An explanation of why turbo cars like high octane gas

Post by **jblackburn** » 10 Apr 2012, 12:36

...but why you notice the difference the most in the summer. I normally run 89 in the S70 and it does fine, but during the summer with the AC on, it feels slow unless I use 93.

The technical explanation more or less is the knock sensors compensating for knock it hears under heavy load or in hot weather by retarding the timing - a LOT. This leads to crappy gas mileage and poor power. This will affect any high compression engine - an NA has a high compression ratio, but turbocharged models even more under boost.

Running 87 probably won't HURT the Volvos or any modern car , BUT I know from experimenting around with a tank or two of it that the turbo models just hate it and bog down like a weedeater with an ill-adjusted carburetor. 87, while it contains more "power" than higher octane gas will actually pre-ignite on its own before the spark plug fires easier when compressed on the cylinder up-stroke than 89, 91, or 93.

Here's the forum post and some related reading if you're interested:
http://www.cruzetalk.com/forum/34-1-4l- ... -fuel.html

Some of those guys are actually pretty sharp.

Post by **asimba2** » 10 Apr 2012, 15:44

We can thank the knock sensors for giving us the most ignition timing advance (which equals the most power) for the knock resistance of the fuel we are running (octane). I recently supercharged my 4Runner and when I moved from 87 octane to 91 I also had an increase in fuel mileage, even with the SC in place. More timing advance means more power and efficiency. Higher octane fuels gives us the timing advance in the first place.

Post by **matthew1** » 10 Apr 2012, 16:03

Summer performance in my 850 between 87 and 91 octane is no contest. My 850 T5 is a dog in hot weather with 87. A Basset Hound.

echase76 · Post by **echase76** » 10 Apr 2012, 20:05

Thanks for the explanation. I haven't noticed too much difference between octanes for winter blends. When my wife filled up with 87 there was not a noticeable difference.

Being new to the turbo game, what are thought on octane additives? Do they work? How much is too much octane for a Volvo? My first summer with the T5 is fast approaching and I'm looking for more of a greyhound than a basset hound, but don't want to break the bank running 93.

Post by **jblackburn** » 10 Apr 2012, 20:41

Most people probably aren't as demanding on their cars or notice as much as I do, but it really bugs me when a car doesn't perform like I know it's capable of when I want it to. 89 or 91 octane should do you fine; you will probably only notice a difference on the hottest days.

The AC being on feels like dragging a boat behind my t5 and knocks down MPG by 3-4 consistently. I believe part of this is due, in addition to cutting back timing at low RPM to avoid knock, to the heat soak from the AC condenser to the intercooler, and partly from the drag on the engine by the compressor.

Try 100 octane in a turbo sometime and take it on a race track. It's VERY fun

obayha · Post by **obayha** » 15 Apr 2012, 18:21

I am cheap, cheap, cheap when it comes to the gas I run. I average aroung 28 mpg all the time, summer /winter, a/c on or off. I dont't push the 265,000 miles I have on the car and just hope it stayes around a little longer. The wobble and blue smoke keep me in the, "don't push your luck mode". It's a comute car and needs to get me there every day. I just hope it doesn't burn the valves up.
Shane