Hi:
New to the forum. Just ordered a 2007 328i Graphite Sports Package and expecting it in August sometime. Now I have owned Hondas for 10 years and was wondering why the BMW need to run on Premium (Supreme I assume) and not Regular or Mid Unleaded. Is there a particular reason for this? And if regular unleaded does this affect the performance of the car (i.e. mileage)? Thanks a bunch

Higher compression requires higher octane fuel, otherwise you get detonation (knock).

Lower oct in these engines will reduce power (you'll notice the Euro cars sometimes have more HP...example 306 for the 335 there, and that's a result of higher OCT fuel)


Notice that the Honda's run on 87, and Acura on 91...and the Acura engines that are the same have more power.

you'll notice the Euro cars sometimes have more HP...example 306 for the 335 there, and that's a result of higher OCT fuelIt's true that most euro pumps offer higher octane options, but if you get regular euro fuel it may be lower octane than our mid or premium. When I was over there 91 was the regular, subtract five for our stricter Octane rating system and euro 91octane = US 86 octane. They had 91, 95, 98 at most pumps, which would be 86, 90, 93 here. I usually buy premium here anyway (come on, it's like two bucks more! your car deserves it.), which would be better than their mid-grade.

He's right though, that the engines are tuned for more power/performance and thus require higher octane. You can take the same engine, tune it to run on crap gas and get 100 hp, tune it to do 150hp and require high octane(among other things), or anywhere in between. It's all a part of creating a graduated product line.

It's true that most euro pumps offer higher octane options, but if you get regular euro fuel it's lower octane than our mid or premium. When I was over there 91 was the regular, subtract five for our stricter Octane rating system and their 91 = 86 over here. They had 91, 95, 98 at most pumps, which would be 86, 90, 93 here.

I don't think they rate the fuel the same...is that all the difference is is -5?

Higher compression requires higher octane fuel, otherwise you get detonation (knock).

Lower oct in these engines will reduce power (you'll notice the Euro cars sometimes have more HP...example 306 for the 335 there, and that's a result of higher OCT fuel)

The reason that you will get less power on a lower octane fuel is because the knock sensors will detect detonation and adjust the fuel-air mixture to be more conservative so that the detonation/knocking stops. Subsequently putting higher octane fuel in a car with a lower octane rating will not give you more hp though because the engine ECU does not adjust itself accordingly. You would need to reprogram the ECU to be able to take advantage of the higher octane (which is what you can do with PROcede).

I don't think they rate the fuel the same...is that all the difference is is -5?They use RON, which is about 10 points higher than MON, the stricter standard for jet fuel, etc. Most pumps in the US use the average of RON and MON, as noted on the pump by (R+M)/2 method, meaning it's about 5 points off.

The main difference between the RON and MON measurements is that MON has a more regimented test procedure. RON simply runs a mixture through an engine and compares the results with known mixtures, while MON specifies the temp the fuel must be tested at, tries out higher engine speeds, and takes into account variable engine timings.

Lower oct in these engines will reduce power (you'll notice the Euro cars sometimes have more HP...example 306 for the 335 there, and that's a result of higher OCT fuel)

No it's not. European engine output measurements are different from U.S. standards. The 306 HP stated for European trim is in DIN measurement format and the 300 listed for U.S. engine is in SAE format. Different standard results in slightly difference in number. 306 DIN HP = 300 SAE HP. HP rating for the same engine that requires 91 octane doesn't actually change if you go with a HIGHER Octane gas.

Unless your car specifically has the compression ratio and air-fuel mixture that requires 91+ octane fuel, it doesn't do the HP squat to run higher octane than what the engine specifically require. On a modern engine with knock sensors, it will retard spark timing to prevent detonation and pinging on a car running too low of an Octane therefore leading to reduced efficiency and reduced engine output.

But it doesn't work the other way around. You don't get more HP from running higher octane gas.

He's right though, that the engines are tuned for more power/performance and thus require higher octane. You can take the same engine, tune it to run on crap gas and get 100 hp, tune it to do 150hp and require high octane(among other things), or anywhere in between. It's all a part of creating a graduated product line.

No, that is incorrect. You're probably thinking of Nissan's VQ35DE engine which was installed in the 350Z, G35, and Maxima. In the Maxima trim it makes 265 HP, in the G35 auto it makes the same 265 but in the manual trim it makes 270, while in the manual 350z it makes 286hp-300hp. It's not the octane it's tuned to run at, since all three cars with their different HP ratings and transmission choice ALL require premium fuel. The way the Japanese alter the engine trim is via cam profile, exhaust restrictions, intake plenum, and software tuning. In the Maxima it's tuned to make more torque down low and less HP up top, while in the 350Z manual it's made to maximize HP but give up on Torque.

Fuel octane rating is more of a function of the engine's compression ratio. Higher compression engines (also force induced engines) require a higher octane fuel to prevent pre-detonation and pinging. Lower compression engines can use a lower octane fuel. That's it. There's no nefarious scheme to tune one car for one set of octane rating for higher trim models.

You're confusing the performance of an engine with the metering out of that performance with valves and timing. The performance of a given engine can be described by it's compression ratio, which as you've already denoted, hinges on it's fuel mixture. If you want a good example, read up on the Nissan A engine.

With a 2-barrel Hitachi (http://en.wikipedia.org/wiki/Hitachi) carburetor (http://en.wikipedia.org/wiki/Carburetor) and an 8.5 to 1 compression ratio this engine produced 62 bhp (46 kW) and 61.5 ft·lbf (83 Nm). The Coupe, introduced in Sept 1968, was equipped with an uprated engine boasting a free flow dual out exhaust manifold, & increased compression, now 9 to 1.Lower compression engines can use a lower octane fuel. That's it.Lower compression engines are lower performing. Higher compression == more power extracted from the air/fuel mixture, which requires higher octane fuel.

The compression ratio is a single number that can be used to predict the performance of any engine (such as an internal-combustion engine (http://en.wikipedia.org/wiki/Internal-combustion_engine) or a Stirling Engine (http://en.wikipedia.org/wiki/Stirling_Engine)).
It is the ratio (http://en.wikipedia.org/wiki/Ratio) between the volume of the cylinder (http://en.wikipedia.org/wiki/Cylinder_%28engine%29), when the piston (http://en.wikipedia.org/wiki/Piston) is at the bottom of its stroke (http://en.wikipedia.org/wiki/Stroke_%28engines%29), and the volume when the piston is at the top of its stroke. A high compression ratio allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. High ratios place increased oxygen and fuel molecules into a reduced space; thus, they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree. Higher compression ratios, however, also make engine knocking (http://en.wikipedia.org/wiki/Engine_knocking), also known as detonation or pre-ignition (http://en.wikipedia.org/wiki/Pre-ignition), more likely and this can reduce an engine's efficiency and even physically damage it.

So there you go. Hopefully now there's no doubt that higher octane requirements are due to higher compression set for a particular engine and thus higher performance from that engine.

You're confusing the performance of an engine with the metering out of that performance with valves and timing. The performance of a given engine can be described by it's compression ratio, which as you've already denoted, hinges on it's fuel mixture. If you want a good example, read up on the Nissan A engine.

Lower compression engines are lower performing. Higher compression == more power extracted from the air/fuel mixture, which requires higher octane fuel.



So there you go. Hopefully now there's no doubt that higher octane requirements are due to higher compression set for a particular engine and thus higher performance from that engine.

There are a lot more variables then just compression dude.

To the original poster...you can run 87 in a 328 no problem. Your gas mileage will go down a tad and it will lose some power in warmer weather but it won't blow up. I ran 87 in all my 325/328's with no ill effects.

FYI, BMW's alleviate knock by pulling all the timing out of the ignition. Which is why they will feel "flat" when running 87.

There are a lot more variables then just compression dude.Not really. See my post above regarding the definition of compression. I'd go into further detail but it seems that even the little I've given hasn't been understood. I will say this though that I'm talking about the performance for a given engine, it's unfair to compare different engines by only their compression ratio.

The initial question was why certain engines require higher octane. The short answer is simply that they're engineered that way. Some engines are designed to extract greater power from the fuel/air mixture by using a higher compression ratio, and require higher octane to achieve the engineered result. A more standard engine will have more average compression and thus lower performance.

Can you run lower octane? Sure, but you risk damaging the engine. The knock sensor is supposed to sense this and step up the amount of fuel the engine burns and/or retard the ignition timing to make up for it, so in theory it will run fine but if you get pinging I'd stop immediately. You can read up on the cause and potential damage here (http://en.wikipedia.org/wiki/Engine_knock).

It makes you think though, if you're going to spend $45 or more to fill up your tank, and your engine runs a richer fuel mixture and you get lower gas mileage/power because you cheaped out and bought regular, why not just spend an extra $3 or so per tank and get the premium?

Okay, specifically, is it absolutely necessary to use the 93 octane? HP aside, will this screw over my engine over time? Let's say I pump the mid-grade in for the next four years, what are my "told you so!" effects going to be?

93 RON or 93 (r+m)/2? I don't see more than 91 regularly in the states. Some carry 93 and others up to 114, but they're rare. My VW gas door says 91 RON or 87 (R+M)/2, which is fairly equal, just euro vs US measurements. If it's 93 RON I'd look for 89 in US measurement, some stations in my area have 85/87/89/91 options.

I don't think it would matter in the long run due to the safe guards put in place, but you do lose a little in gas mileage, and given the price of fuel these days the difference between regular and premium is almost negligible. 20 cents more was a big deal when gas was $1, but now that it's $3+ per gallon the difference isn't that great. You might spend $55 for premium and $52 for regular on a full tank.

Here in CO we never hardly get above 91 octane because of the admixtures put into the gas to clean up emissions. You will be fine with using mid-grade, just like SomeBMWGuy said, all that will happen is that you will not be running at optimal performance/gas mileage (at least with the newer BMW's with knock sensors, you may not want to do this with older cars)

What some of you are missing is that you get poorer MPG than you would with premium (that's why the call it MIN requirement :) ) and while you are saving $5 at the pump you're giving it back for less MPG and less performance. So why not protect the engine from the start. If you're crying about $5 at the pump you probably bought a car you can't afford.

What some of you are missing is that you get poorer MPG than you would with premium (that's why the call it MIN requirement :) ) and while you are saving $5 at the pump you're giving it back for less MPG and less performance. So why not protect the engine from the start. If you're crying about $5 at the pump you probably bought a car you can't afford.

Not even $5! (unless you have a 25 gallon tank)

ok thanks, my question was geared more towards the MPG and engine performance, cost is not an issue - I just wanted to know if it does harm to place mid or regular unleaded at the pumps. Thanks

Okay, specifically, is it absolutely necessary to use the 93 octane? HP aside, will this screw over my engine over time? Let's say I pump the mid-grade in for the next four years, what are my "told you so!" effects going to be?

It's my understanding that it will have no longterm negative effects on the engine. We hvae a new 328xi that we're running on mid-grade Sunoco - and I cannot tell the difference in any way from performance on premium.

I am almost positive that the ecu's even have a map for lower the 87 octane for those weird back woods parts of the country.

I am almost positive that the ecu's even have a map for lower the 87 octane for those weird back woods parts of the country.

I'm not sure about every car, but in my 350Z the manual said you can run 87 bu to "Avoid full throttle" - detonation can be a killer, and it's really bad at full throttle and high RPM :)

I am almost positive that the ecu's even have a map for lower the 87 octane for those weird back woods parts of the country.

Hello "SomeBMWGuy":

I carry a few bottles of the best octane booster I can find for those times. (Don't forget a small funnel.)

My X5 "recommends" 91 but the manual says it will run on lower. My 335 says 91 is "required". So far, I have only had a couple of instances where I could not get 91.

My 335 manual says that 91 or greater is recommended and 89 or greater is required with some possibility of knock at startup.

My X5 "recommends" 91 but the manual says it will run on lower. My 335 says 91 is "required". So far, I have only had a couple of instances where I could not get 91.
I couldn't find your quoted "91 is required". My BMW 3 Series Driver's Reference Guide, p21, states (emphasis mine)
For optimum engine performance and durability, premium unleaded gasoline with a minimum 91 octane level is recommended.

And the Owner's Manual for Vehicles (great name, eh?) says on p103,

Super PremiumGasoline/AKI 91
This gasoline is highly recommended.

However, you may also use gasoline with less AKI.

The minimum rating is [...] 335i: 89. [...]

If you use gasoline with this minimum AKI Rating, the engine may produce knocking sounds when starting at high outside temperatures. This has no effect on the engine life.

Note: Do not use any gasoline below the specified minimum fuel grade. Otherwise the engine could be damaged.

Hello "SomeBMWGuy":

I carry a few bottles of the best octane booster I can find for those times. (Don't forget a small funnel.)

My X5 "recommends" 91 but the manual says it will run on lower. My 335 says 91 is "required". So far, I have only had a couple of instances where I could not get 91.

I know people who put 87 in their 335i's so I'm sure it will be fine if you just put a little bit in. 89 is fine for the 335i's as well. It should say that in the owners manual.

I know people who put 87 in their 335i's so I'm sure it will be fine if you just put a little bit in. 89 is fine for the 335i's as well. It should say that in the owners manual.


So the people you know are all mechanical engineers working for BMW.

It makes sense to have it run safe on 89 because you wouldn't want to tune without some room for error, but saying you can run whatever fuel you want isn't good advice.

It will run on 80 OCT too, but how long do you want it to run for?

So the people you know are all mechanical engineers working for BMW.

It makes sense to have it run safe on 89 because you wouldn't want to tune without some room for error, but saying you can run whatever fuel you want isn't good advice.

It will run on 80 OCT too, but how long do you want it to run for?

No, they are not engineers.

My point was that if you are stuck in the middle of nowhere and can only get 87 octane, I wouldn't hesitate to get a few gallons of that. It's not like the engine is going to instantly explode from detonation.

For the non turbos 87 runs perfectly fine with no knocking. At least it always has for me.

No, they are not engineers.

My point was that if you are stuck in the middle of nowhere and can only get 87 octane, I wouldn't hesitate to get a few gallons of that. It's not like the engine is going to instantly explode from detonation.

For the non turbos 87 runs perfectly fine with no knocking. At least it always has for me.

I agree, but realistically I've never seen a gas station with just "87"...so pay the extra $5 and put the right stuff in there.

The chances of something going wrong are slim, but if something does and BMW can say you didn't run the required fuel...guess who loses.

If you are planning on keeping the car for a long time, you might want to consider the higher octane and top tier fuel (Shell, Texaco, Chevron, few others). Top tier fuel is recommended by BMW and a number of other top auto manufacturers. They may just be covering their butts, but theoretically, detergents should help out in long run to keep engine clean and performing well. To reduce costs, many oil companies have stopped adding detergents to their gas.

If you are planning on keeping the car for a long time, you might want to consider the higher octane and top tier fuel (Shell, Texaco, Chevron, few others). Top tier fuel is recommended by BMW and a number of other top auto manufacturers. They may just be covering their butts, but theoretically, detergents should help out in long run to keep engine clean and performing well. To reduce costs, many oil companies have stopped adding detergents to their gas.

Hello "forrestpilot":

Somewhat related...

A local dealer had about a month old 335i that they were trying to sell for about 2K off list. The story was a local bought it and then his company transferred him to the Mexico office. Apparently this vehicle will not run on their gasoline so he could not take it with him.

335I doesn't run off of tequila?

Sounds like they made up a story for a car somebody couldn't afford or they bought back ; )

I couldn't find your quoted "91 is required". My BMW 3 Series Driver's Reference Guide, p21, states (emphasis mine)


And the Owner's Manual for Vehicles (great name, eh?) says on p103,


Hello "azhrei":

I stand corrected. I checked the manual again. 89 it is.

In a feeble attempt at self defense, the label on the filler door says "Minimum octane rating (AKI) 91".

I completed an unscientific test this weekend comparing gas mileage using 87 vs 93. I went on a 165 mile trip which I had made two weeks ago running on 87 octane, only this time before leaving I filled up with 14 gallons of 93. For both trips I had cruise set at 78 mph throughout and both trips I was using the AC. End result was that on both trips I got 28.6 mpg, rather uncanny that it was exactly the same, but anyhow, if you are using higher octane and expecting better gas mileage you may be disappointed. FWIW, the cost difference for 14 gallons was only $2.24 (2.99 for 93 and 2.73 for 87) so it's worth using the higher oct for better performance.
Pat

I completed an unscientific test this weekend comparing gas mileage using 87 vs 93. I went on a 165 mile trip which I had made two weeks ago running on 87 octane, only this time before leaving I filled up with 14 gallons of 93. For both trips I had cruise set at 78 mph throughout and both trips I was using the AC. End result was that on both trips I got 28.6 mpg, rather uncanny that it was exactly the same, but anyhow, if you are using higher octane and expecting better gas mileage you may be disappointed. FWIW, the cost difference for 14 gallons was only $2.24 (2.99 for 93 and 2.73 for 87) so it's worth using the higher oct for better performance.
Pat
Not trying to criticize your method, nor to doubt your findings, but I just wonder if it would have made a difference if you drove at say 90mph or maybe kept it in 4th gear at a higher RPM at 78mph. I am guessing that the effects will be more prominent at higher engine revs and maybe sublime or non-existent at lower to moderate revs.
Anyway, thanks for sharing.

Not trying to criticize your method, nor to doubt your findings, but I just wonder if it would have made a difference if you drove at say 90mph or maybe kept it in 4th gear at a higher RPM at 78mph. I am guessing that the effects will be more prominent at higher engine revs and maybe sublime or non-existent at lower to moderate revs.
Anyway, thanks for sharing.
If the engine runs more efficiently on high octane, I would think that a difference would be apparent accross all engine speeds. Because, more fuel is used at higher RPM, the value of the difference may be greater but percentage-wise, it should be the same. IMO.

Pat

If the engine runs more efficiently on high octane, I would think that a difference would be apparent accross all engine speeds. Because, more fuel is used at higher RPM, the value of the difference may be greater but percentage-wise, it should be the same. IMO.

Pat

Where the higher octane gas will make a bigger difference, is if you're constantly slowing and/or stopping and accelerating, or if you're doing some high performance driving where engine is under heavy load and sustained high RPM.


Pinging or detonation are more likely to happen when engine is under heavy load. Cruising conditions does not cause the conditions required for higher octane gas. In the real world, it's perfectly fine to be using 89, even 87 octane gas if all you're ever going to be doing is cruising near speed limit on the freeway most of the time, and taking 10 minutes to get up to speed by accelerating really slowly.

A more appropriate test would probably be to run a single tank of 89 in daily commute and then run a tank of premium in the same commute, and calculate your mileage based on figures at the pump.

Where the higher octane gas will make a bigger difference, is if you're constantly slowing and/or stopping and accelerating, or if you're doing some high performance driving where engine is under heavy load and sustained high RPM.


Pinging or detonation are more likely to happen when engine is under heavy load. Cruising conditions does not cause the conditions required for higher octane gas. In the real world, it's perfectly fine to be using 89, even 87 octane gas if all you're ever going to be doing is cruising near speed limit on the freeway most of the time, and taking 10 minutes to get up to speed by accelerating really slowly.

A more appropriate test would probably be to run a single tank of 89 in daily commute and then run a tank of premium in the same commute, and calculate your mileage based on figures at the pump.
This was a trip that involved construction slow downs, idiots in the passing lane doing 5mph under the speed limit, speed traps, etc, so it was not 78 mph for 2 hours. Either way, the point here was that people were inferring that the reduced cost of low octane would be neggated by worse gas mileage, which based on my experience is not true.

If you spend 35K+ on a luxury vehicle, why would you even consider putting in 87 octane? I can understand if you're on a road trip and the only station has 87 or 89, then I'd be concerned. But if I was into just filling up my e92 it'd be 91 or 93 octane all the time.

2.99 for 93???

Where are you? Iraq?

2.99 for 93???

Where are you? Iraq?
I live in PA but work in NJ where gas is about 20 cents cheaper and they do not allow self serve, so you don't even have to get out of the car.:D

It's pretty insane to buy a 40K car and get cheap on basic things like gas.

It's pretty insane to buy a 40K car and get cheap on basic things like gas.
Agree, especially since it's only a couple bucks difference for a full tank.

I live in PA but work in NJ where gas is about 20 cents cheaper and they do not allow self serve, so you don't even have to get out of the car.:D
Yeah I drive to NJ from Ottawa about 2 times a year and pass through ON, NY, PA. The cheapest between here and there is NJ. Upstate NY 91 oct is 3.29 or so! Almost, but not quite, as expensive as in Canada.
I was in N. Cali this spring and ouch, gas is too damn expensive there. Still cheaper than Canada though :mad.

Yeah I drive to NJ from Ottawa about 2 times a year and pass through ON, NY, PA. The cheapest between here and there is NJ. Upstate NY 91 oct is 3.29 or so! Almost, but not quite, as expensive as in Canada.
I was in N. Cali this spring and ouch, gas is too damn expensive there. Still cheaper than Canada though :mad.
I head up your way every summer as we have a cabin about 45 minutes east of Kinsgton, we always fill up in Watertown before heading through the 1000 Islands so that we can make it in and out without fueling. (provided I don't need to siphon a bunch out for the boat)