What grade gas you running in 3.0's

[Jiga jace1]

what do you mean by misfire?

[JCL]

Originally Posted by crosvs

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.

Essentially you want the lowest octane possible at which you can get all the performance out of the engine. Ignition timing is one factor, so is compression ratio. Leaving aside internal engine mods, putting in fuel with a higher AKI than necessary will not give you any more power. If you go faster, it is only because your wallet is lighter.

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.

91 isn't bad, it just isn't necessary. It can be bad, but that is usually related to cold weather operation only. If you added a chip that takes advantage of the better fuel, fine. Did you also change the compression ratio?

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!!

Forget Euro-calibre fuels. Not sure what they are. In Europe, octane is measured according to a different formula. Although it isn't a precise conversion, there is approx a 6-7 point spread. So, a fuel that is 98 RON in Europe is essentially the same as a fuel of 91/92 pump octane in North American. The fuels are the same, the octane measurement is different. Let's keep using a consistent measure here, in this case pump octane. If you put a better fuel in than the 91 that the vehicle was designed for, without doing any engine modifications (compression ratio, porting, forced induction system, etc) then there is no benefit. None. Nada. You can put in any fuel you like, but it won't help. Some will argue that better fuels have more of the additives that keep the combustion system clean, but that was more true ten years ago than today. When Techron first came out at Chevron, the % varied by pump grade. That difference has largely disappeared. The entire marketing program called Top Tier is about eliminating that variable, which had largely disappeared in any case, if you are looking at name brand fuels.

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.

Not true. Your X5 can handle 100 pump octane fuel, but it can't calibrate for it, that is to say that it can't take any advantage of it. Go ahead and put it in if you like, because driver confidence is a big part of racing, and if you think you are faster, you may be. Just don't attribute it to the fuel.

[JCL]

Originally Posted by Jiga jace1

what do you mean by misfire?

While there are many potential reasons for a misfire, or missing, the following refers only to situations caused by fuel.

AKI refers to the resistance of the fuel to ignition, or rather uncontrolled ignition, also called predetonation. A fuel with a higher octane rating will be harder to ignite. That is a good reason to have high performance ignition systems on race cars. On a modern street car, the problem usually only shows up in cold weather, when the engine is cold; the engine can misfire, or miss. When the piston compresses the mixture, and the sparkplug does not ignite that mixture, that is missing. It can happen on one or two cylinders, usually until the engine is at operating temperature, or at least partly warmed up. The BMW fix for this is to advise the owner to move to 89 pump octane until the weather warms up. This is not a problem in Southern California, but not all of us live there.

[JCL]

Originally Posted by 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?

Sorry, the answer was buried in the long-winded explanation. Your 100 RON is the same as 93 pump octane in North America. It is essentially what the manufacturer recommends. Use it, it is fine. It is no better than 98 RON in theory, but if the truck runs better on it, buy all means keep on.

[JCL]

Originally Posted by DINANM3

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.

DINANM3: Thanks for that. Yes, with those modifications I am sure you can use all the knock resistance you can find. My comment was that we should keep clear what the figure 100 means. Your 100 pump octane isn't the same as 100 RON.

Jeff

[crosvs]

JCL- thank you for all the constructive feedback. And so I think one pressing question -- and one lesser question! -- remains:

(1) Why does our fuel door say "91 MINIMUM?"

(2) Same logic for my Dad's C4S then too, right? Don't bother with 100?

Another note: I know for a fact that, as someone mentioned earlier, turbo- or supercharged engines necessarily DO benefit from higher octane fuel. Apparently the Mitsu Evo is a RADICALLY different beast when running on 100 octane fuel.

And finally: as I mentioned earlier, 91 octane fuel *DOES* produce *SOME* knocking at low RPMs under more than 1/4-throttle position. Any thoughts?

[Quicksilver]

I'm no expert but I will add this to the Mix. Comment as you see fit. But someone brought up the fact that there are different fuel for different regions. If thats so then one would need to know that when making a choice. Correct? http://www.straightdope.com/columns/041008.html

[at4689]

i use the premium or the best the gas station has becuase a couple of dollars more is not worth to not get it and maybe in the long run ruin ur x. idk

[JCL]

Originally Posted by crosvs

JCL- thank you for all the constructive feedback. And so I think one pressing question -- and one lesser question! -- remains:

(1) Why does our fuel door say "91 MINIMUM?"

Cut and pasted from earlier in this thread:

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).

(2) Same logic for my Dad's C4S then too, right? Don't bother with 100?

No comment. I don't know anything about the ECU programming on the C4S. What does the manual recommend? Don't bother to go above that rating, as a rule of thumb. Is the C4S known to take advantage of fuels with a higher AKI than that recommended by Porsche?

Another note: I know for a fact that, as someone mentioned earlier, turbo- or supercharged engines necessarily DO benefit from higher octane fuel. Apparently the Mitsu Evo is a RADICALLY different beast when running on 100 octane fuel.

Can benefit, but don't necessarily benefit. The forced induction system allows a higher effective compression ratio, which you may be able to take advantage of if the engine doesn't experience pre-detonation. Depends on how the system is designed.

And finally: as I mentioned earlier, 91 octane fuel *DOES* produce *SOME* knocking at low RPMs under more than 1/4-throttle position. Any thoughts?

Just a guess, but either there are build-ups of deposits in the combustion chamber, raising the compression ratio and causing hot spots that light up the fuel before the piston reaches TDC, OR your fuel supplier isn't really supplying you with 91. You could try other brands, same AKI, and see if there is a difference.

[JCL]

Originally Posted by Quicksilver

someone brought up the fact that there are different fuel for different regions. If thats so then one would need to know that when making a choice. Correct?

True. That is why, without time to determine local fuel quality when I travel, I nearly always buy 91. It is just the easiest thing to do. No analysis involved. I never buy better than 91. I also make sure that I am buying premium from a station that is likely to have sold a lot of it. If I am in a rural area, at a station with two pumps and not much traffic, I won't touch the premium. It is likely to have been there for awhile.