Cold weather octane rating?

[doobie]

Does anyone subscribe to this methodolgy:

Hot weather = high octane to prevent retardation of firing (anti-knock)

Cold weather = lower octane engine less likely to knock due to low temps.

From what I'm told you can switch to either 89 or 87 in cooler weather and not worry about loss of engine performance. Wy less than scientific tests seem to back this info up.

Any input? Thanks!

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[HndaTch627]

WRONG!! they do not change the octane they merely add ethanol(grain alcohol) to the gas to make it want to burn. You don't lose any power, but it'll run like crap if it's warm out. You'll also notice worse gas mileage if the temp is about 50 degress.

Jeremy

[Richard L.]

Originally posted by Doobie:
From what I'm told you can switch to either 89 or 87 in cooler weather and not worry about loss of engine performance.

Yes, in theory, you can switch to a lower octane grade in cooler weather and not worry about loss of engine performance. You might want to check out this link for more info. Here are some paragraphs taken from the aforementioned link:

7.6 What is the effect of temperature and load?

Increasing the engine temperature, particularly the air-fuel charge
temperature, increases the tendency to knock. The Sensitivity of a fuel can
indicate how it is affected by charge temperature variations. Increasing
load increases both the engine temperature, and the end-gas pressure, thus
the likelihood of knock increases as load increases. Increasing the water
jacket temperature from 71C to 82C, increases the (R+M)/2 ONR by two [111].

7.10 What is the effect of air temperature?

An increase in ambient air temperature of 5.6C increases the octane
requirement of an engine by 0.44 - 0.54 MON [27,38]. When the combined effects
of air temperature and humidity are considered, it is often possible to use
one octane grade in summer, and use a lower octane rating in winter. The
Motor octane rating has a higher charge temperature, and increasing charge
temperature increases the tendency to knock, so fuels with low Sensitivity
( the difference between RON and MON numbers ) are less affected by air
temperature.

7.12 What is the effect of humidity?.

An increase of absolute humidity of 1.0 g water/kg of dry air lowers the
octane requirement of an engine by 0.25 - 0.32 MON [27,28,38].

--
Richard
http://www.whitetyphoon.com/impreza/

[doobie]

Thanks for the link, Rich. Some pretty informative stuff. I figured there was more to it than me simply being, "WRONG!"

Steve

[cvalle-sd]

The weather here is pretty consistent, and rarely changes quickly.

If the peak temps during the day are 80 or higher, I'll go with at least 89. When I had my ITC installed (pre-stolen, I guess) I went 89 in cool weather, 92 (NOW 91) When it was hot.

Now, I'll go 87 most of the time, and 89 if it's in the 80s or higher.

[Richard L.]

Steve, you are welcome. Unlike some other folks on this list, I always try to be helpful without being rude or offensive.

--
Richard