Originally Posted by Chris L.
There is no more energy in a gallon of 91 octane gas than there is in a gallon of 87 octane gas. Higher octane gas simply is more resistant to expoding at higher temperatures. So in higher compression ratio engines (such as the X-fire's), the owner's manaul calls for 91 octane fuel. The higher the compresion ratio....the higher the heat caused by the compression process. Low octane fuels may explode (ping) just from the heat of compression.....for instance .....diesel fuel might be described as having a super low octane rating ( explodes much easier than gasoline....that's why it's used by terrorists in bombs!)...diesels don't need spark plugs except to start the engine I think.....they fire just from the heat of compression.
So, higher compression ratio engines call for premium.....most nowadays have computerized "Knock-sensors" and will adjust timing if you put lower octance fuel in.....BUT.....you won't get the maximum horsepower from the engine.....
The whole purpose of engines designed with higher compression ratios......you're shoving more AIR & FUEL into the volume of the cylinder....before you light the match......power is determined by how much 'Stuff" you can blow up.....to put that into perspective ....you can have 2 engines with the same displacement...say 3.2L (like the X-fire).....if one has a 9.0 compression ratio & the other a 10.5 ratio.....you are shoving more fuel-air mixture into the 10.5 one on each firing = more power!!
If you put 87 in....you will either get premature detonation (ping).....or, the engine's sensors will adjust to dumb down the process to prevent the ping.
Informative reply, Chris. My buddy, Smitty, was a Merchant Marine and spent many tours hauling gas. He always told me to buy the cheapest gas you can find because it's basically all the same. He delivered to all the major brands and the only difference was what they put into it at their tank farms.
I've used 87 octane in many of my V-8's including my company car. They always performed well with no problems. I have a Hemi in my Dodge Quadcab but I use 89 octane in that. Occasionally, I use premium and I can actually feel a slight difference in performance. I've only used premium in the XFire, either 92 octane Gulf or Ultra 93 Sunoco. It would be hard for me to put a lesser octane in my XFire. I just don't feel like I want to try a lesser octane in that car.
Bob