Originally Posted by Tom K.
While BMW has tuned the 328i for 91 octane, it will run satisfactorily on 87 (see the owner's manual) with possibly some performance/economy degradation under certain circumstances.
^^ This is about as succinct a way of putting it as I’ve seen.^^
I don’t agree with the alternating tanks suggestion though.
Here's my long version:
You may or may not be able to tell the difference, but there is definitely a difference. And the occasional tank of premium isn't going to resolve the reduction in power and may therefore just be a waste of money.
The higher the octane rating, the less chance of knock (detonation). As a general rule, for the purposes of discussing whether it is "OK" to use regular gas when the manufacturer recommends premium, you have to take into account the circumstances in which detonation is most likely to occur. Generally, it tends to occur in a high compression engine, in high ambient temperature (like a 100 degree day), and under load (e.g., sudden acceleration, carrying several passengers and their luggage, going uphill).
When a manufacturer recommends regular in a particular model, using premium won't hurt anything, but it is a waste of money. If all you need is regular to run at designed efficiency, anything more contributes nothing to engine performance.
When a manufacturer recommends premium in a particular model, it can and will run with regular. In this situation, if you put in regular gas, the car will continue to run normally and I doubt anyone will be able to tell the difference. However, once you start pressing down on the accelerator under the conditions in which detonation tends to occur, the knock sensor will signal the PCM that detonation is occuring and ignition timing will be retarded until the knocking stops.
As a result of the timing retardation, engine torque is reduced. Again, the difference may not even be noticed, but the reduction in power will probably affect fuel economy (e.g., you need to give it more gas to climb the steep hill).
As Tom noted, the stock 328i engine is tuned for 91 octane. Although that engine doesn’t run as hot as the stock engine in the 335, it is a high compression engine (higher compression than the 335 engine if I recall correctly) and, as mentioned, high compression is one of the conditions conducive to detonation.
The process of retardation of the ignition timing because of the detection of detonation by knock sensor is an adaptation, i.e., the PCM will "learn" that regular is being used and keep the ignition timing retarded to avoid detonation. The adaptation can be cleared or reset if you have a scanner not an OBDII scanner/code reader, but a "real" one, i.e., having much of the scope and function to one a dealership service department may have, such as a GT1 or similar. I know the Bavarian Technic has this capability and there are probably others.
Most people do not have a means of quickly resetting the adaptation. The car computer will eventually reset the ignition timing on its own, after a preprogrammed drive cycle. Thus if you start using premium gasoline, the PCM will gradually permit the ignition timing to advance. It may take several tanks of premium to restore normal timing, to regain optimum fuel efficiency. At least this is the process for other types of engines. I don't know specifically how quickly timing is advanced back to normal in a BMW after retardation from driving around with a tankful of regular and would appreciate hearing from anyone who does know this.
So if you use regular when the manufacturer recommends premium, the timing will retard, available torque will decrease and merely running the occasional tank of premium will pretty much have no affect in restoring engine power and fuel efficiency.
Real world example: Suppose I have a stock 328i, 10.7:1 compression ratio. BMW recommends premium fuel and I drive around on regular. Within a short while the engine (specifically the ignition timing) will adapt. I will probably only notice when merging onto a freeway or pressing down on the accelerator to change lanes. The same amount of power won't be there. I'll have to give it more gas (which will lower the mpg's). Why run a powerful engine if the full power is not there when you ask for it?
As Tom said, if BMW calls for premium, you're not getting the performance you paid for if you use regular. So even though I can use regular I always use premium so the power to be there when I need it, even if I do not need it every day. As an alternative, I suppose it's possible to save a little bit of money on gas by alternating tankfuls of regular and premium, but only if I use my BT to reset the knock sensor adaptation after filling up with premium.