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6G Celicas Forums > 6th Generation Celica > Engine/Transmission/Maintenance
richee3
I feel like I've seen enough questions asked about fuel between the forums that I'm on that I feel like a thread like this may help point someone in the right direction when they have questions. So, first things first:

Q: What is octane?

Before getting into what octane is, we need to discuss knocking or pinging. Pinging is what happens when the air/fuel mixture ignites in the combustion chamber before the spark plug ignites it. This can be very damaging to the engine. Imagine the piston coming up in the cylinder, and having the fuel ignite while the piston is still traveling up. The pressure from the ignition will be trying to push back down on the piston, rod, and crankshaft. (This is just a rough explanation. If clarification is needed, I will be happy to pull quotes, GIFs, etc. from other sources.) Modern engines can cope with this by detecting pinging through the knock sensor and pulling ignition timing, which robs power and raises EGT's, but that's a different topic of discussion.

A higher octane rating will have anti-knock additives and have a higher resistance to pinging, allowing your engine to run more advanced timing and avoid potential damage. The trade off is that a higher octane fuel takes more energy to ignite, and will actually lose power in an engine that isn't designed to to run on it.


Q: But "premium" fuel is "better" fuel, right?

Not necessarily. Fuel stations love to use labels like "premium" and "super unleaded" to convince unwitting customers to fill up with a higher octane fuel, even though they don't need it. Marketing aside, let's discuss better fuels. Here in the US, the EPA has set a standard for fuel. The companies that meet or exceed these standards are known as "top tier" companies. These are the stations that are the best to fill up at. Further research, as well as a list of these companies, can be found at:

http://www.toptiergas.com


But my car gets better gas mileage when I fill up with premium!

There are two different possible causes for this. The first possible cause is that you are not getting better mileage at all. As stated earlier, a higher octane fuel takes more energy to ignite, which will result in a power loss in an engine that isn't designed for a higher octane. The second possible cause is that some companies will add more detergents to their higher octane fuels, which may help clean your engine. A clean engine is an efficient engine. However, these detergents cannot work miracles and will not clean the carbon out of your engine after just one tank, if they can clean it at all.

Keep in mind that with more anti-knock additives and detergents in your fuel, you are actually getting less fuel in each gallon (or liter, depending on which side of the pond you're on.)


Q: So does my engine need a higher octane fuel?

A good rule of thumb is that a turbocharged and/or high compression engine should always use a higher octane fuel. Our lower compression economy engines like the 5S-FE and 7A-FE will run perfectly fine on regular fuel. The 3S-GE and 3S-GTE should always fill up with the highest octane available, and 3S-GTE owners should do further research to match their boost levels with the octane level that is readily available. (For instance, on 93 octane, I can run 17 psi in my Supra but if I can only get 91 octane, I need to keep the boost below 15 psi. These numbers will be different for you depending on your engine, boost, and octane ratings where you live.)

Q: Will it hurt my car to fill up with a higher octane fuel if my engine doesn't need it?

No. Your ECU may advance your ignition timing a bit, but more than likely nothing will change. Filling up with a higher octane fuel if you don't need it will hurt your wallet but nothing else.

Q:What about off the shelf octane boosters? Do those work?

The short answer is no. They will raise the octane rating of your fuel, but only by "points" and not numbers. If you read the back label, they will almost all explain clearly that they may raise the octane rating by 3 points instead of 3 full numbers. Example: You have 20 gallons of 87 octane fuel and add a bottle of octane booster. Now your 20 gallons of fuel is 87.3 octane. There are a couple of brands that seem to legitimately work and raise the octane by full numbers, such as Royal Purple and NOS. But for the safety of your engine, it's best to avoid any faith putting unknown and untested claims.

Q: What about E85? Can I fill up with E85 because of reasons?

Again, the short answer is no. Because of its higher ethanol content, it is known to be corrosive to rubber fuel lines and aluminum. Plus, it takes more ethanol to make your engine run, and our cars simply weren't tuned for the added demand in the fuel system. Here in the US, E85 is not guaranteed to be 85% ethanol, which mandates the need for a flex fuel vehicle- one that is designed to be able to withstand the corrosive nature of ethanol as well as monitor ethanol content and adjust the tune as necessary to run efficiently. However, E85 is also known as cheap race gas due to its high octane rating of between 100-105. With the right modifications to your car and a tune, E85 can be very beneficial. But for those of us with stock or nearly stock cars, E85 is out of the question.


These are the only topics I could think of to discuss. If you guys think of anything, I'd be glad to add it in!
Bitter
Petrol? Are you gone all limey on us mate?


Good copy/paste though.
Box
Don't worry I call it petrol too, that or fuel. I tell people I'm going to the petrol station and they look at me funny for some reason.
richee3
I actually did consider doing a big quote of more well-written info but the wife was driving and I was bored. Since I had nothing better to do, why not write a thread about octane? Besides, the couple of threads I've seen about fuel were really too detailed for all 6gc.net really needs.

I plan on adding a brief paragraph on E85 when I get home and get settled down. I'm debating briefly touching on meth injection as well but I may pass on that as it doesn't really apply to most of us here on the forum.
Box
You might could do a separate post on meth injection over in the forced induction section and could touch on it here and then have it hyperlinked to the main post.
njccmd2002
Meth is illegal in tn
Bitter
Meth, not even once.
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