Is Octane 94 an overkill

[h317]

Hi to all!

I traded in my Accord for my first ever BMW a few weeks ago (a sapphire black 330xi with extended yellow leather package, automatic transmission). Never in my life have I been sooo eager to 'drive' to work

I am not knowledgeable on cars/ mechanics so please be easy on me. My first question is: I was told by my sales rep that bimmers need high octane fuel. Is 94 an overkill. Does it really 'hurt' the engine with low octance fuel?

Thanks.

 

[frolf]

run premium or 89, don't pay extra for 94 vs. 91-93 because stock that is overkill.

 

[Big Daddy]

Use what your owners manual tells you to, unless like Dan said, you chip the car, then use what the chip requires. I would not pay extra for 94 if all that is required is 91.

 

[wood_e]

89 should be sufficient

 

[Tom]

I agree, welcome aboard and congrats on the new car.

 

[h317]

Thanks for the advice. Just curious, why is there an octane 94 grade? Who/what kind of automobile need that? Is it just another tatics that gas companies ripping us?

 

[frolf]

various modifications (chip to begin with) require use of 91 octane, and lets say you did a lot more than a chip, to avoid knocking you might just play it safe with that kind of fuel.

 

[theBMWenthusiast]

congrats on the car and welcome to the forums...my family and I just use regular (but then again we drive an extreme amount and gas prices are getting exspensive) but never had any problems with it

 

[wood_e]

Thanks for the advice. Just curious, why is there an octane 94 grade? Who/what kind of automobile need that? Is it just another tatics that gas companies ripping us?

Chip tuned engines and high compression engines need higher grade fuel that resists pre-ignition due to the higher compression within the combustion chamber.

 

[Bmw 325i 7803]

It's ok to run 89 in an e46????? I'm not sure about that....

 

[Big Daddy]

I had a 79 Chev Pickup with a built 350, I mean built. It had 11:1 compression ratio and loped at an idle. I went through three transmissions before Summit Racing finally built one I could not destroy with the torque. This beast used 94+ octane.

 

[codex57]

It's ok to run 89 in an e46????? I'm not sure about that....

You can, you just lose gas mileage and power. If you're using regular to save money on gas, it might actually be counterproductive depending on how much difference in gas mileage your engine makes. I know the octane makes a huge, huge difference on my g/f's Camry V6.

 

[h317]

Isn't there an equilibrium point in octane grade vs performance. I don't mind paying extra for higher octane fuel but is it worth/necessary to go as high as 94. Maybe 91 as good enough

 

[codex57]

I suspect every car is different (due to options, engine size, tire sizes, etc.) and every gas station is different (due to different suppliers, regulations, ethical/unethical owners, etc.). You're probably just gonna have to do trial and error until you find your particular acceptable equilibrium point.

 

[panzershreck]

basic question: i've read up on engine compression ratios & octane mixes of petrol fuels, but according to a friend, "the mechanics dont matter, 91 octane will make your engine perform better than 89 or 87" is this true?

 

[bahnstormer]

if that's what the motor needs then yes
otherwise it'll cut back power

 

[panzershreck]

you think a 14 year old M20 engine with only 135k miles on it would really need premium? (car didnt come with a manual)

also whats best for an engine's longevity? (is it not that over time engines eventually require premium?)

 

[Epi330]

basic question: i've read up on engine compression ratios & octane mixes of petrol fuels, but according to a friend, "the mechanics dont matter, 91 octane will make your engine perform better than 89 or 87" is this true?

False, mechanics do matter.

"Octane" number used in US is also called AKI (anti-knock index), and it only reflects the maximum compression ratio that can be used without premature ignition (aka "knocking").

Now, if the engine does not have high comression ratio (user manual says "regular is Ok"), then using higher octane does not help the engine at all. It does not make it perform any better, nor increase mpg, nothing.

For engines with higher compression ratio (as E46 have - manual calls for "premium only", i.e. 91), using lower grade gasoline will result either in engine "knocking" (which damage the engine), or in sub-optimal performance when car software compensates for the different fuel.

In other words, every engine has it's own optimal AKI ("octane"). Using higher grade does not help, but using lower one does hurt. Granted, the damage to engine from using 89 instead of 91 (and the performance loss) is not very significant. But, on the other hand, the price different is not that big either.

 

[Epi330]

(is it not that over time engines eventually require premium?)

It is very unlikely that compression ratio can increase over time. Quite the opposite - when parts wear and loosen, the compression is not that great anymore, so you can use lower grade than when car was new.

 

[panzershreck]

just for general future reference: which has a higher compression ratio? 8:1 or 10:1? (as an example)

and what would you say is normal for an 8.8:1 engine (m20)?