Race gas?
 

So can the motor take or use anything higher then 93
octane and be ok with it? I tried searching didnt find any info.

I'd see no reason why it couldn't. Just don't use leaded gas. In some areas here in the bay they have 100 octane at the pump.

Why?

The engine is designed to run on premium pump gas. Putting in higher octane fuel will not produce more power, in fact it could produce less HP along with less fuel economy and worse emissions.

It's not the most accurate way to say it but think of Octane as a measurement of the fuels "resistance to ignite", or resistance to detonate.

If your car was designed to maximize the benefit of a fuel with high octane like race gas (over 98) then it wouldn't be able to run 91-93 octane, or maybe it could but with great struggle or great retardation in ignition timing.

Putting high octane fuel in your street car won't hurt anything other than your wallet. Just like having a car that is designed to run on regular won't be adversely affected by running premium, it won't be benefited either. The only thing that will be affected is the wallet.

It will run on Race Fuel, but you will likely see your mileage go down as the ECU isn't programmed to take advantage of the higher octane. Best stick to 93.

Why would mileage go down? That depends on properties of the fuel independent of octane...most driving happens in stochiometric closed loop mode.

Incomplete combustion would leave a surplus of oxygen in the exhaust indicating a lean condition which the ECU would compensate for with more fuel. Hence, lower mpg.

What incomplete combustion? Are you trying to imply that the race gas won't burn as well as normal pump gas?

If the greater resistance to ignite causes the "kaboom" of the combustion to be weaker or later in the phase of the stroke this can decrease power. I would imagine the real world decrease would be relatively negligible, however over the course of a full tank of gas and difference of 8-10 octane points then change could be noticeable or measurable.

The combination of ignition time+Octane rating creates the point of ignition and burn rate to maximize combustion power and efficiency. If you change the octane rating it is very similar to retarding the timing because the higher octane fuel won't ignite as readily and since the combustion chamber pressures aren't any higher to necessitate the higher octane, the ignite point and burn rate will be slightly decreased.

So yeah... What Draco said.

The whole point of race gas is that it's harder to ignite. So it won't burn as readily or as completely if the cylinder pressures aren't at optimal for the octane.

So in a normal engine, yes, race gas won't burn as well as normal gas.

I doubt that the rate at which 100 octane gas burns is significantly slower than the rate at which 93 octane gas burns. There is plenty of time, relatively speaking, for the fuel to "fully" burn in the cylinder, and I don't think there would be a problem with the oxygen sensor calibration if say E85 vehicles don't have a problem, and ethanol burns very slow. If the engine isn't set up to take advantage of the increased octane rating then you won't see an mpg increase, but I think it's crazy to say it'll go down. Even with variation across different fuels, the heat per unit equivalence oxygen is very similar for all hydrocarbons, even alcohols.

There's enough of a difference between 94 and 93 that you can no longer get 94 in most of the US since no cars could fully utilize it, leading to more pollution and lower mileage.

I'm not saying it's a drastic loss, but it might account for 1 or 2 lost mpg average across the tank. But the big issue the lack of any power gains to justify the large jump in price. Not sure about the OP's sources, but most race gas pumps I've seen are around $7/gal.

for street driving it is a waste of $$$....no doubt....

However if you are planning on an autox or especially a track day in high heat, its not a bad idea....

For example my old E92 M3 was a 12.0 CR engine that spun to 8400rpm & also required 93 octane..but in the republic of Kalifornia we only get 91...so when I took it to the track I filled it up with unleaded 100, just to be safe..... Some other M3 forum members did back to back dyno tests in hot weather and found the 100octane DID make more power, probably due to reduced knocks, and the engine running on its max timing map......vs detecting a knock and pulling timing....not much...around 10whp I recall on an engine putting down 360+whp

The M3 did have 8 knock sensors or 1 per cylinder...I don't know how many the FA20 has....prolly 4 since it is a newer design....and has higher static CR....

When I take my FR-S to the track it will have unleaded 100 in the tank...

DracoREX sure, maybe a tiny bit, but I don't even think the fuel consumption should be affected. 94 is pointless since cars don't ever "need" more than 93, blending the 94 down into more 93 is a good idea for the refineries, I don't think 94 and 93 are different enough to affect emissions though. "Octane rating" is something pretty vague and you can't really tie it to burn speed definitively. I don't know how 100 octane and 93 octane gas usually differ, but I would think that they're not different enough to cause a noticable change in performance on the same tune.

1 mpg on a 25-30 mpg car is less than the drop in fuel economy you see from 0% ethanol to 10% ethanol, and that's replacing gasoline with a much much lower (volume) energy density fuel.

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

If the race fuel does not contain ethanol, fuel economy may go up on account of that. The rest of the engine probably won't take advantage of the additional knock resistance.

And yes, it IS possible for the ECU to estimate the octane of the fuel. There are many stock ECUs that do just that. You can also calculate the MBT timing using an onboard model.

Assuming the BRZ isn't drastically different from previous Subarus, it is using a very simple timing control system which doesn't model flame front propogation or MBT.

So on ECU's that have the ability to proactively work in conjunction with knock sensors to "test" ping and advance timing accordingly, how far will they advance?

From what I've been reading the engine will advance itself if you go from 87 to 93 octane and thus take advantage, like has been stated, but will it go beyond the advanced timing map that's optimal for 93 if you put in 100? It it my understanding that's not part of the program. So while and engine with an adaptive ECU like that of the Camry will see a benefit with 93 over 87, it doesn't appear that it's programed to provide benefit for any octane above that without telling it do so. Is this accurate?

You could be right, and I'm sure there are limits to how far it goes.

can we assume that it will advance timing until it starts to ping and then back off?

It's possible that the ECU will add in some more advance, but it's not even worth trying without a dyno.

Just stick with 93.

I tend to assume too much. I was hoping someone knew the answer. It doesn't seem to be readily google'able.

Honestly Toyota tends to keep certain info like this under lock and key.

the only way to find out would be to dyno test and data log.

I think that's a bad assumption, the ideal amount of advance for a certain engine speed is fuel dependent, and the only thing about the fuel the ECU can see is if it knocks and what the exhaust temperature looks like. Someone correct me if I'm wrong, but I think the ECU would have a target spark advance map and pull back from there.

You could broadly categorize the modern strategies as having two types: those with an MBT calculation/lookup, and those without. Subaru traditionally doesn't use an MBT calculation. MBT = Minimum spark advance for Best Torque. The well-known Subaru software doesn't use this kind of calculation but other manufacturers do.

There are different ways to do it and it varies with the manufacturer and how new the software design is. The well-known Subaru software up to now starts with a base timing ignition timing map, and then it has a 3D lookup table that gives authority to advance timing a certain number of degrees based on rpm and load. There is a multiplier that determines how much of this total additional spark advance will be used. Then there's short-term knock sensor feedback and also another learning table that's populated by rpm and load.

Many different manufacturers use some kind of "high octane" and "low octane" map. Mitsubishi does for example. You start on the high octane and interpolate to the low octane table based on knock sensor feedback. There are a number of ways this is implemented across manufacturers. You can accumulate some kind of knock value that counts up and counts down as a timer increments.

The other style of system uses an MBT lookup table or an MBT calculation. Bosch was an early adopter of the fully torque-based engine control systems that also directly implement the concept of MBT.

http://www.ft86club.com/forums/attac...1&d=1340332995

So in the Motronic 7 system diagram above (late 90s/early 2000s German cars) for example you have an MBT map and then a spark efficiency transfer function. That function tells the ECU how much torque output it's losing by deviating from the MBT value. Knock control is also implemented into the torque request logic.

http://www.ft86club.com/forums/attac...1&d=1340332995

A lot of Hitachi based control systems implement MBT with an actual simplified MBT calculation. It's based on the concept that, as a general rule, MBT spark advance typically results in the 60% cylinder mass fraction burned occuring between 12-15 degrees ATDC. Working off a basic differential equation you go backwards and fill in a bunch of variables based on sensor signals and constants based on engine geometry.

http://www.ft86club.com/forums/attac...1&d=1340333502

That results in your real time MBT calculation. Since you can't achieve MBT under normal conditions with pump gas, you have to have compensation maps to push your timing advance curve out to the borderline detonation range. This could involve advancing or retarding in some fixed increment with a counter, or it could use a more sophisticated way to actually model the fuel octane value in RON (Research Octane Number).

So running on unleaded race gas could actually result in more timing advance but only if it's been set up that way. It depends how much authority the ECU has to advance the timing in a given area.

Awesome post. :thanks:

Okay, so I think it is time for some education.

Every OBD2 ECU has the ability to track fuel octane. The ECU has a minimum of two fuel maps, High octane and Low octane. Based off of the amount of knock present the ECU either raises the Octane count, or lowers it. What does this mean? Well fuel calculations are based off an interpolation between the two maps, for instance...

If a given load and RPM access a lookup cell in the high octane fuel map and it is calling for a 12.5 AFR, and the corresponding low octane fuel map calls for an 11.5 AFR, based on the octane rating (or more correctly the amount of knock over time the engine has witnessed) the actual cell maybe somewhere between the two, or it no knock has been detected 12.5, or consistent knock 11.5. Remember pulling timing isn't the only way to fight detonation, an increase in fuel lowers the cylinders temperature and decreases knock also. If you don't believe me go pick up a scan tool and log "Fuel Octane"....

If you had been reading along you would have noticed that I got schooled about this before you got to me. Also, are you SURE every OBD2 ECU does this?