91 or 93 Octane?

[BJH1]

I was just browsing through my owners manual and noticed the manual recommended 91 octane. The confusing part reads, "The purchase of higher octane is not recommended."

It's the "not recommended" part that perplexes me. Shouldn't 93% be better?

 

[Tom F&L GoR]

Not really. Your engine has an octane appetite, and once satisfied, a higher value will not provide more power or anything.

When different cuts of refined products are used to blend a gasoline to higher octanes they are often they are less volatile components. The result is less atomization, poorer driveability, higher hydrocarbon emissions, cylinder wall wetting, oil degradation... I know the list sounds pretty dramatic, and in each case the problem is only directionally worse, not tremendously worse. But the problem is noticed enough that dealerships (especially Ford) frequently recommend against higher octane to cure driveability problems.

 

[DChan415]

Tom, you're like an encyclopedia.

 

[Got Venom]

I have always heard the same thing at Tech Sessions I have gone. to. They say the lowest possible octane your car can run on without detonating is the best octane to go with. So if you here your car pinging under a load, go up to the next grade of octane. Otherwise stick with the 91 octane. Try punching your car when you are in third gear at around 1800 rpm . In other words under a load. If you here it ping here especially on a hot day, then go up a notch on the octane if not stay where you are. You will actually get better performance out of your car. Now if you add a supercharger etc. , you will naturally have to go to a higher octane. Out here in California, the highest octane we can get is 91, and it is working with my Sean Roe. So if I can run 91 octane, so can you , unless your car is modded.

 

[Tommy K]

Brannon,

Octane is a fuels rating of it's "resistance to burn". Anytime you compress gas, it makes heat. The more you compress it, either via high compression pistons or forced induction, the more heat it makes. The reason you run high octane fuel when recommended is that it won't pre-ignite simply under the compression heat...it resists it, and waits for the spark plug to light it off. When you're running too high compression, forcing air, and running too little octane, the fuel won't be able to resist pre-igniting, or detonation, and will light the mixture off prematurely. When a piston is still on it's way up, and it decides to blow, and force it down...boom. That's when stuff breaks. So, Dodge figured that with the factory 9.6:1 comp. ratio or thereabout, that 91 octane should be enough. Running higher octane, 100 or maybe even 93, under "normal" conditions will just make it harder for the engine to burn it off. Fuel economy and performance may suffer. That's why if you have a family car rated for 87...anything more is a total waste, and maybe costing performance. But if you have a high comp. engine, especially with turbo, sc, or nos, usually the higher octane the better.

Tom

 

[BJH1]

The knowledge base on this forum amazes me! I will look for 91% from now on. I'll check on my wife's G35 Coupe as well. Although, its a higher compression engine and mostly requires the 93%.

Thanks.