why 91 octane in the prius?

There is no benefit in using gasoline with a higher than recommended octane rating. In fact, there is less energy in high octane gasoline, so your mileage may actually go down.

 

I smell "dealer disinformation" here...
.
_H*

 

No need for 91 octane gas and no need for syhthetic oil either. The engine never overheats, produces 76 hp and has a 5000 rpm limit. Keep those numbers in mind when thinking about high octane gas and synthetic oil.

 

<div class='quotetop'>QUOTE(jamarimutt @ Jul 28 2006, 09:03 PM) [snapback]294121[/snapback]</div>

Agrees on the higher octane; using anything higher than the minimum recommended octane draws more cash out of your pocket, not more power out of the engine.
Synthetic oil, however, does maintain its viscosity better/longer than standard oil, especially at temperature extremes....

 

<div class='quotetop'>QUOTE(ZenCruiser @ Jul 28 2006, 09:36 PM) [snapback]294138[/snapback]</div>

Synthetic oil also has better shear strength at the same viscosity as dino oil. It also tends to form less acid byproducts due to heat breakdown.

A good question to ask is if you are using synthetic oil, can you safely drop the viscosity down to 5W20 or 0W20 and get a slight pickup in MPG? The lower viscosity will produce a bit less flow friction, allowing for the oil pump to produce less drag, yet the oil (being synthetic) should have excellent shear strength (the ability to keep a part coated under pressure is related to shear strength, I think). Anybody well-versed on this subject?

 

Who is telling you to use 91 octane, and why?

 

Some people are used to using 91 like having a performance car or maybe an older car that needed 91 in the later years.

Maybe some people think that by putting 91 in you'll have more power. After all, it's for performance cars so in theory your car will perform better.

Consumer Reports even suggested seeing how lowering the octane would do on your car. If it didn't knock then use the lower rating and save money.

 

I am about two thirds through a 5000+ mile trip from Florida to OH, IL, Co and back. I usually use 87 gas, but put in gas in IA that must have been 10% ethanol. My mpg went from 48 ave down to 45. Then on the return trip I stopped at a station that had 89 oct for 6 cents cheaper than 87, so I tried it. It was also 10% ethanol. THe mpg went down to 44. I was happy with the 48 mpg for Interstate driving but not 44 or 45. So stay away from any ethanol added gas if possible. Also, in CO reg gas is only 85 oct. I used it with no noticable effects, including mpg.

 

I have noticed the following in my car.

First few tanks were 87 and I averaged about 44 MPG

Then I switched to 91 and it averaged about 48

I had a few tanks of 91 and it was about 43

Last couple of tanks of 91 and I have been about 51 and this was during the 110 degree heat we had in So cal over the past couple weeks.

with no change in driving habits

All gas was from the same shell station


Go figure

 

<_< Howdy Heir Butt

Sounds like your Prius is virtually NEW, in which case, the improved MPGs can well be from your Prius "loosening up." Most Prius owners have posted an increase in MPGs as the miles on the odometer accumulate. In my case, the better MPGs came after 7,000 to 9,000 original miles.

Insofar as 91 octane, I use what Toyota recommends, 87. In my '85 Jaguar, I also used 87 with no problems for the 120,000 miles I drove it before it went to a new home.

 

<div class='quotetop'>QUOTE(pete bogumill @ Jul 28 2006, 08:22 PM) [snapback]294072[/snapback]</div>

I remembered reading somewhere that the compression ratio was 13:1 so my first fill up I used Exxon 93 octane. Not only was it a waste of money but the ICE shook terribly every time it powered off. : :blink: Almost like the old days with post ignition (run on). No foolin' ! The next tank I went to 89 and it went away.

 

The interesting thing about the mid octane gas is that there is no car manufactured which requires it. Most cars run fine on 87 octane, and a few require 91. The 89 octane gas is just there to make the oil company more money by appealing to the "more expensive must be better" thinking.

 

<div class='quotetop'>QUOTE(BobZ @ Jul 29 2006, 01:38 PM) [snapback]294370[/snapback]</div>

It is true that the mechanical compression ratio is 13:1, but, due to the miller cycle valve timing, the effective compression ratio is less.

 

<div class='quotetop'>QUOTE(Heir Butt @ Jul 29 2006, 07:35 AM) [snapback]294279[/snapback]</div>

Since you never switched back to 87 to exclude engine break-in, your conclusion is incomplete.

FAQ.

Same as all tests which were not tested for the negative to exclude masking factors.

 

<div class='quotetop'>QUOTE(FL Buckeye @ Jul 29 2006, 09:50 AM) [snapback]294270[/snapback]</div>

Pure grain alcohol has an octane rating of 106, so if you mix alcohol with gasoline, you're going to raise the octane rating of the gasoline. The ethanol station near where I live (Ottawa, Canada) sells regular gas at 87 octane, and its E-10 is 89 octane (and is slightly more expensive).
The octane rating is a measure of the pressure at which the fuel spontaneously explodes ("knocks"). If you run an engine at a higher octane, the engine needs more oxygen to combust as efficiently, so unless the Prius can add additional air (e.g. turbocharging), then going for a higher octane will result in less power. I understand the new Prius being designed will likely be turbocharged, but at present it is not. So using octane higher than 87 is not going to improve your mileage. The Prius' computer can adjust the spark timing to the higher octane to prevent knocking, but that doesn't mean you'll get any better mileage.

 

I use 87 octane in Chicago. When I go to Rocky Mtn. Nat'l Park, do I use the 85 octane or a higher octane?

 

<div class='quotetop'>QUOTE(Prius Maximus @ Jul 29 2006, 09:33 PM) [snapback]294470[/snapback]</div>

Use 85 octane; at the higher altitudes here in Colorado it is the same as 87 octane at lower altitudes in terms of knock-prevention (which is what octane is designed to measure).

Have fun in Rock Mtn. National Park - we live in Fort Collins and visit several times a year.

 

If the engine does not ping or knock you have enough octane for your application. Higher octane produces a bigger "bang" in the cylinders but it requires more compression and more spark to make it happen. The Prius does not have enough "oomph" to make use of higher octane fuels. Too much octane will reduce the performance of the vehicle and make it more difficult to start in cold weather.

My Camaro, which I race is designed for 93 octane pump gas. At the track I achieve the best results when mixing pump 93 with enough 103 unleaded to obtain an octane rating of 95... On eht street 93 is fine and perhaps I could get away with 91, but i have not tried it.

In regards to synthetic oil, it is simply more slippery than conventional oil and holds its prperties better in both hot and cold conditions making it easier on the engine starting in cold weather and less likely to break down in hot weather. The additional slipperyness of synthetic is generally good for a couple of extra horsepower as well.

Although the engine will run fine on the proper weight conventional oil, properly changed, it will run better on synthetic. The only question si whether the vehicle owner cares enough about the couple of hp to justify the additional cost.