Here's my question.
I drive a Mercury GM with a 4.6L V-8 engine, factory stock, second set of plugs and have put about a half million miles on 3-4 of those things in the last 15 years. I like'em. I don't think the engine has a knock sensor, or at least if it does, it doesn't work too well, because in the summer month especially that engine will ping just as long as you want to keep your foot on the foot feed at that position. I don't like to drive it pinging down the road. I do often put the next grade up in or at least do it on an every other tank basis, as I am basically in agreement with the consensus that higher octane fuels are a waste in engines not designed for them. But I do notice a ever so slight improvement in the mileage when I do it. It may be that now that the engine is not pinging itself down the road it is delivering more horsepower to the wheels, which is what I want it to do anyway.
I also have some engineering experience in fuel pipeline stations where the tanker trucks load, and I have many times seen the tankers from all brands all filling from the same tank filled from the same pipeline and have talked to the drivers as they did the additive thing. That to me is suspect, as drivers are as human as the rest of us and if his wife beat him that day before he left for his haul, he might just forget to add the package, or might do it twice. What about rainy days when it is coming down in buckets-does he bother? Who knows and where is the quality control guy checking him-I never saw one. How good does it mix? Did he add it before he filled so that he got good mixing, or put it in when the tanker was full as an after thought?
In my former life, I drove lots; sometimes requiring several fill ups a day. As a practicing ME, I think I have the ability to understand the variability of fill up levels and take into account the variations. Mostly, however, since I am basically lazy and don't count time standing at a gas pump as productive (not to mention the frustration of having to try to figure out how to operate modern pumps-the guy who designed kitchen microwave controls and television clickers designed the gas pump controls) I fill it to the brim. Multiple click off's until it is full and won't take any more (even with the occasional rocking the car). I also note whether the car is on a mostly level surface, or if it is tilted in any manner. I know just about how much it will take to get to the first click off and just about how much I can 'stuff' into it by working at it. Those of you who drive your cars lots of consistent miles know what I mean.
If I had a dollar for every time I had filled a vehicle, I'd be long retired.
That all said, every now and again, the mileage will suddenly jump noticeably. While on a good day when I am driving decently, mostly turnpike driving, not much in town driving, no excessive speeding (rare, but I can do it) I get a steady 22-23 MPG (I have the power package with a different ratio rear end so I get less than the std. GM.)
But from time to time, not often, but often enough to raise up a question in my mind, the mileage will jump to the 24-25 MPG range, and I didn't do anything in particular to obtain that. Remember determining this is always after the fact on the next fill up.
Anybody got any idea what might bring this on????
I have a theory. My theory is that if a jobber has an excess amount of higher octane fuel in stock that he has to move, that he might drop it into the 'regular' tank and sell it at 'regular' prices just to turn his inventory. I can't prove it, but I have never come up with any better explanation of why the random increase in fuel mileage. It certainly wasn't anything I did.
Usually this happens to me in a situation where I can't go back to the station where I go the 'hot' load and try to repeat the results.
I'd be curious to hear the experts on this.
Pat-I got into this thread late, but gave you a star for your first post in it.
rmw
