Join Date: Mar 2007
Location: Old Forge, PA (Scranton)
It seems like this topic comes up every so often on the boards, so after retyping it repeatedly I decided to save it on my computer and copy + paste whenever necessary. This is originally my response to somebody on the boards a few years ago who vehemently defended his stance that using higher octane fuel was good for his engine. What I've found over the years is that people who have been using high octane fuel in their regular cars are the ones who defend their stance the loudest when provided with evidence to the contrary. I get it.... Nobody wants to admit what they were doing for YEARS was wrong (plus the added embarassment of having been throwing away money as well). I am not a fuel engineer. I am a high school chemistry teacher. So take that at face value.
So here we go:
The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. The higher the octane number, the more energy it takes to ignite the gas. Technically, an octane rating measures the proportion of isooctane to heptane in a fuel. But as a practical matter, a fuel's octane rating relates to how much energy it takes to ignite that fuel. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Lower-octane gas can handle the least amount of compression before igniting. The compression ratio of your engine determines the octane rating of the gas you must use in the car, which is why we should really follow the advice of the engineers who designed our engines. If it says use 87 octane, then use 87. You're not doing your engine any good by using what you perceive to be "better" fuel.
Unless your car is explicitly designed to run on high octane gas, using a high octane gas will NOT give your car better mileage NOR will it give it more power. That's because, once again, the rating refers to how much energy it takes to ignite the gas, not directly to how much energy the gas puts out.
Of course, with anything, there are exceptions to the rule. One exception is with engines designed for high octane gas. In that case, using high octane gas WILL improve performance and mileage. The reason has to do with the compression and ignition timing characteristics of the engine. Those specially designed engines will only perform efficiently with higher octane gas.
The second exception is if your car has a lot of engine knocking or pinging. This is a sign that the gas is not igniting when it should (i.e. the timing could be off). This reduces the power and efficiency of the engine. In this case, use a higher octane gas and see if it helps. (And by higher, I mean 89 and not 92.)
In conclusion, in most cases, there is no reason to use and pay for expensive high octane gas. So please don't waste your money. You're actually harming the durability of your engine over its lifetime if you use higher octane fuel and your vehicle doesn't explicitly call for it.
94 YJ w/ stuff. All the work/money I put into it, and the comment I get most is my $20 mod: "Hand throttle?!? That's awesome."
You know you drive a YJ when it is no longer getting older, but slowly becoming brand new part by part.