Fuel Requirement

I don't doubt you noticed a difference with your cars. Not all cars are made the same way. My last car was a 04 BMW 330 and my present car is a '07 Lexus GS 350. I could not notice any difference caused by different fuel octane.

FYI - The very first car I saw with computerized mapping was the '67 Audi 5000. So, it's not like it's a new science. Bare in mind, the manufacturers have to legally protect themselves...reason for all that mumbo-jumbo and they MUST recommend whatever fuel octane was used for their EPA certification, no exceptions allowed!

If it makes you feel better by using high octane in your new Toyota...by all means do so, you will not be alone!

 

I see a lot of 85 octane at high altitude, where traditional engines don't need as much octane as at sea level.

 

I sure hope you're right. I also hope someone from Toyota can clear this issue up.

 

The fuel pump is in the fuel tank and doesn't care what it pumps. Its just a pump - as long as it's liquid, it's happy. Keep more than 25% fuel in the tank to ensure that it's always submerged in liquid for cooling.

The injectors are in the cylinder head intake ports, before the valves, and also don't care much what liquid they're squirting.

In terms of flow and other liquid characteristics, the grades of fuel are exactly the same. The energy content is more or less the same - I've actually seen some sites indicating lower energy content in higher-octane fuel.

Higher octane fuel just means it can be compressed more without spontaneous combustion - exploding. The timing of the flame is very important to ensure that all the force generated by the expansion of the gases pushes the piston down the cylinder. Too early, and some of it is wasted pushing against the piston trying to return to the top of the cylinder. If a fuel is used that is lower in octane rating than the engine was designed for, it may explode when the piston hasn't yet reached top dead centre, causing damage.

To prevent this, cars are equipped with knock sensors, which detect the explosions and, via the engine management computer, automatically retard the timing, sparking earlier, which wastes energy but at least doesn't cause damage.

Turbos add greater pressure, in effect greater compression, and are more susceptible to knock for that reason.

The Prius has a high stated compression ratio of 13.0:1 which would require a very high octane fuel in a conventional Otto-cycle engine. However, the compression ratio is effectively varied by the Atkinson cycle - the intake valve stays open through the beginning of the compression stroke, so the final pressure stays lower. The greater expansion ratio extracts more of the fuel's energy as motion, but torque and power are reduced. Instead, low-end torque comes from the electric motor.

I speculate that the car could have been designed for higher-octane fuel, with a greater expansion ratio, and get even better economy as a result. Our lowest grade in the UK is 95 RON (probably about 91-92 using the US octane rating).

 

Mike,
Very good information.

Here's something I just found and I hope will be useful to some:

How can you tell if you’re using the right octane level?
Listen to your car’s engine. If it doesn’t knock when you use the recommended octane, you’re using the right grade of gasoline. Knock occurs when cylinder pressures are high. It is normal for an engine to ping a little at full throttle because cylinder pressures are very high at full throttle. Engine knock, however, should not be ignored since it can result in serious damage to the engine. High octane gasoline burns slower than low octane gasoline. The slow burn prevents engine knock when cylinder pressures are high.
If your engine runs well and does not knock or ping on low octane gasoline, there is no advantage in switching to higher octane gasoline.
If your engine knocks or pings, it does not necessarily mean something is wrong with the gasoline. It could be a problem with the car's ignition timing or exhaust gas recirculation. On high mileage engines, a carbon build-up in the cylinders can increase cylinder pressures and cause knock.
Will higher octane gasoline clean your engine better?
As a rule, high octane gasoline does not outperform regular octane in preventing engine deposits from forming, in removing them, or in cleaning your car’s engine. In fact, the U.S. Environmental Protection Agency requires that all octane grades of all brands of gasoline contain engine cleaning detergent additives to protect against the build-up of harmful levels of engine deposits during the expected life of your car.
Is all "premium" or "regular" gasoline the same?
The octane rating of gasoline marked "premium" or "regular" is not consistent across the country. One state may require a minimum octane rating of 92 for all premium gasoline, while another may allow 90 octane to be called premium. To make sure you know what you’re buying, check the octane rating on the yellow sticker on the gas pump instead of relying on the name "premium" or "regular."
Does high octane gasoline improve mileage?
In general, if your car is designed to run on 87 octane gasoline, high octane gasoline will not improve mileage. If switching to high octane gasoline does improve mileage, you might find that a tune-up will give you the same improvement on 87 octane gasoline.
Does high octane gasoline achieve quicker starting?
No, it doesn't.
Does high octane gasoline increase power?
If your car is designed to run on 87 octane gasoline, you shouldn't notice any more power on high octane gasoline. Again, if it does make a noticeable difference, you may need a tune-up.
Is high octane gasoline more refined -- is it just a better product?
Additional refining steps are used to increase the octane; however, these additional steps do not make the gasoline any cleaner or better. They just yield a different blend of hydrocarbons that burn more slowly. The additional steps also increase the price.
Handyman USA - Gasoline Octane Rating

 

NEVERMIND.. Mike beat me to it as I didn't see the second page ..


To those that have said their engine requires 91 or higher but use 87, you are taking a risk.

From science 101, we know that pressure creates heat. Octane is a measurement that signifies how much pressure it can take before spontaneously combusting. The higher the rating, the higher the pressure it can withstand. A turbo/high performance engine is built to a higher compression (generally 9.5:1 and up). A rating of 87 octane cannot withstand these pressures when the piston is forced up. What happens is the gas will combust before the spark hits, which forces the piston down, and in turn, forcing other pistons up or down in their respective cycles, then the spark fires in those cylinders. This puts a HUGE strain on the crankshaft. Not only that, but since the valves are controlled by the camshaft, which is controlled by the timing chain, which is controlled by the crankshaft, the valves are not in the correct places when detonation occurs. Valves in wrong places + detontation + pistons in wrong places = a big bill in service. If your car REQUIRES a certain octane, please use it. If it RECOMMENDS a certain octane, it is your choice. But using a lower then recommended octane will lower performance, mileage, etc because the anti-knock sensor will retard the timing based on what it is "hearing" from the engine. The anti-knock sensor is simply a tiny microphone that can "hear" knock before you do.

Enough of the Mr. Wizard stuff...

Back to the original question. I think it is not being translated properly. A RON (research octane number) of 92 is generally equal to a MON (motor octane number) of 82. The gas we use in the states is AKI (Anti-Knock Index). AKI = (RON + MON)/2. 92+82 divided by 2 is 87.

 

Long-Term diagnosis reveals a different story.

E20 has been in testing here for awhile now (with very encouraring results) and E10 has been the only fuel available since the 90's. And because there is a mandate in Minnesota for E20 to become the standard, I'll be using it in my Prius... without any concern.

My guess is older vehicles that have never used any ethanol encounter the rare problems we hear about. Ethanol is a fantastic cleanser. Imagine the crud built up over the years from dirty fuels.
.

 

I just took my 07 Lexus GS 350 in for a recall last month. The 10% ethanol was causing corrosion on the fuel rails. That recall concerned 06-07 GS; 07-08 LS and 06-08 IS Lexus. Corrosion of the fuel pipes could result in fire!

Check with Toyota before proceeding.

 

Knock sensor, is a device using piezoelectric effect, it’s also known as KNK. This knock sensor will always make sure your vehicle run close to that detonation point, and to ensure to deliver fuel efficiency and power. When engine detonation occurs, vibration of the cylinder blocks causes the sensor to generate a voltage signal. Normally 1 sensor for 4 cylinder engine and 2 sensor for 6 and 8 cylinder engine. Obviously knock sensor are critical for turbo and supercharged cars.
There are two types of knock sensors used on Toyota engines.
i) The mass type sensor, which produces a voltage output over a wide input frequency range (vibration frequency is about 7KHz). The ECU uses a filter circuit to identify between background noise and actual engine knock.
ii) The resonance sensor, is tuned into a very narrow frequency band and only produces a certain signal voltage when exposed to vibration in the range of 7KHz. The ECU requires less complicated filter curcuit with this type of sensor.
When there’s a detonation taking place, it retards ignition timing until the knocking stop. The the timing advanced back to normal or calculated value. Due to this, ignition system will operates at maximum performance. If the ECU noted that the knock retard is not working it will enter a fail-safe mode and fix the retard angle to prevent engine damage.

According to the service rep at the local BMW dealership, they only used 87 octane gas.
I traded my '04 330 for my '07 Lexus GS 350. I don't know if they are still using the same type of gas.

 

I don't see how the owner could be at fault if the only gas sold in your state is 10% ethanol. It's not like we have a choice here. It's mandated by the State Govt.
So if there is any issue regarding E10 then Toyota and the State would have to figure it out.
I'd buy 100% gas if I could.

 

I was under the impression that the 20% mandate had to be postponed. Fuel pumps throughout the state were only certified for 10% ethanol. But, that was as of a few months ago. Maybe by now this problem has been resolved.

It will be interesting to see what will happen in the long haul.

 

I've worked on performance vehicles both privately and professionally for the last seven years. I can promise you that with the conservative tuning that cars come out of the factory with, one degree of octane is not going to make a difference in longevity. If toyota rates the car for 88, it will be fine with 87. Unless a specific, individual vehicle is retuned (like at a dyno shop) for maximum performance at a specific octane rating, you're safe to use a slightly lower octane than recommended from the factory. After tuning dozens of vehicles, I've never seen one that wasn't conservatively tuned from the factory.

My weekend driver, on the other hand, is tuned specifically to make maximum horsepower at 93 octane on 12 pounds of boost at sea level. If I run one tank of regular in that car, I'm playing with fire. It's two different worlds.

 

Jabber, I knew you could set this fools straight. I did not know that Priuschat had so many scientists and chemists.

One guy tried to say that all fuel pumps are in the gas tank. I had a 1973 porsche and the fuel pump was behind the passenger seat along with all four of my last Audi's. Maybe he is thinking of a sump pump in a well.

 

Ethanol is known for being much harder on gaskets, steel, etc. E85 definitely will cause havoc if used on cars not designed for it.

True. My Audi with a reflash prom HAD to have 93 octane. Not even 91 would be OK.

Actually, most fuel pumps are in the gas tank now. It serves two purposes. 1) the fuel in the tank keeps the pump cool. 2) If there is a problem, you aren't shooting fuel all over a hot engine.

 

Don't you mean sparking later?

 

No, sparking earlier. It's necessary to ignite the air-fuel mixture before it ignites itself due to the compression.

It took me a while to understand that myself.

The problem is when it self-ignites, it doesn't burn nicely with moderate progression as the normal combustion process does. It explodes, producing a shockwave and very hot burn temperatures. This is what causes the damage.

Igniting the fuel early still causes the pressure to oppose the motion of the piston, but the expansion time is much greater.

 

Driving cross-country we noticed that "regular" was 85 octane at high elevations. The octane can be lower because there is less air mass to compress and less mixture to ignite.

That said, if RON is roughly higher than AKI by 5, a minimum RON of 88 would indicate a minimum AKI (pump sticker) of only 83, ergo no problem.

My experience some years ago in a car that recommended premium (Porsche 356C) was that some regulars (Union, Chevron, ARCO) were consistently fine, and others (Shell and "white box" brands) often not. I could hear and feel the pings if the octane was too low. The engine was rebuilt at ca. 115,000 miles after losing oil (pan bolt sheared off), with no apparent signs of damage due to low octane.

 

I would still like to get an official response from the Prius team on the required Octane otherwise we are all guessing.

 

You have it backwards, re-read the OP. Minimum RON = 92.