Ok...so every single day I see a thread asking, "what will happen if I run 87 octane," "is 91 octane ok?" etc.

Now I am taking my time to explain all of this, so hopefully people will read it before posting such threads. I am going to go into more detail than needed because I know how hard it is for some people with little understanding of internal combustion engines to understand.

First of all, octane ratings and what they mean.


Octane is C8H18. Basically, just a molecule made up of 8 carbon and 18 hydrogen atoms. Other examples of H/C molecules are butane, hexane, heptane, etc. The reason we use octane as a reference is because of it's capability of handling compression.

Most cars that any of you will be driving have 4 stroke internal combustion engines. These strokes are intake, compression, power/combustion and exhaust. A gasoline/air mix is pulled into the combustion chamber during the intake stroke, then compressed, ignited by the spark plug and expelled during the exhaust stroke.




link to above image http://www.timloto.org/pictures/motor_wout3.gif

Other examples of h/c molecules are far less capable of handling compression without combustion. If you were to use butane, for example, it would likely ignite during the compression stroke. When the air/fuel mixture ignites before the spark plug is intended to ignite it, this is called pre-ignition (not detonation!). You can imagine the stress on the piston and rod when it is being forced by the rest of the rotating assembly's momentum to move upward, but combustion is trying to force it downward.

Basically, the perfect gasoline for your car is one that does not ignite until the desired ignition timing, but is combustible enough to burn cleanly and completely once ignited. Too much resistance to combustion = incomplete burn. Too little = pre-ignition.

The other instance of "knock" is detonation. Detonation is combustion of the end-gas or the remaining air/fuel mix. This happens after ignition BY THE SPARK PLUG. Detonation is far less destructive than pre-ignition and some engines are actually tuned with a small amount of detonation from the factory, but run fine for thousands and thousands of miles, but these are usually engines with very low specific outputs. Detonation is usually caused by excessive heat and a bump in which octane rating you use can help. You want to avoid both detonation and pre-ignition if possible.

So, now you understand that the higher the octane, the better the gasoline handles compression and the higher the resistance to combustion. Now, octane ratings you see at gas stations are stating the EFFECTIVE (not actual) percentage of iso-octane vs heptane in the gasoline. A 93 octane rating means the gasoline from that pump has the same characteristics, concerning knock, as a 93% octane 7% heptane fuel. You are actually getting many different chemicals and additives other than octane and heptane, but if the rating is 93, it will perform similarly to a true 93% octane fuel.

Now which one is best for your car? If your manual says 93 octane, use as close to 93 octane as possible. If it says 91 octane, use as close to 91 octane as possible. Your car is not going to explode if you don't use the right one. What is possibly going to happen is this. If you use 87 instead of the recommended 93, some type of knock may be experienced in certain driving conditions. Your knock sensor(s) will detect the abnormal sound and KR will be increased by the ECU. KR = Knock Retard, basically pulling timing to decrease the chance of more knock. This is the "fix" to stop you from cracking a ring land or any other issues. Of course, that doesn't mean you should run lower octane gas to save money. If KR exceeds 3-4*, you are gonna want to be very careful, but there is no way of knowing this without a data-logging. If you have too high of an octane, you will have an incomplete burn which, unless you are using something far beyond what your car is tuned for, will only result in an issue of carbon deposits over a long period of time, or other potential problems, but again over a looooong period of time, so if recommended is 91 and only 87 and 93 are available...go with 93.

If you have an aftermarket tune, again, use higher if what you are tuned for is not available, but slightly lower will not blow a rod through your oil pan so don't drive 20mph in a 60 just because you got 89 instead of 91.

to sum it all up, use what you are tuned for!!! You will NOT pick up power by using a higher octane (you may actually lose some) and you will just be increasing your chances of a mechanical problem by using lower octane. However, if you can't get exactly what you are tuned for, don't freak out! Go for the closest possible option (higher is preferable to lower if possible).

 

Well said!

Sticky!

 

good write up.

 

what he said^^^

 

nice.

sticky.

 

^^ x4

Sticky

 

sticky this piece for the ones who are still learning about everything... like sticky this in the 2.0/general/2.4/2.2...

 

Stick it already mods

Good writeup without getting too techie for most peeps

 

Thanks that helped me a lot.

 

 

Great write-up. Chalk one more request up for a sticky.

 

x1mill on the sticky. sweet write up.

 

Stickers.

 

Fix image then what everyone else said, sticky...I'm getting sick of posting links explaining this .

 

Very good write up. I vote sticky this as well.

 

the animation won't stay working for some reason. It's not really necessary anyway, but the link is to an animation showing the 4 stroke cycle.

 

Very detailed explaination.

Unfortunately, no matter how many times this has been explained already people still for some reason think that running premium in a car tuned for 87 will net you more horsepower and gas mileage....

 

I vote sticky. This explains a lot.

 

I had about 1/2 a tank of 93 and put about 3-4 gallons of 98 octance in it and after about 15-20 minutes it ran awesome! I felt more acceleration. I'm not tuned but it did work.

 

bump.

 

Most of the threads are just cheap people asking if it's okay to put 87 in their SS/SC cause they don't wanna spend the extra few bucks a tank. At least this thread will let them know they possibly could damage if they drive their cars hard.