Best octane?

Yes, it could flag an emissions error as it tries to adjust for the higher octane.

Octane is the ability of the fuel to withstand higher compressions without detonating spontaneously. Higher performance engines are often designed with higher comression ratios to obtain that performance and so needs a higher octane fuel that can tolerate it. There is actually a bit LESS energy in higher octane fuel because of the octane boosting additive.

Only if you hear excessive pinging do you try the next level octane. If pinging stops and MPG/performance improves, use that octane. Most cars actually don't need it though.

Prius was designed for 87, so save even more money and use that. If you want cleaning power of a fuel, use a Top Tier fuel, of which Shell and Chevron are most common.

 

I just learned that octane ratings above 87 can mess with your starting system.

Wow. How about that. You really HAVE to use the cheap gas. Most cars are the other way around, you HAVE to use the expensive stuff.

 

Oddly enough, sometimes at some of the gas stations around here the 89 octane is actually a few cents cheaper than the 87. My best guess is that ethanol is cheaper than gasoline so the E10 blend winds up cheaper.

 

ethanol is much cheaper than gasoline.

however, it doesn't seem that way when i go out to the bars here in town....

 

hmmm we have E10 in WA and it AINT CHEAP here

 

At least here in Manitoba, the ethanol blend is a good 3.5 cents a litre cheaper. That's about 13 cents a U.S. gallon cheaper.

I gas up at Mohawk, which is part of the Husk Oil group. Their lowest grade blend is 90 octane, and they offer 94 octane as a premium fuel.

When I used to tow with my 2000 GMC Sierra, I found that using the 94 octane cut out all the pinging and the truck felt much stronger, with around 1.5 MPG better towing economy.

The Vortec 5.3 uses "stereo" knock sensors, there are two of them under the composite intake manifold. Under heavy loads my local dealer recommended Premium, apparently the PCM will compensate for load and octane.

Otherwise it makes no sense to run a higher octane fuel. With a car like the Prius use a quality mass-market fuel. At least in the United States you don't have to worry about MMT buggering up the O2 sensors and catalytic converter.

 

If the octane is higher because the gas is E10, you can use it in the Prius. Ethanol inherently has a much higher octane.

 

hmmm well its says its 87 octane...and i would have to think that ethanol would lower the octane rating rather than raise it. the smaller the chain the lower the rating as i understand it.

 

From what I have read, Ethanol has an octane of 113.

Although octane gets its name from the chemical formula, its practical definition is how well it resists spontaneous detonation at high pressures.

See http://www.gov.mb.ca/est/energy/ethanol/et...ethanolfaq.html

 

must be the effect of the "ol" verses the "e" that accounts for the difference in octane rating.

 

part of me is an organic chemist. "octane" means something entirely different to me. can anyone explain these ratings a little more in depth? thanks

 

octane is the rating that is determined by the ratio of heptane to octane in the gas you burn.

heptane is easily compressible which causes an increase in temperature and pressure and lowers its flash point.

octane is not. so the higher the octane rating, the higher the ratio of octane verses heptane in the gas.

higher octane DOES NOT have more power nor does it increase gas mileage in cars that dont require it.

higher octane gas is used in cars that have higher compression ratios and are prone to "ping". ping is when the gas/air mixture ignites too early because of the heat in the cylinder combined with the increasing pressure of the mixture causes a partial detonation of the mixture before the spark plug ignites.

this causes a loss of power since the full benefit of the expansion due to explosion is lost when it happens at the wrong time. (actually having it happen too early pushes against the momentum of the engine)

increasing the octane rating increases the flash point or temperature at which spontaneous combustion happen OR iow, it forces the spark plug to ignite the fuel. in a car that does not have a ping problem like the Prius, it would be just a waste of money as it would provide absolutely no benefit at all. this has been tested at altitudes up to ...well dont know...

 

thanks dave. that makes more sense now.

 

Dave, good explanation on octane as it relates to automotive.

One thing about high ethanol content is that cold weather performance can suffer, especially cold starts. I'm not sure how folks could start their E85 cars on a -40 day unless they were plugged in.

Typically a high ethanol content will cause starting problems much below -20 C.

 

i lived in Alaska several years ago and cars of ANY kind had problems starting when it was that cold...of course a lot had to do with batteries not keeping a charge and oil being a bit thicker than normal

 

Dave:

Actually, E85 vehicles are very difficult to start in cold temps. GM has maintained that in temps colder than +20 F or around -10 C, an E85 motor will not start unless "assisted."

Please review the following links:

http://www.osti.gov/energycitations/produc...?osti_id=659061

http://www.pugetsoundcleancities.org/Ethanol.htm

http://www.westbioenergy.org/market.htm

For the last link: Kind of scary that at 0 F or -18 C, 13 of 16 test trucks on E85 refused to start after 40 secs of cranking.

The factory E85 vehicles usually carry warnings about this.

Jay

 

hmm maybe they should install glow plugs. when in Alaska our neighbor had a mercedes diesel. it would start without too much of a problem on cold days (ALL vehicles were plugged in there) but it had to idle for what seemed like hours before he could drive it.

 

hmm maybe they should install glow plugs. when in Alaska our neighbor had a mercedes diesel. it would start without too much of a problem on cold days (ALL vehicles were plugged in there) but it had to idle for what seemed like hours before he could drive it.

another thing about the use of high octane gas. several years ago i had a "quasi-legal" access to jet fuel. it was at 104 octane and we all decided that it would be the ideal solution for my car which was a 1969 Camaro that had been tricked out.

well, the car ran like total crap on it. sooo, that idea bombed. cost me a set of plugs, points, etc because the gas fouled up everything. might be ok for cruising at 800 mph in a jet but it sucked in a car.