I put plus gas in my 2000 XJ a few times and then heard it was bad for it, what could it so to my engine? I think I filled it up 2 or 3 times with plus

 

What u heard is bad, misinformed, mythical info. Google "octane myths" and get good valid info. "Plus" gas will in no way be "bad" for your XJ......it's just NOT necessary and wastes your money.

 

Here in CO, we have 85, 87, and 91 I believe it is. You can run 85 without knocking due to elevation, I guess, but I've been running 87 and get much better mileage. It costs 15-20 cents more but I get 4-5 MPG more.

 

Here in Ohio it's 85,87 and 93. I use the mid range on my 97. My 94 was very fussy and would only get decent gas mileage with the high test.

 

A common misconception about octane is that it is the flammability or volatility of the fuel, when it's the opposite. The higher number is higher resistance to knocking or pinging due to high combustion chamber temps and pressures. I run 87 in my 90, no additives, and get about 15-16mpg on 5" lift with 3.73 and 35s, regardless of gas grade

 

There's always a post or 2 saying/claiming better mpg with higher octane than what the motor was designed to efficiently run on. There r article after article saying higher octane than necessary WILL NOT improve mpg. Octane myths claiming better mpg using a higher number just keep on keepin' on.

 

If anything, I have been told that running too high of an octane fuel with colder plugs in low compression engines can result in incomplete burns.

 

Another myth. And a colder plug has nothing to do with spark power or heat. Plug temp has to do with how hot/cold the porcelain tip is.

 

Yup. This always comes up in a super sport motorcycle forum I belong too. Most people can just not grasp this and insist on "running the good stuff" even though the bike is engineered to run on 87.

 

Using higher octane than what the book recommends will not improve mpg/make the motor run cleaner/increase performance or in any way damage/harm the motor. Running higher octane than needed is simply a waste of money. All this presumes the motor is well maintained and tuned to factory specs.

Running lower octane than recommended is when problems can start.

 

I've never heard that myth before! One for my records.

The ONLY time running a fuel with a "higher than recommended" Research Octane Number is going to have any effect is

- When the more expensive fuel has better detergent additives (cleaning injectors over time).
- When the more expensive fuel is a different blend (eg 91 here is pure gasoline, 87 is 10% ethanol). This is highly dependent on vehicle, some vehicles run better with ethanol, some better without, others no difference.
- When some internal engine problem is causing pre-ignition knock, such as carbon buildup.

Otherwise, it does nothing.

 

Which myth r u referring too.....there r several myths in this thread already.

 

In the US and Canada, octane is posted in AKI. RON is used in Australia.

If he put 87 RON in his XJ, it would struggle.

 

Actually most us gas stations use Ron+Mon/2

 

Good point LOL The first one from the OP.

Interesting. Gonna have to poke in the eye whoever told me that one. Internet tells me that Anti Knock Index is (RON + MON) / 2. That explains why Australian lads report much higher "octane" numbers.

Why can't people standardize on this ****???