What grade of fuel works best for you?

toyota says to only use regular. there are a millions other discussions here on the forums about higher grade fuels. what it comes down to is that you're wasting money and may have problems down the line. high octane fuels can cause starting problems in the prius.

so in all, i wouldn't recommend it... i use the lowest octane rating and get 48-54 mpg in not-optimal driving conditions.

 

I've used all three grades of Shell. If anything the Premium had the best mpg, but I think all 3 are about the same. I definitely saw no drop with premium, and no error codes.

I only bought the non-regular because I had a coupon for it one time, and the other time I got mid grade because they were out of regular.

 

unleaded regular gets me an average of 57 mpg in the summer... good enough for me

 

Oh well I will be the bad guy, as I am a chemistry major and am supposed to understand this hydrocarbon stuff. Octane ratings are not about the amount of energy in each gallon, it is about how easily the gas ignites prematurely with compression. High octane gas has less energy per gallon.

 

My owners manual says to use a minimum of 87 octane. I use mid grade as the engine has a 13 to 1 compression ratio and toyota warns of prolonged detonation. I live were the temperature is high and with hills all around, and have seen the results of preignition and or detonation. And with the mileage I get I feel midgrade or even premium is still cheap.

hdrygras you are right in that less octane gives more instantaneous power, however, higher octane can give the same amount of power, although it does take a longer cycle. That is why race engines searching for horse power use such advanced timing figures and high comression ratios. I am sure you know there is more to it than what I have put down such as chamber design, squish band, flame fronts, dual ignition systems, etc.

 

Opps I pushed the mouse twice.

 

Yeah, but the Atkinson valve timing winds up giving it an
*effective* compression ratio of more like 6:1, and an expansion
of the 13:1 or whatever. That's the whole point of Atkinson/Miller.
You shouldn't have to worry about detonation at all.
.
_H*

 

OK but why does Toyota warn about detonation? This warning would tell me that Toyota has had some experience with that problem as they specifically state to avoid prolonged detonation.

 

IMO I think you are wasting your money, but if it makes you happy go for it.

The gas will not give you the boost in power or fuel economy unless the engine is designed to take advantage of the higher compression. Performance cars specifically have their engines tuned to this. Toyota has the Prius engine tuned to 87 octane.

If you are getting better mileage you will not be able to determine that from one tank. You will need to test the theory over multiple tanks and under the same conditions. You would also need to empty the tank to ensure you are solely using that specific gas. This generally cannot be done outside a controlled environment. Heck, I had two tanks of exactly the same commute no other side trips, etc and still got a 8 mpg difference (same gas type and rating). Too many possible variables for error (heck the gas meter and odometers are note exactly accurate either.

Also power and fuel economy contradict each other/are trade offs. If you want more power you will sacrifice in fuel economy and vice versa. Otherwise you would see more Ford Mustangs with 60 mpg fuel economy ratings.

This is a quote from the Federal Trade Commission on octane ratings: in most cases, using a higher octane gasoline than your owner's manual recommends offers absolutely no benefit. It won't make your car perform better, go faster, get better mileage or run cleaner. Your best bet: listen to your owner's manual.

Several other webs sites (chemistry, environmental, and auto sites) state that using a higher octane that recommended can cause extra stress on emmissions systems. It may result in a rotten egg smell from the exhaust. They even suggest for high performance cars to use low octane gas as long as it does not knock to extend the life of the engine, reduce emmisions, and improve fuel economy.

 

There are a ton of threads on here, and the other Prius sites about this already

Bottom line is:

The Prius Atkinson Cycle Engine is tuned specifically for 87 Octane (or 85 in high elevations).

Higher Octane fuel will not help your mileage. It actually contains less energy than Regular, and is more likely to decrease your mileage.

It also burns slower, and can result in un-burnt fuel getting out the exhaust, which will at best pollute more, and at worst shorten the life of emissions components. Long term use can also cause carbon buildup on valves and such.

Because it's not burning as fast, or as completely, you will in fact have LESS power available too.

Contrary to what you might intuit, Lower Octane fuel actually burns hotter than High Octane and provides better functioning of the Catalytic Converters. They stay hotter longer without then need to run the engine as often to keep them up to temp.

About the only possible argument for "Premium" fuel is if your provider puts a lesser additive pack in their lower octanes to entice you to pay more. That's the purpose of the 'top tier' gas program, to convince fuel manufacturers to put adequate detergents in all grades of gas.

Toyota is very clear on using only Regular. Consistent High Octane use can result in an "Check Engine" warning light coming on with an "Emissions System" warning code because of the unburnt hydrocarbons.


One tank won't hurt you, but it's not something I'd do on a regular basis.

 

may i see the source where you got the figure 6:1 effective ratio? i have always believed that it was an effective ratio of 10:1

 

I always use low grade like Toyota states. The dealer told me they filled her up with high octane and I was like "why" but figured they were just telling me that to make me like them. It was right when the Katrina hikes went into effect so who knows. I do know my first was my worst MPG.

 

Where do you see that? Yes, if you have excessive pinging, try the next higher grade to prevent pinging. Pinging is caused by premature detonation where the fuel mixture spontaneously combusts during compression, before spark is applied. Are you getting excessive pinging? If not, then a higher grade is not necessary.

The Atkinson/miller engine reduces the effective compression stroke by allowing some fuel/air mixture to escape through the intake valve at the early part of the compression stroke. The objective is to effectively extend the power stroke in relation to the effective compression stroke.

 

Someone also posted once that the Prius Tech Manual or Hybrid Tech Training course has a strong warning against using High Octane fuel, because it could cause "Failure To Start" errors on the ICE.

BUT, I have never head that confirmed, so I don't quote it as gospel.

Does anyone have a Prius Tech Manual they can check for this? Would be interesting to get a scan of that warning if it exists.

 

Can we please create a PriusChat FAQ and put it somewhere where we can just fill it with info, like Tempus's with no divergent threading, for the common recurring questions, or is that what the Knowledge Base is for?

 

Yes, and I was the one who posted it.

Lemme see what I can do about a scan... might take me a while, i warn you.

 

I have used all 3 grades of Shell gas here in Canada. I have had no significant change in mpg or Km/L. However, when I visited my aunt in New York, I filled up on Chevron and DID notice a much better mpg or Km/L. I had mentioned this to a buddy who came back from Florida and he also noticed a much better mpg in his Dodge Ram. He stated that he was getting 22 mpg on the highway. I will have to visit my aunt and the wonderful U.S.A. more often.

 

http://www.chevron.com/products/prodserv/f...letin/motorgas/
“The power an engine develops depends on its design. In general, the more air an engine can process, the more power it can produce. Major design considerations for power are the displacement of the engine, the compression ratio, and the presence of a supercharger or turbocharger. Other factors affecting power are the number of valves per cylinder, valve timing, and spark timing. Because different grades of gasoline have essentially the same heating value, they all provide the same power in a given engine as long as their antiknock performance meets the engine’s requirement.â€

Per the 2005 Toyota Prius Owner’s Manual, page 204, Octane Rating: “Select unleaded gasoline with an Octane Rating of 87 (Research Octane Number 91) or higher

MMT was commercialized in 1959 and was used in gasoline alone or in combination with the lead alkyls. The Clean Air Act Amendments of 1977 banned the use of manganese antiknock additives in unleaded gasoline unless the EPA granted a waiver. MMT continued to be extensively used in unleaded gasoline in Canada at concentrations up to 0.068 g/gal. (18 mg/L). In 1996, after several waiver requests and court actions by the manufacturer, the courts ordered the EPA to grant a waiver for MMT. Its use is limited to a maximum of 0.031 g/gal. (8.2 mg/L). California regulations continue to ban the addition of manganese to gasoline.