Quote:
|
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.
Quote:
|
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?
Quote:
|
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.
Quote:
|
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.