Can a higher octane fuel harm the M30 engine?

[gj1968]

I have been using a 93 octane( BP/Amoco) gasoline for my 1993 535i since I got the car almost a month ago. I just read from the Roadfly E34 forum this posting regarding the 535i:



However, I'm happy to trade some power for a car that runs on 87 octane (that's what it REQUIRES -- higher octane will reduce its performance and even harm the engine long-term), and is simple and straightforward to work on.

Is this for real? or just another IMO/2 cents worth baseless opinion. If so I must be foolish to pay more money on the pump for no return and might even harm the car's engine in the long run.

[Torque]

That's a myth. Higher octane won't harm the engine or reduce its performance. Only time a engine is 'aware' of octane is when it's lacking some.

[genphreak]

I have been using a 93 octane( BP/Amoco) gasoline for my 1993 535i since I got the car almost a month ago. I just read from the Roadfly E34 forum this posting regarding the 535i: Is this for real? or just another IMO/2 cents worth baseless opinion. If so I must be foolish to pay more money on the pump for no return and might even harm the car's engine in the long run.No, but bad fuel will harm it a lot (kludging up injectors and filters). Bad fuel comes in all octanes (avoid service stations with a chance of old rusty in-ground tanks). That said, I am not sure the performance is worth the extra if you potter round town most of the time. If you chip the car you can't use 87- so pay the extra and hope the economy buys back the extra cost- on an old M30 that is not in new condition... I doubt that will happen. :) Nick

Perhaps someone can correct me (I hope) :)

[632 Regal]

correction torque, higher octane can reduce power but you are correct that it wont hurt anything. Its basicly a cleaner fuel with different additives to boost octane.
That's a myth. Higher octane won't harm the engine or reduce its performance. Only time a engine is 'aware' of octane is when it's lacking some.

[Torque]

I'd love for someone to enlighten me on how it can reduce power.

[Qsilver7]

ahhh...against my better judgement...I'll jump in. Octane discussions are contentious at the least because everybody has an opinion and lots of them are based in misconception, and marketing deception.

The M30 engine (unless chipped) can not gain anything from fuel with an octane rating higher than 87 AKI (91 RON if in Europe). It was designed by BMW to run on that type of fuel. It has no technology to adjust to any other type fuel rating. As someone already stated...your owners manual tells you to run 87 AKI (and some fuel filler caps have a sticker on them or somewhere inside the fuel door). Here is the info from my old 91 735iL (M30 engine):
http://memimage.cardomain.net/member_images/1/web/433000-433999/433594_229.jpg

If you do some research on octane ratings and what they mean...you'll find that the rating has nothing to do with the gas being "better" or "worse"... the higher the octane rating the harder it is for the gas to spontaniously combust. Or you could say...the higher the octane, the higher the resistance to pre-ignition. The gas company's marketing departments have driven the "higher is better" misconception home...and has many people confused into thinking the 89 AKI and 91 AKI is progressively better fuel.

When you understand how higher octane fuel's resistance to "pre-ignition" works and how modern BMW engines are designed to use 89/91/93 AKI fuel and have knock sensors that adjust to octane fuel levels to prevent pinging (or pre-ignition)...you can begin to understand how using 93 AKI fuel in a car engine designed to run on 87 AKI can be harmed in the long run.

An engine tuned to run on 87 AKI but is continually filled with 93 AKI...all of the fuel in the chamber is not igniting because the engine wasn't designed to adjust to the higher octane fuel's characteristics and propensity to resist ignition. When you use a fuel with a higher octane rating than your vehicle requires, you can send this unburned fuel into the emissions system.* It can also collect in the catalytic converter.* When you over stress any system, it can malfunction or not do what it was designed to do properly.

I'll stop there because I'm trying to reduce something down to terms everyone can understand and recognize...so for those who get it...don't put me in front of the firing squad if it is not perfectly correct. :D

What you should do...is use the fuel that is recommended by BMW for your car...or if you've chipped your car...use what the mfg of the chip recommends. The M30 minimum requirement is 87 AKI that's it...it can't get anything out of higher octane fuels because it wasn't designed to...(unless you have a chip which remaps the timing etc. etc.) The M50 and M60 engines are more modern engines and BMW designed them with knock sensors to adjust to various fuels to reduce pinging...but both engines were designed to run on 89 or 91 AKI (check you manual or sticker on the fuel filler cap)..AND they have the ability to adjust (they have knock sensors) to a minimum of 87 AKI or up to a higher 91/93 AKI. These engines will also adjust there performance and fuel usage accordingly, too. A lower AKI will lower performance & fuel consumption...and a higher AKI will increase performance and fuel consumption.

Here is what my e38's owners manual says...note the text highlighted in yellow (and also note the typo at the beggining of the paragraph...BMW recommends 89 AKI for my M62 engine...87 AKI is the minimum and 91/93 is the maximum):
http://memimage.cardomain.net/member_images/1/web/433000-433999/433594_369.jpg

[Chowchilla Skunkwerks]

Excellent explanation Q....

[Jay 535i]

http://memimage.cardomain.net/member_images/1/web/433000-433999/433594_369.jpg

Careful. Not ALL E34s have knock sensors.

[Torque]

Careful. Not ALL E34s have knock sensors.Which ones do?

[Jay 535i]

Which ones do?

I dunno, but I've been told that mine doesn't (1990 535i).

[genphreak]

Careful. Not ALL E34s have knock sensors.Which is (one of the reasons why I want to install a Megasquirt and dump the Motronic. No M30 has a knock sensor, only later 525s (the ones with VANOS, I think) , 530 s and 540s. Not sure about 518s and 520s. Great explanation too qsilver7, (I enjoyed checking out your space on cardomain the other day too...) all good info! :) Nick

[632 Regal]

it burns slower.
I'd love for someone to enlighten me on how it can reduce power.

[motorman]

M20's and M30's dont, M50's and M60's do

[SRR2]

Also, chances are that the specific energy content of the higher-numbered fuel will be lower due to the low energy content of the octane-enhancing components.

There's also the details of the refining process. Basically all fuels have a specified acceptable range of specific gravity to meet API and federal standards. It's in the refiner's best economic interest to produce fuel that's at the lowest possible point of that range -- IOW, they're selling you fewer molecules of burnable fuel per unit volume. Years ago, when I knew a bit about aviation fuels, the additive packages for higher octane fuels had higher SGs than the base fuel stock, so in addition to their lower energy content, they enabled the use of even lower SG base fuel (fewer molecules/unit volume again) due to their density. It's not a lot though, just a couple percent at most, but in the long run, it's enough to be measureable.

[Paul in NZ]

its a sad fact tho that the overall quality of higher octane gas is higher.I run 95 ron in my 535 occasionally and it sure FEELS smoother and i get slightly better consumption.98 tho does not seem to make any extra difference.The marketing hype claims that the higher octane fuels "clean" your engine

[Anthony (M5 in Calgary)]

its a sad fact...The marketing hype claims that the higher octane fuels "clean" your engine

That and the psychological trick of putting the high-octane selection button nearest to where the nozzle sits so you're more likely to "default" to premium...

[BMWDriver]

Well, if I want more power, I just rev her up to 5-6000 rpms and get blown away. 87 octane is just fine for me, my wallet and my baby (M50).