when i first got my 2004 hemi i was putting regular 87 gasoline in it..i read the manual and it suggested 89 gasoline does it make a real big difference between the two? and if so how big is it? thanks...

 

Yes there is a difference. The octane rating is how resistant the gas is to detonation. In a higher performance engine, such as high compression or turbocharged engines, you would need to run higher octane to prevent detonation. Detonation is when fuel detonates or combusts prematurely, which can damage the engine.

If the manual recommends 89 octane, I would stick with it

 

02DodgeRam1500, you hit the nail on the head. Great explaination...

 

The chemacists on here should answer. Octane refers to the atomic structure and the rating is how many of those "octane" (meaning 8--I think in this case refers to Oxygen bonded to carbon and others for molecule) are in the fuel sample by Parts Per Million (PPM) (I think).

The qualities of the fuel rated at a higher octane means that it actually burns slower. Hence--for building power ounce for ounce of fuel lower octane is better because it burns faster. However, higher compression engines essentially don't get far enough in their stroke to use that fast burn completely and knock and can rattle the piston causing damage--this is the knock you generally don't hear (If I remember correctly). Also, a high compression engine with lower octane rating can "pre-detonate" as is mentioned. If it fires too early during compression stroke and the fuel burns fast you'll get another type of knock or reverberation which is the knock you can hear (I think). This one isn't as damaging but often if you hear the one you're probably getting the other too.

One more thing to consider is that because higher octane burns slower you can advance timing a little bit. This actually helps build power by igniting the charge while still in motion from the compression stroke. If the burn lasts long enough to excert force during the duration of the power stroke then you tend to build more power. The difficulty here is if you don't have some way to retard during startup then it is a lot harder to ignite----IE: hard to ignite fuel in a great deal of advance when the crank is only rotating at 400 or 600 RPM.


END POINT: most modern cars' computers adjust by a knock sensor how much advance your ignition can take and will adjust when slower burning fuel (higher octane) is used and it will retard to start the car. IE: if it recommends a fuel, use it (except when you're using cruise control for 300 miles in Nevada--then use the cheapest dang stuff you can find.)

 

Red91RT - interesting chemistry class you took. The post a couple lines up addressed the question correctly. You got a little too excited. I dont know if people would refer to me as a chemist (not chemacist), but I have a BS in it. Anyway, Octane is the organic chem structure (carbon based) formed from what is called "cracking" crude oil and creating whatever hydrocarbon chain desired. The octane reading of gasoline indicates how much can be compressed (in what units I am unsure) before it ignites. The octane has nothing to do with the stroke of an engine, but the compression of the engine dictates exactly what octane is needed. Using a lesser octane results in combustion instead of a fully evolved ignition. You said using a lower octane increases horsepower. That's a scary thought. Stick 87 octane in a Nascar and see what happens - it would probably not get over 110mph. Adding lead dramatically increses the octane (pressure required to begin ignition) but is BAD for fuel-injected engines and your reproductive system. Have a nice day.

 

Of course it makes a difference! That's why they make it in different "grades". It would be a lot cheaper for refineries to make just one type/grade of gas, but there's too many different kinds of cars and trucks out there, all with different octane requirements...

 

Say Hello premium and 2.10 a gal.

 

My HEMI would spark knock on 87 octane.

 

Im not sure.. i read the lil black book.. and i aggred with everything in it.. i just cant bring my self to feed in reg gas.. Prem all the way! $1.06/L Candain style.

 

yeah thats why i have a 91 crx when i gotta roll to far away places...california gas aint no joke! but thanks for all you chemistry scientist fellas for the REAL DEAL on gas...peace