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Old 02-03-2008, 11:30 PM
 
Location: Ken Caryl
49 posts, read 133,920 times
Reputation: 44
Default 85 Octane Gas

Please explain why Denver is the only city I have visited that sells 85 Octane gas.

Is it that because of the altitude or some other reason that maybe it's OK to use 85 Octane in cars that have a minimum 87 Octane rating?
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Old 02-03-2008, 11:37 PM
 
11,728 posts, read 23,683,519 times
Reputation: 7093
Yes, its the altitude. Lower air pressure acts like a natural anti-knock agent. Running 85 in a car calling for 87 is fine as long as you stay at high elevation. I believe 85 octane is found in other mountain states too.
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Old 02-04-2008, 08:43 AM
 
1,176 posts, read 2,878,659 times
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Less air requires less fuel.
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Old 02-04-2008, 10:01 AM
 
Location: Las Flores, Orange County, CA
26,362 posts, read 53,431,728 times
Reputation: 16290
Quote:
Originally Posted by steveindenver View Post
Less air requires less fuel.


Doesn't 85 octane mean less energy per volume of fuel? That is, aren't you using less fuel (you'd use just as much 89 or 91 by volume), and getting less energy per volume of fuel AND you are paying less for that volume?
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Old 02-04-2008, 10:58 AM
 
1,176 posts, read 2,878,659 times
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Yes, the reason you can run lower octane is, in general terms answered in post one. The lower absolute pressure of the cylinder results in less heat when compressed and thus the ability to run with lower octane without predetonation.

The point I made, and not well is that another side effect of having less oxygen up here is that we burn less fuel. In theory your fuel consumption should be better at altitude, although you are also making less horsepower. This is of course assuming there is no manner of forced induction.
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Old 02-04-2008, 11:15 AM
 
Location: South Central PA
1,561 posts, read 2,817,679 times
Reputation: 336
Quote:
Originally Posted by steveindenver View Post
Yes, the reason you can run lower octane is, in general terms answered in post one. The lower absolute pressure of the cylinder results in less heat when compressed and thus the ability to run with lower octane without predetonation.

The point I made, and not well is that another side effect of having less oxygen up here is that we burn less fuel. In theory your fuel consumption should be better at altitude, although you are also making less horsepower. This is of course assuming there is no manner of forced induction.
Solve the US's dependence on fossil fuels! Elevate the entire US to 5000+ feet!



Ok.. i'll go back to the PA forums now lol.
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Old 02-04-2008, 02:46 PM
 
Location: Aurora
357 posts, read 798,583 times
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okay, i have a question. My honda crv (2005) is not happy in cold weather on 85. I bought it in CA and live in HR, just driving around here doing mommy stuff. I've started putting higher octane in it just on the really cold winter days. any explanation?

please help me, I'm a girl .
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Old 02-04-2008, 03:00 PM
 
Location: Las Flores, Orange County, CA
26,362 posts, read 53,431,728 times
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Quote:
Originally Posted by LisaCACO View Post
okay, i have a question. My honda crv (2005) is not happy in cold weather on 85. I bought it in CA and live in HR, just driving around here doing mommy stuff. I've started putting higher octane in it just on the really cold winter days. any explanation?

please help me, I'm a girl .
Did the higher octane help? (You don't write that it did.)
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Old 02-04-2008, 03:06 PM
 
5,748 posts, read 7,013,820 times
Reputation: 4361
Quote:
Originally Posted by LisaCACO View Post
okay, i have a question. My honda crv (2005) is not happy in cold weather on 85. I bought it in CA and live in HR, just driving around here doing mommy stuff. I've started putting higher octane in it just on the really cold winter days. any explanation?

please help me, I'm a girl .
Lisa, I have no idea why a lower-grade works in our Acura (Honda), just that it does. We have successfully switched from high-grade (91) to mid-grade (87) without any knocking or other problems. Maybe a trip to the Honda Service Department is in order?
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Old 02-04-2008, 04:08 PM
 
1,176 posts, read 2,878,659 times
Reputation: 445
Quote:
okay, i have a question. My honda crv (2005) is not happy in cold weather on 85.
Colder air is more dense. In theory under most circumstances as the temperature drops you are making more power; a consequence of running lower octane in your vehicle under these circumstances is that you can get some pre-detonation (the "bang" of the piston which should occur as it reaches it's highest point of compression occuring too early), often noticed as running rough.

There are a myriad of factors that determine octane requirements. These include:

humidity (absolute), temperature, air/fuel ratio, atmospheric pressure, timing (spark) head and cylinder coolant temperature and a few more I'm leaving out.

All modern vehicles have knock sensors and will generally pull timing to stop pre-detonation. This generally results in a very sluggish vehicle.

You might notice the car runs smoother once it warms up and the intake is heating the cold air enough so as to reduce density. The safest bet is to simply run a higher octane in cold weather situations.

Anyhow, keep in mind when you are driving around on hot summer days in Denver and the density altitude is approaching 9,000 feet that you are losing close to 30 percent of your horsepower at sea level.

hp loss = elevation x 0.03 x hp @ sea level / 1000 (rough equation)
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