How long does it take the computer to adjust to octane change?

<div class='quotetop'>QUOTE(Les Gas @ Apr 13 2007, 12:09 PM) [snapback]422873[/snapback]</div>

If the engine has a knock sensor, then it can adjust to a low octane fuel by retarding the timing to prevent knock. However, I'm not aware whether the Prius actually has a knock sensor (and I'm thinking it doesn't which I will explain below). My GF has an Avalon which usually requires Premium gas but can adjust to lower octane with slightly lower performance as a result.

The reason that I don't think the Prius has a knock sensor is because it is designed to run on 87 octane fuel (low octane) so it isn't really necessary to adjust like the Avalon does. In fact, there is no benefit to using higher octane gas in the Prius because it doesn't require it. In some vehicles (like my Ford Explorer) it is actually detrimental to use higher octane fuel because it can result in more carbon deposits in the engine because the fuel burns more slowly than the engine was designed for.

To sum up, no benefit from using higher octane because the Prius was designed to use the lower octane fuel. In particular, the Prius uses the Atkinson cycle, which makes pre-detonation unlikely. Doubtful (but possible) that the computer can do any adjustment to make it run better on the higher octane stuff.

Octane is not a measure of quality; it is a measure of resistance to detonation - another way of stating this is that it is a measure of how quickly the fuel can burn.

 

Don't use mid grade. The Prius is specifically designed for the lower octane fuel, and as such, there is no advantage and some disadvantages to using higher octane fuels. As you burn off the mid grade fuel, top off the tank with the normal stuff and things will gradually get better.

Tom

 

midgrade is just 50 % regular and 50 % superunleaded mixed
gas stations typically have 2 tanks 1 for regular and 1 for super
and the gas pump draws both

 

<div class='quotetop'>QUOTE(qbee42 @ Apr 13 2007, 09:32 PM) [snapback]422890[/snapback]</div>

This can be not exactly true. My manual says one should use 95 octane or higher, otherwise the engine can be broken B)

- Piotr

 

<div class='quotetop'>QUOTE(bulek @ Apr 13 2007, 12:51 PM) [snapback]422912[/snapback]</div>

The US spec model uses 87 Octane fuel. I think that the Octane number may be calculated differently in Europe than it is in the US.

 

<div class='quotetop'>QUOTE(dogfriend @ Apr 13 2007, 09:59 PM) [snapback]422916[/snapback]</div>

You are right... I did not realize this before. It seems US 91 is more or less EU 95.

- Piotr

 

<div class='quotetop'>QUOTE(Les Gas @ Apr 13 2007, 01:25 PM) [snapback]422933[/snapback]</div>

I seem to recall that the gas companies sometimes use Ethanol to raise the octane rating so its possible that you did get more Ethanol.

 

<div class='quotetop'>QUOTE(dogfriend @ Apr 13 2007, 12:59 PM) [snapback]422916[/snapback]</div>

Correct.

Wikipedia® - Octane Rating

 

the Prius has a knock sensor it's located just below the intake and just above the oil filter mount.

 

We have now 2900 miles and our prius likes the 91 octane here it is called super unleaded

the 87 oct and 91 cost the same but we get more pep and power and mpg from the 91 oct.

I believe there is just a plain difference in where gas is bought and what brand. We have tried the walmart (murphy oil) and get terrible mileage. I will try shell 87 octane today it's that time to fill up .
I think the owners book says MINIUMUM 87 octane I dont recall it stating ONLY 87 I will check that today also

 

Any more octane than what is required to keep the engine from pinging is a waste of money, will produce less power, and be harder to start in cold weather. Many newer cars will run on the lowest octane just fine as long as you don't do jackrabbit starts. Higher octane makes a bigger bang when fired. It also requires more spark.

I find it far more likely you received a load of bad gas than the octane level changing the driving properties of your Prius in a seat of tha pants quantifiable manner.

 

The Higher octane is required in my Pontiac GTP with a supercharger because of the higher pressure in the combustion chamber. The higher pressure will cause the 87 octane to pre-detonate and cause a dieseling sound heard form the engine. The higher-octane burns slower and giving a more even presser release at the higher cylinder pressures. I I did use to years ago in my four cylinder Toyota 4X4 truck when I was four wheeling out at NC beach. The higher stress loads and heat caused the lower octane to pre-detonate and diesel.
bottom line; the premium spent on high octane (.20 to .50 cent per gallon) is not worth the extra MPG if any is seen in a Prius, so save your money and get the 87 octane.
I’m not a mechanic but I slept in a holiday in last night.

 

More ethanol in the gas will not make the car "sluggish" as you describe. You will get slightly lower mileage with more ethanol, but the engine will stay cleaner as higher ethanol fuels burn cleaner. The engine is often easier to start as well, in cooler climates, as the ethanol vaporizes easier.

If the Prius does have a knock sensor, it should take 4-8 start-stop cycles for the system to reset to the new fuel.

I wouldn't have expected the car to notice the small difference between regular and mid-grade (two octane points - 87 vs 89). Are you sure it's real, or is it perhaps imaginary? If real, it could just be a "bad tank of gas". It happens. I would expect a slight drop in mileage. Perhaps 2-5%.

 

You could have gotten some gas with water in it. I had that happen on my "other car" after running it almost empty and then filling it full completely. I noticed it almost immediately with a loss of power on acceleration and sputtering. I called my brother who is a mechanic, and he said to wait until morning; there is often an additive in gas that will absorb water, or the water may "sink" in the tank below the fuel pump. He thought that rather than "bad gas", I may have accumulated condensation in the tank that I mixed into the gasoline when I filled it up completely.

I don't know if that scenario is possible with the Prius and its gas tank bladder, but its another reason not to overfill any car.

 

Water in the gas, unfortunatly, is a problem with ethanol, even the E5 or E10. The ethyl alcohol will absorb water from the air, and then support certain bacteria, which make a living converting iron to iron oxide. This eventually punches a hole through the tank, allowing groundwater into the underground fuel tank. Then everyone who fills up gets a nice tank of hydragas. The real downside is it includes rust dust (mud), which makes things not work properly anymore. Ask me how I know.

The other side of this is, if you do have some water in YOUR fuel tank, the ethyl alcohol in ethanol will absorb it, allowing the engine to cleanly burn it. So it can -remove- water from your fuel tank.

 

Higher octane gas also has less energy than lower octane. Thus with higher octane gas you'll experience a slight drop in mpg.

 

<div class='quotetop'>QUOTE(daniel @ Apr 15 2007, 07:06 PM) [snapback]423766[/snapback]</div>

Yes! The octane is there to keep the fuel from burning; any extra octane actually inhibits combustion.