So I've scoured the brickboard's archives for any posts relating to octane and turbo performance as I try to pin down an intermittent mystery problem I've been having with my 1985 745t lately. I think it might have to do with my intermittent use of a particular inexpensive gas station, and I'm looking for some help to understand why. On a sidenote, I would recommend anyone who wants a better understanding of the elements involved read this post:

http://www.brickboard.com/AWD/?id=144708

To recap quickly, whenever I boost (during the problem) the car stumbles horribly. The more I boost the worse it becomes, with the best performance (during the problem) coming with the boost gauge at the top of the black (right before the turbo spools up). BTW I do not believe it is a vacuum leak or hose problem as I have replaced all vacuum hoses and inspected the turbo plumbing closely. Besides, I don't think the pattern fits with that type of problem (frequency and duration doesn't match).

At said gas station I always use 91 octane. This is because I have had trouble running lower grades (87, etc.) in the past; the car stumbles and doesn't seem to like the lower grades, with the turbo seeming to "disappear". The similarity in my current problem to what it's like on lower octanes makes me think that this station is dishing out lower octane fuel than is labeled on the pump (I'm alread planning on calling my state's AG). So I have four questions:

1) Why would a lower octane cause a turbo vehicle to stumble under boost?
I have a meager understanding of higher octane increasing the burn evenness across the fuel and the temperature (avoiding pre-igintion), but I still don't really understand why such a small difference could literally kill my boost. Does the increase in compression cause an increased likelihood of preignition, thus magnifying the inherant weakness of the lower octane?

2) Even if this station is pushing a lower octane out higher octane pumps and that is the reason my car is stumbling, shouldn't the knock sensor detect any pre-ignition or whatever and trigger the ECU to compensate? Or is that what it's doing when I lose power under boost (retarding the timing)? If it IS doing this is that proof positive that my KS works? If not does that indicate that one reason I have such trouble with the volvo recommended octane for my brick might be that the KS is dead?

3) If the station I intermittently buy from isn't pushing low octane out 91 pumps is there something else they might be doing to the fuel to cause my problem? I have experience truly bad gas (varnished an engine with it once), so I know it isn't that, but could tehy cut it with something that would cause problems under boost?

4) Finally, a number of posts I've read discuss the dangers of pushing an engine that is pinging or knocking because of the fuel. I don't hear any such noises coming from my engine during the problem, however I haven't encountered them before and don't know what they would sound like. Does not hearing something indicate the problem is elsewhere, or just that the KS doing its job and retarding the timing to prevent damage?

Thanks greatly in advance for any help and information you can provide. I'm kinda in the dark with all this fuel information and trying to ramp up on a great deal as quickly as possible.

Thanks,
rt