knocks on the lowest octane, go up one level until it stops knocking. Neither my bike nor M roadster knock with low octane.

[Kram71]

In part Octane is a rating on how slow and hot the gas will burn. Higher the number the slower the burn rate. This allows time for the higher compression piston to get pushed back down the cylinder. If the air fuel burns too fast you are basically smacking the top of the piston with out of control explosion. This gives you that ping sound. Lower octane burns faster and hotter. So if the motor can handle the lower octane, it is better performance wise. But to protect the engine, higher within reason, is always recommended. Like almost everything else in car performance, it is a balance.

These engines have knock sensors built in to retard the ignition if knock is detected. It will not harm your engine to use a lower octane as the engine management will adjust, but it could result in slightly decreased performance.

While we're on this topic. If one puts 87 in the car, won't the car compensate to avoid knock? I guess this would mean less power, but still driveable.

yes. Engine management will compensate. 20+ years ago when I was in college, I worked part time as a mechanic at an independent BMW shop. I still keep in touch with the owner regularly, and I've discussed this topic with him numerous times. He also regularly gets this question from clients. His standard response is as stated above, the engine management will compensate, so you do not risk damaging your engine. Then, he also challenges anyone to be able to detect the difference from one tank to another of gas with varying octanes.

So, I'm on BMW number 7 for me, and I've used varying octanes in all of them. I've never been able to notice a difference. Perhaps if I had an aftermarket tune or where doing regular performance driving I would, but my cars have been daily drivers. I've not noticed any difference. Maybe some can depending on the car and the gas in your local market, but for me in KS, there's no meaningful difference.

I've explained the concept of octane multiple times very in-depth so won't do so again here but all octanes of gas have the same energy density. Higher octane fuel does not contain more energy. Higher octane fuel is ONLY harder to combust than lower octanes. That's it. As PfFut mentioned, this means the engine can compress these fuels more. More compression = greater power. There are other things like burn rates that affect it too but the critical piece is how compressible it is. Higher octane fuels can be compressed further before igniting, which means simply they are harder to combust. For this reason, it is often times more dangerous to run high octane in a low octane engine, vs. running low octane in a high octane engine. Low octane in a high octane engine will combust easily, so easily it may knock. Knock detectors will detect that and pull timing to compensate. High octane in a low octane engine though, the engine does not physically have the means to compress the fuel to where the fuel was designed to be compressed to. It will ignite it later, which in these types of engines is often a double edged sword, because later ignition also means less time for the fuel to fully combust. In extreme circumstances, a low octane engine will guzzle high octane gas because it is not fully combusting it.

As such, I have often advised people on long road trips to run a lower octane. On road trips you arent usually flooring it, so knock isn't a concern. Running a lower octane fuel will allow easier and more complete combustion and will often see (slightly) better mpg.



You are only likely to notice the difference when flooring it near redline, even then, you may not always notice. Because there may not always be a difference. It will pull timing depending on multiple factors, but fact is - there isn't an octane sensor. The car will behave normally, unless it has a reason not to via the knock sensors detecting knock. So unless the knock sensors are triggered, the timing will remain advanced. So if you normally drive and cruise around, particularly in an automatic, shifting at 3 or 4000 or whatever, it wont ever have to pull timing and it will behave exactly the same with any octane.

Based on all the advice, it's your choice. Flip a coin and fill up. I went through exactly the same feedback on my BMW motorcycle gas options. If it knocks on the lowest octane, go up one level until it stops knocking. Neither my bike nor M roadster knock with low octane.

Thanks guys. I appreciate all the explanations!

[R Shaffner]

Whoa guys. An important factor has not been mentioned here, which is that the car's computer will adjust the fuel and spark timing to compensate -- not just to avoid knocking but to improve efficiency and performance. That means higher output and better gas mileage (relative to the performance). That's why BMW's recommendations are for higher octane gas.

If you drive in a spirited way, at all, then you can definitely tell the difference between a tank of 87 and 93. If you drive in a way that is sedate and never spirited, and minimal cost is your goal, then sure you can get there cheaper with 87 (or with a true economy car, of course).

[R Shaffner]

I've explained the concept of octane multiple times very in-depth so won't do so again here but all octanes of gas have the same energy density. Higher octane fuel does not contain more energy. Higher octane fuel is ONLY harder to combust than lower octanes. That's it. As PfFut mentioned, this means the engine can compress these fuels more. More compression = greater power. There are other things like burn rates that affect it too but the critical piece is how compressible it is. Higher octane fuels can be compressed further before igniting, which means simply they are harder to combust. For this reason, it is often times more dangerous to run high octane in a low octane engine, vs. running low octane in a high octane engine. Low octane in a high octane engine will combust easily, so easily it may knock. Knock detectors will detect that and pull timing to compensate. High octane in a low octane engine though, the engine does not physically have the means to compress the fuel to where the fuel was designed to be compressed to. It will ignite it later, which in these types of engines is often a double edged sword, because later ignition also means less time for the fuel to fully combust. In extreme circumstances, a low octane engine will guzzle high octane gas because it is not fully combusting it.

As such, I have often advised people on long road trips to run a lower octane. On road trips you arent usually flooring it, so knock isn't a concern. Running a lower octane fuel will allow easier and more complete combustion and will often see (slightly) better mpg.



You are only likely to notice the difference when flooring it near redline, even then, you may not always notice. Because there may not always be a difference. It will pull timing depending on multiple factors, but fact is - there isn't an octane sensor. The car will behave normally, unless it has a reason not to via the knock sensors detecting knock. So unless the knock sensors are triggered, the timing will remain advanced. So if you normally drive and cruise around, particularly in an automatic, shifting at 3 or 4000 or whatever, it wont ever have to pull timing and it will behave exactly the same with any octane.

I have no desire to start a fight, but I disagree with at least half of this -- probably every-other statement and conclusion. WAY too simplistic and not well-stated, if well-reasoned. Sorry.

For example, the octane rating has nothing to do with how "hard" it is for an engine to compress a mixture of fuel/air. (The volume of the air/fuel charge would, of course. When the throttle is wide open and/or when the air charge is boosted, then sure it's harder to compress. But that would be true regardless of the octane rating.)

Yes, higher octane gas is less likely to pre-ignite (knock) and it burns a little slower. But our BMW engines aren't stupid. They know that and adjust. They'll add even more gas when the demand is there. And they'll advance the spark for whatever the fuel/air charge is. Or they'll maintain the advanced timing for high octane gas when the car would have to retard it for lower octane fuel. More advanced timing, without knocking, increases power AND efficiency.

In short, the ONLY reason for ever running lower octane fuel in your modern BMW is because it's cheaper.

[Adker]

Due the the blend used in higher octane fuels, it will contain slightly less energy per gallon than lower octane fuel. Lots of reading on the subject, best summary I have run into is:

"Octane does not offer any better fuel mileage, increase engine horsepower, or make the engine start quicker. Higher octane only reduces the likelihood of engine knock or ping."

How the engine management system uses knock detection effects the engines ability to generate all the HP the manufacture designed into it.

[BimmerBreaker]

I've heard that too about the higher octanes having slightly less energy density due to the additives. I've also heard this applies to any top tier gasoline though, so I'm not really sure about that. Really hard to find info on gasoline additives... (not surprisingly)

Whoa guys. An important factor has not been mentioned here, which is that the car's computer will adjust the fuel and spark timing to compensate -- not just to avoid knocking but to improve efficiency and performance. That means higher output and better gas mileage (relative to the performance). That's why BMW's recommendations are for higher octane gas.

If you drive in a spirited way, at all, then you can definitely tell the difference between a tank of 87 and 93. If you drive in a way that is sedate and never spirited, and minimal cost is your goal, then sure you can get there cheaper with 87 (or with a true economy car, of course).

Really? Nobody mentioned that?

It will pull timing depending on multiple factors, but fact is - there isn't an octane sensor. The car will behave normally, unless it has a reason not to via the knock sensors detecting knock. So unless the knock sensors are triggered, the timing will remain advanced. So if you normally drive and cruise around, particularly in an automatic, shifting at 3 or 4000 or whatever, it wont ever have to pull timing and it will behave exactly the same with any octane.

I have no desire to start a fight, but I disagree with at least half of this -- probably every-other statement and conclusion. WAY too simplistic and not well-stated, if well-reasoned. Sorry.

For example, the octane rating has nothing to do with how "hard" it is for an engine to compress a mixture of fuel/air. (The volume of the air/fuel charge would, of course. When the throttle is wide open and/or when the air charge is boosted, then sure it's harder to compress. But that would be true regardless of the octane rating.)

What I meant, as you probably could have discerned from reading the rest of my post, is that higher octane fuel CAN be compressed before pre-igniting (knocking). Engines designed for lower octanes will have trouble burning higher octane fuels properly. Octane rating is a measure of how much work is required to pre-detonate a fuel.

IYes, higher octane gas is less likely to pre-ignite (knock) and it burns a little slower. But our BMW engines aren't stupid. They know that and adjust. They'll add even more gas when the demand is there. And they'll advance the spark for whatever the fuel/air charge is. Or they'll maintain the advanced timing for high octane gas when the car would have to retard it for lower octane fuel. More advanced timing, without knocking, increases power AND efficiency.

In short, the ONLY reason for ever running lower octane fuel in your modern BMW is because it's cheaper.

BMW engines can adjust timing but only up to a certain point. You can't just run race fuel in your car and gain 50hp. Each engine has a specific octane it is designed for, anything higher than that is useless to it. S54's for instance can run 96 - so you can mix 91 and 100 race fuel to get close to this, and the engine can make use of it. Run 96 in an M52 though and it will just have a lot of trouble with it because it is designed to burn a lower octane fuel. Higher octane fuels are NOT beneficial at all to engines DESIGNED for lower octanes and can cause trouble in extreme circumstances.

I mentioned all of this in my post though...

[Adker]

I've heard that too about the higher octanes having slightly less energy density due to the additives. I've also heard this applies to any top tier gasoline though, so I'm not really sure about that. Really hard to find info on gasoline additives... (not surprisingly)

Search on Methylcyclopentadienyl manganese tricarbonyl (MMT) which is a common additive for octane boosting. I have used toluene (RON 121) (not good to the lungs) before going the ethanol(RON 120) for a turbo track car. Purchased toluene by the gallon at the local paint store.

As far as BTU energy, it all depends on what they mix the gas with to increase the octane rating. Using toluene, you increase the energy per gallon and using ethanol you decrease the energy per gallon. Since most fuel now has ethanol, it will have slightly less energy per gallon. Octane boosters MMT, ETBE and ethanol all have similar octane/energy ratings if memory serves me correctly.

Gasoline = 150,100 (Btu/Gal)
Gasoline (10% ethanol + 90% gasoline) = 145,200
Ethanol= 84,000

[R Shaffner]

BMW engines can adjust timing but only up to a certain point. You can't just run race fuel in your car and gain 50hp. Each engine has a specific octane it is designed for, anything higher than that is useless to it. S54's for instance can run 96 - so you can mix 91 and 100 race fuel to get close to this, and the engine can make use of it. Run 96 in an M52 though and it will just have a lot of trouble with it because it is designed to burn a lower octane fuel. Higher octane fuels are NOT beneficial at all to engines DESIGNED for lower octanes and can cause trouble in extreme circumstances.

I mentioned all of this in my post though...

OK, so in this context, on a BMW forum where the discussion is about the benefits of using higher octane in cars that ARE designed for it, and for which the manufacturer recommends higher octane, then what? Higher octane works better in these cars.

[Adker]

Read post 5 to answer your question. Using 89 in my engine in my town with my street driving style works with no knock or performance issues for my car. I do believe the car performs as good or slightly better on 89 octane.

[BimmerBreaker]

OK, so in this context, on a BMW forum where the discussion is about the benefits of using higher octane in cars that ARE designed for it, and for which the manufacturer recommends higher octane, then what? Higher octane works better in these cars.

You are only likely to notice the difference when flooring it near redline, even then, you may not always notice. ... So if you normally drive and cruise around, particularly in an automatic, shifting at 3 or 4000 or whatever, it wont ever have to pull timing and it will behave exactly the same with any octane.

I feel like I tried to address that before. Not everyone is driving their cars to the redline all the time or flooring it everywhere. I at no point said to people who drive their cars hard to run a lower octane. What I did say was that for long roadtrips I will sometimes advise lower octane because knock isn't a concern and oftentimes there will be an increase in mpg. Cars designed for higher octane can safely run lower octane. It is in many cases worse for a low octane engine to run high octane. Put race fuel in a lawnmower and see waht happens

On cars asking for 89, they usually can't make use of 91 or 93 and it can be counterproductive because the engine will be less efficient. The differences are less notable with such a small difference in octane, in reality I have to imagine that any modern engine can safely run any octane of gasoline from a normal gas station. But there is reasoning for higher octane being worse in low octane engines. For performance you should use the maximum octane the engine allows. For cruising around, you are fine to run a lower octane.

[melville]

So how much timing will the management system pull when presented with knock? Paging Abel!

If the ignition timing is sufficiently retarded, some of the combustion will be taking place in the exhaust manifold. There will also be flame passing between the exhaust valves and their seats. This is not a good thing.

In extreme cases it will burn the exhaust valves. Extreme in my experience being idiot hippies in VW Buses with non-functional advance in their distributors, so that timing at speed was TDC and not the preferred 32 BTDC.

[BimmerBreaker]

Pretty sure I have a tune on my M3 that does exactly that on deceleration - pull enough timing to burble and pop in the exhaust. Not a good thing sure, but sounds cool. Cats are deleted as well.

[JustinHalverson]

Pretty sure I have a tune on my M3 that does exactly that on deceleration - pull enough timing to burble and pop in the exhaust. Not a good thing sure, but sounds cool. Cats are deleted as well.

Interesting.... I run 93 octane in my S52 M, because I like to think I have that little bit of extra performance there when I'm in the mood to run it to redline. I have noted a burble in the exhaust on deceleration, which I don't particularly like (everyone has their own opinion on these things). I'm going to try some 89 octane to see if that makes a difference.

[rf900rkw]

The first requirement to get over-run burble is to disable the over-run fuel cut in software.