Hi guys! I hate to start a thread on what gas to use for my car when I know there are conservatively 200 threads on this topic but I searched and couldn't find an answer to this question..

Basically, the gas stations around where I live have 87, 89, and 93 octane. It's my understanding that the IS is optimized for 91 octane. Instead of paying more (albeit only a little more) for 93 octane that my car doesn't need, would it be bad for the car to fill it half way with 89 and then fill the rest of the way with 93 octane?

Apologies if this is a stupid question but any thoughts would be appreciated. Thanks a lot!

 

seems like a waste of time to do such a thing, but I guess it would equal out.

 

I think the Lexus manual say 91 or higher?

Anyway, I'm not sure if 83 + 93 would = 91 octane, but I just don't think its worth the hassle to save couple pennies nickles and dimes especially when you drive a Lexus.

 

made my day

 

You don't need to worry about it, running 93 is perfectly fine.
It's optimized for "premium' fuel' 91 or 93 doesn't matter

 

good question, i ve thought about this before. not really sure

 

Octane is a measure of the fuels resistance to knock. That's all it is.


the 2IS is designed/tuned/programmed to require 91 octane to insure it does not knock.

Anything below that and the car will knock under certain conditions,

Anything higher is of no benefit since it can't knock any less than not at all.


So there's no harm in running 92, 93, etc and there's potential (very small) harm from running 89.


Mixing exactly 50/50 of 93 and 89 should produce 91, basic math.

That said, you can't insure the fuel is "mixed' evenly in the tank or that you got exactly half of each each time (and even trying you'd have to be constantly keeping track how much of each you put in last and so on). Hardly seems worth it to possibly save 70-80 cents per total tank.

 

Im hoping your not trying to save a couple of cents by doing this. Dont waste your time and just use 93

 

89 + 93 would equal = 91 Thats the way they figure the octane rating: (R+M)/2 so
(89+93)/2= 91.

The numbers come from If the gas was pure Octane: 100% = Pure octane, 87 = 87 % octane 7% other ( lighter) hydrocarbons that would detonate.


Octane is a chain Hydrocarbon in gas: C8 H18, where as Heptane is C7 H 16 . The shorter the chain the more likely detonation. Of course there is all kinds of stuff in our gas like HC rings (benzene C6H6), ethanol C2H5OH; {raises Octane index} Hexane {lowers octane index} etc. These raise and lower the Octane or "Anti Knock" rating.



The gas would get mixed up pretty quickly driving around town, I would pump the 93 first though.

Does anyone know if the car's ECU would advance the timing to take advantage of the 93?

 

91 is all its programmed to advance the timing for......that is, unless you have a piggy

oink oink. My car smells like bacon every day!

 

I'm so doing this.

I hate sending my money to the middle east oil companies or just oil companies in general.

Take that BP, and Haliburton!!!

I'm going to be rich after one year of making my own 91 gas...

 

take it to the Man!!

 

We should start an east coast 91 Octane Gas Club.






-D
_________________________________________
91 Octane Gas Club Member #0000000001

 

I think you are confused... neither 89 nor 93 are R or M.

This is much simpler math. The average of 89 and 93 is 91... the octane formula you quote doesn't enter into it. The formula you quote is for something else entirely I'll explain in a minute.


Not quite, no.... Your definition (and it's iso-octane in the test, not "pure octane" for the RON number. This is the number used in lots of places outside the US (and why you see stuff like 95 and 98 octane in those countries).

The US uses AKI, anti-knock index, which isn't the same thing....it's the adding of RON plus MON then dividing by 2. That's what the (R+M)/2 means.

MON is tested differently and the results are usually (but not always) roughly 8-10 points lower than RON on the same fuel.

None of this is especially relevant to the topic though.





I'm not quite as confident as you that you'd get a homogeneous mixture rapidly.... and you'd always need to know exactly how much of each to pump at any given time, unless you've got something way more precise than the factory gas gauge you won't know that.... so unless you just put in say 5 gallons of each every time (never actually filling the tank fully because you can't know how much exactly half the right amount to fill it is if there's anything in there now) you'll end up with a little more of one than the other anyway.

It seems like an insanely silly waste of your time given you'll also need to pump twice, insuring you don't go over X gallons on either fill... but if your time is worth less than the 50 cents you'll save on those 5 gallons, knock yourself out.

As an added FYI, most of the gas in the US doesn't come from the middle east ... roughly half of our oil is produced in the US, and the next biggest chunk comes from Canada (and a decent bit from Mexico and elsewhere too).


No, it won't. The car has no idea what fuel you put in it. It only knows the stock timing advance is set for X (which is the timing expecting 91 octane fuel), and if it senses knock, it pulls timing.

 

It's not only about octane....it also depends on the additives that each "brand" gas uses