[kaval]

Originally Posted by 01TrublUGT

LOL paragraphs are not illegal if you look back in my original post all of the "\r\n\r\n" were where I tried to skip lines and start a new paragraph, but for whatever reason it did not show up.....Hopefully it won't do it this time too.

While some would think that the computer can adjust more than I stated believe me it can't. There is NO WAY for the cars computer to tell what octane fuel you are using. The ONLY side effect of using a lower grade octane fuel than what the factory specified is pinging/detonation. So if that is the only side effect then the only way the computer will know what is going on is by the knock sensor input. Most all modern computers can "Learn" to a degree, so if you take a car that was programmed to run premium fuel and start feeding it a diet of 87 after a while it will learn what is going on and make corrections to compensate. While most people take that as the cars ability to adjust for different fuel grades it isn't and that is not what it was intended to do. While your computer is not running in safe mode per se, it is backing off timming to prevent any damage. Backing off the timming from factory specs will both hurt performance and MPG.


As for the Caddy, most of them are programed to run on 87, IIRC the only Caddy that states Premium fuel is the XLR. As for the SLK32 while it may check out with the dealer, it is getting just that...checked out. They are not testing the fuel in any way as there is no reeason to. Now if something were to happen and it went in for warranty work on the motor they would hook up the computer and see that the knock sensors were pulling timming enough for the computer to permanently re-adjust its fuel maps, from there they would take a sample of fuel from the tank, have it tested, and when it came back under 91octane your friend would get the apologetic phone call saying that the cost of a new motor would not be covered.

I will do you one better here. My car runs 10.1:1 compression and from the factory it states premium fuel only. I had my car tuned on a dyno with 93 and it was putting down 330rwhp, at that I was getting about 23mpg on the freeway. I had to drive my car from La to Tx so in an effort to save cash I brought it back to the tuner and had the tune adjusted to run on 87. The end result of that tune took my 330rwhp down to 293rwhp, no pinging and the car ran fine the whole trip. Once I got home I totaled up the miles and the fuel I used, my MPG dropped from the 23 I was getting running 93 to about 17-18MPG running 87. While I saved money at the pump, I spent more in the end needing more fuel to get the same distance.

While it would seem that running 87 would save you money over running 91, IF the car in question came from the factory tuned for the 91, it won't. You will be giving up the motors efficency to spend less at the pump so while you are leaving the gas station thinking you just saved $4.00, you will be making a return visit to the pump sooner as the less efficient a motor is the less MPG it will get.

I realize that gas is not getting any cheaper but if you spend $25,000.00+ on a car that suggests you run premium fuel, why would you not in an effort to save maybe $250.00 a year?? Look at the amount of time and money we spend keeping the outside of our cars looking like they are brand new, then skimp on the things that keep that car running the way it was designed. If you are going to buy a car and either can't afford to or don't want to do what the maker suggests maybe you should look for another car that does not need the premium fuel.

Finally compression ratios are brought up.

What is the compression ratio on your friend's car?