What grade gas you running in 3.0's

[JCL]

Originally Posted by accella4.4

So there is chemically almost no difference between 89 and 91? I'm askin' not debatin'...and maybe I missed this, but the reason for 91 on the long runs is why? Fuel quality control or slightly more efficiency from a tank? I would ask, why not give a car the better stuff when it is working harder in stop & go city driving, no? Or, is 89 all the car requires? And what about sea-level driving vs higher altitude?

First of all, all the numbers quoted below are pump octane, not RON. Also, the examples apply to engines built by the factory, not those that may have been modified with forced induction systems (where all bets are off, and the higher the AKI the better)

I used 89 and 91 in my example to illustrate theoretically the difference between what the engine requires, and what the fuels actually are. 89 may be all the car requires. It is certainly sufficient in my part of the world. It is also what is used by my BMW dealer for all of their fleet (M vehicles excepted) There is a difference between a fuel of 89 and one of 91, but it is only in terms of resistance to detonation. There is no other significant difference (latent heat of combustion, etc). There may be a difference in the amount of cleaners/additives added to the higher cost fuel, depending on your supplier.

The biggest reason for 91 is to maintain the safety margin that BMW calculated into the octane recommendation. Even with the worst regional fuel supplier, you will most likely be fine on 91 (apart from contaminants, water, etc). Turn the argument around? How do you know that the fuel you are buying is 91? Because the pump label says so? Have you tested it? Even the refinery data is based on tests at the refinery, not the service station. The biggest reason for variation at the service station is mixing of various fuels (they may be delivered in the same pipe, or same truck).

You only need enough AKI to prevent predetonation in your driving conditions, and in your engine. Summer and winter can be different. Lots of carbon build-up in your engine is a factor. Light throttle vs heavy throttle is a difference. High and low altititude is a difference. The only way to know for sure if you have a sufficiently high AKI in your fuel is to test and see whether the engine operates as well as it does on a known high octane fuel. Too high an octane rating, or AKI, isn't any better. It isn't treating the engine to something it can appreciate. The fuel will be harder to light up, potentially causing misfires. More importantly, it is most likely not sold from a location that pumps as much of it as a standard grade fuel. That means it can be more likely to have contaminants, be stale, etc. At a service station, premium fuel is often the lowest volume product. That is why it is stored in the smallest, oldest, tank. Statistically, the cleanest freshest fuel will not be the high octane product.

When we talk about the engine knocking, or pinging (which it would do if there wasn't an ECU to adjust the engine parameters), it isn't like it does it all the time. If we think back to cars that used to ping, or knock, they didn't tend to do it all the time. Frequently, a car would ping on a long uphill grade with a light throttle, for example. Heavier throttle would reduce the pinging. All this talk about predetonation seems to assume that it will happen all the time. Consider that it may only have happened under a small percentage of driving conditions. Usually, city driving wasn't one of them, particularly at sea level. Having said that, carbon build-up in the combustion chamber can promote predetonation, so some driving on the highway at high temperature/load factor is a good way of cleaning it out.

I am not advocating that we should all go buy 87 AKI fuel. I am simply pointing out that all of the hype over buying fuel with a high AKI, and sometimes even beyond what the manufacturer recommends, is just that, hype. If one doesn't want to go through all the trouble of testing various fuels, one should simply buy 91, nothing more. End of discussion. That is what BMW assumes its customers will do. It is just that as an enthusiast, who has run single cylinder test engines to determine AKI in a lab (many years ago), who has worked on vehicles for decades, and who has a background some years back in the wholesale/retail gasoline business, I just find it incredible that so many enthusiasts here continue to believe that a higher AKI is somehow better for an engine, particularly when it is higher than even BMW recommend. It is marketing hype most of the time. And, it seems to be working.

Have a great day.

Jeff

[crosvs]

It is a waste of money over purchasing 91, but it won't generally hurt the engine. If you have a misfire in cold weather, stop using higher octane fuel, as it can cause the misfire.

The biggest mistake made is that people assume that a higher AKI is somehow better. If you have a sufficient AKI to resist predetonation, anything else is lost. Your engine can't tell the difference.

This needs to be refined further: my understanding is that you want to use the LOWEST octane possible to which your car's computer can calibrate to prevent knocking.

Obviously, in a car without a knock sensor -- like my old Mustang -- 87 would prevent it from knocking just fine. Hence, using 91 would have actaully been bad -- carbon deposits in cats, etc -- until I added a chip recalibrating the engine for 91.

However, in our X's, I imagine the computer can calibrate itself all the way up to Euro-calibre fuels of 100 octane or so. Let's assume for sake of argument, however, that our computer could only handle up to 98 octane. Well then, I'd say put in 98 -- but do *NOT* put in 100!!

For instance: on May 10 when my Dad and I head up to Thunderhill Raceway in Willows, CA, we're going to fill upon 100 octane at the track because my understanding is that our X's *CAN* calibrate for 100.

[DINANM3]

Originally Posted by JCL

DinanM3, you seem to be confusing RON with pump octane. The poster who asked the question is in Geneva. 100 RON in Geneva is very close in anti-knock properties to 93 pump octane in North America. Your 100 octane fuel in a road vehicle doesn't make a lot of sense, as the engine was not designed for it.

The M5 and M3 i put them in were modified not stock. The M3 was superharged. And beleive me it made a difference.
I was just saying that 100 ocatane in a stock 4.8 wont hurt it. Not sure if it will help it.

[Riyad Aliyev]

thank you JCL for explanation. but still, nobody realy answered my question, is it ok if i'm going with this "Shell V-power 100" on my little beast named X5 4.8is?

[DINANM3]

No it will not hurt it.

[Riyad Aliyev]

Originally Posted by DINANM3

No it will not hurt it.

Thank you.

[DINANM3]

by the way what do you usually use for gas

[Riyad Aliyev]

What do you mean what do we use for gas? Gas is gas. Didn't quite understood your question.

[DINANM3]

Well you wanted to know if it is allright to use 100. So obviously you are not currently using 100. What do you use now.

[Riyad Aliyev]

ohh excuse me. no, i do use 100. I just wanted to know, do i need to change or is it ok if i continue to use 100. My personal oppinion is that 100(European style or whatever) is exelent, cuz' the ride is just flying.