They are more different than you know. Its why certain JDM cars need re-tunes to drink USDM fuel. Look at the differences between 87 and 93 then say 93 and 100 wont be as significant; actually it will be more.

I don't think people who put 93 into their Yaris or Civic or Corolla see a drop in fuel economy, do they? I believe hydrogen has a very high flame speed, yet its octane rating is very high too. If you want a more earthly example, methanol seems to burn quite quickly too. Octane rating is not burn speed, and engines are not that sensitive to burn speed at low rpm anyways since there's plenty of time for the fuel to burn.

Im very confused by the first sentence. And the second. And also the example.

Im just here to tell you that 93 VS 100 can be the difference of over 25WHP easily with a tune.

Some modern ECUs used closed-loop knock detection, and can take advantage of higher octane fuels. One example is the 2GRFE.

For example the ES350 uses the same ECU as the V6 Camry, but the Lexus is rated 91 octane and 2-3hp higher, while the camry is rated 87 octane with less hp.

The RAV4 V6 (also same engine) is supposed to use 87 octane, but guys have taken them to the drag strip and ran better times on premium vs regular.

Not sure if the FA20 does this or not.

serialk11r

You are fighting a loosing battle, and you wil likely melt your cat with 100 octane in the tank, due to the close placement of the "precat" or whatever we are calling it, to the exhaust valves,

when I changed from 91 to C16 it started trying to make boost at idle due to the fuel still combusting after the exhaust valves, EGT went through the roof, etc.

you need mare timing advance to make race fuel work, its not nice to have more timing it is required for the combustion event to coincide with the ~14 degree ATDC magic number, it will most certainly kill power (especially up top where time for combustion is narrower) because higher octane fuel (as you stated) has the same power density, it is simply harder to ignite (that is exactly the definition of octane "resistance to ignition")

I prefer the AKI method, as it allows Ethanol fuels to be compared also, taking into account the cylinder cooling...nevermind i'm diverging from the topic at hand.

I never meant that 100 was not different from 93, but DracoREX claims that fuel economy will drop, which I believe to be false. At low engine speed the burn speed is fast relative to the engine speed so it's not an issue.

With a tune, 100 obviously can make a lot more power, but I was never disputing that, only the fuel economy part. I don't understand how you can be confused.

Until someone is able to log ignition advance on different octane fuels, we have no idea if this is true or not. For that matter, an advanced ignition table isn't necessarily a guarantee of more power - it'll take dyno runs as well to know for sure.

100 octane fuel is most likely useless for this car. But I see a lot of people talking in absolutes, when we have no idea yet what this ECU is doing. Will there be a difference in power going from 91 to 93? Japan can get slightly higher octane than that at the pump, maybe the car has a stock tune lurking in the ECU that's even more aggressive than for 93. :shrug:

I agree with Daemione. Who is to say that the timing, maps, etc cannot deal with higher octane fuel? Although it isn't likely...

In Canada, well Alberta we only have 94 octane, no stations have 93 and all of the 94 are blended with ethanol.

If the ECU is set up to add ignition advance in the case of no knock or pre-detonation higher octane could be useful.

A lot of people are using the word IF. "If there's a tune, If the engine's built for it, if the timing is advanced... if if if if."

The OP didn't have these IF's, he just wanted to know if it was OK to use in the engine. It is "OK" but pointless.

Also, I'm not a chemist so there's that. :D

I know. The ECU has no way of measuring or knowing the AKI rating of the fuel. The only instrument it reads is a knock sensor which detects "ping" or detonation and retards ignition timing. One might say that's a way of measuring AKI in the fuel but there's nothing on this engine (and this isn't rocket science) that lets the computer know; "Hey, this is 100 AKI octane, let's advance the timing!" If there was, that would be incredible and very very very very expensive. The only way the computer knows you have higher octane is if you tell it so.

The ECU's aren't exactly a mystery as OBD2 ECU's have been worked with for several years now. This isn't something developed by DARPA. The ECU runs the ignition and injection maps based on a series of data points it collects from sensors (Map/Maf/TPS/knock/O2/etc, etc). Unfortunately there's no octane sensor (outside of a knock sensor) but that would be really cool.

So the stock ECU is doing what just about every other stock OBD2 ECU does in a car, this one just has 8 injectors to work with. :happy0180:
So to the OP: 100 Octane in an unmodified street car is roughly equivalent to lighting your wallet on fire with your cash inside of it.

That's right! Good example! The fuel has the same number of hydrocarbons, the same amount of energy but one is more resistant to detonation. So the Lexus' engine is designed to take advantage of that higher octane with A: Increased cylinder pressure, B: More advanced timing, C: Higher compression, or D: all of the above. edit: after further research it looks like the answer is B: timing.

Quote from another forum which explains well what uspspro and Genomaxter say.

Ok, I'm excited to learn how this occurs. Can you please elaborate. The knock sensor can only detect knock if knock occurs and it doesn't occur unless detonation/ping is occurring so how can a closed-loop knock detection sensor instruct the ECU to take advantage of higher octane outside of defaulting to standard mapping?

A lot of Toyota ECUs actually use the knock sensor to decide on what timing it should be using. This has been the case on the 2GR, 2zz, and 1zz.

What the ECU does is advances timing until it detects knock (very slight that you wont notice it) and then it retards back slightly. It will keep doing this over and over constantly. It may not know what octane you are running, but it doesnt matter because it can tell what timing at the current moment works and what doesnt. The stock ECU can gain a little performance from this but not as much as a full tune on an aftermarket ECU.

Good info. Thank you.


http://www.filehurricane.com/viewert...e_you_know.jpg

The stock ECU for the 2GR is pretty damn good.

Here's the dyno (whp) from my 2GR swapped MR-S. It takes advantage of the breathing mods, and premium fuel pretty well.

http://farm8.staticflickr.com/7101/7...b9b5680f_b.jpg