92, 95 and 98 Octane

I remember in high school my science teacher said there’s no benefit to using higher octane than what’s recommended. In fact, you can tune your car so that it can take lower without knocking and pinging. I’m curious why cars here run on higher octane gasoline than in the US, where the pumps typically offer 87, 89 and 92. Is it an EPA thing, like less pollution? My March is supposed to use 95, but lately I’ve taken to filling up with 92 with no perceivable difference. As I mentioned in an earlier post, there seems to be no correlation between brands and performance; my car will purr with pleasure one week and be sluggish the next.

Has anyone had experience retuning their car to run at on lower octane?

I guess it depends upon how much power you want out of your car. Lower octane, more detonation/knock = less aggressive tuning = less output. If I put anything less than 98 in my car it goes into limp mode.

Depends on how is your engine optimized.
The higher the power output per capacity (bhp/liter or bhp/ci), the more octane you need.

In US, you have big capacity engines with low power output, so you don’t need high octane nor very refined fuel.
Typically, if you get more than 100 bhp/liter, you at least need octane 95. Above 120bhp/liter, you’ll need 98 octane.

So if you have a big, low output, low revs engine, it’s useless to put more octane than needed.
If you have a small revy engine, you can use higher octane fuel. However, the difference is pretty small so, unless you’re on a track, just use the lowest octane stated on your driver manual.

In usa they use AKI (Anti Knock Index), while the rest of the world uses RON (Research Octane Number). AKI is an average of RON and MON (Motor Octane Number). If you’re not bored already you can find out more on Wikipedia!

and I think your teacher was right - no point in using higher octane than recommended unless you increase compression ratio or advance the ignition beyond standard.

[quote=“march”]In usa they use AKI (Anti Knock Index), while the rest of the world uses RON (Research Octane Number). AKI is an average of RON and MON (Motor Octane Number). If you’re not bored already you can find out more on Wikipedia![/quote]Putting this in simpler terms Octane is measured differently so the numbers in Taiwan and the US are not directly comparable.

yeah there was a thread bout this on here sometime ago. IIRC regular (87) in the USA is like 92 in Taiwan and 95 in Taiwan is like premium (91) in the USA.

Thanks for the clarification, guys. I was wondering why a subcompact like a March would need 95 octane here when a Porsche 911 only needs 92 in the US.

Correct me if I am wrong.
The octane value tells you at which compression rate the gasoline air mix will self ignite. When this happens before the piston reached the highest point, it’ll cause problems, :fume: :raspberry: - obviously.
My guess; when the engine is a little older the compression drops anyway and there won’t be any problem using a lower octane than advised in the car’s manual.

[quote=“Hamletintaiwan”]Correct me if I am wrong.
The octane value tells you at which compression rate the gasoline air mix will self ignite. When this happens before the piston reached the highest point, it’ll cause problems, :fume: :raspberry: - obviously.
My guess; when the engine is a little older the compression drops anyway and there won’t be any problem using a lower octane than advised in the car’s manual.[/quote]

OK. You’re wrong.

(First bit, up to, and including, especially “obviously”). Only a bit wrong, but still, strictly speaking (which is how you should speak about this kind of stuff), wrong.

Your guess (2nd bit) sounds very reasonable, though, and will often be true.

Sometimes, though, an older engine gets a bit coked-up which can increase the effective compression ratio, which will tend to promote detonation, which the octane number is directly related to and defined by.

The coking may also promote pre-ignition, which the octane number is indirectly related to, and not defined by.

[quote=“Ducked”][quote=“Hamletintaiwan”]Correct me if I am wrong.
The octane value tells you at which compression rate the gasoline air mix will self ignite. When this happens before the piston reached the highest point, it’ll cause problems, :fume: :raspberry: - obviously.
My guess; when the engine is a little older the compression drops anyway and there won’t be any problem using a lower octane than advised in the car’s manual.[/quote]

OK. You’re wrong.

(First bit, up to, and including, especially “obviously”). Only a bit wrong, but still, strictly speaking (which is how you should speak about this kind of stuff), wrong.

Your guess (2nd bit) sounds very reasonable, though, and will often be true.

Sometimes, though, an older engine gets a bit coked-up which can increase the effective compression ratio, which will tend to promote detonation, which the octane number is directly related to and defined by.

The coking may also promote pre-ignition, which the octane number is indirectly related to, and not defined by.[/quote]

Sorry for quoting wikipedia

Anyhow, it burns, it stinks, and it’s all the same. Putputputput toftoftoftof :slight_smile:

[quote=“Hamletintaiwan”][quote=“Ducked”][quote=“Hamletintaiwan”]

Correct me if I am wrong.

[etc see above][/quote]

OK. You’re wrong.

[etc, see above]

[/quote]

Sorry for quoting wikipedia

[quote]

[etc, see above][/quote]

Anyhow, it’s all the same.[etc, see above] [/quote]

Thats OK. The Wikipedia article isn’t wrong

To those of you knowledgeable on the Yaris, what octane gas do you feed it? The manual seems to say at least 92, but translation may be wrong. Is 92 the recommendation octane, or is it saying don’t go lower than that? As if you could knowingly get gas here under 92.

I’m asking about a 2012 model. Thanks!