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Old 11-25-2001, 07:37 PM   #3
Richard L.
Scooby Specialist
Member#: 8465
Join Date: Jul 2001
Chapter/Region: NWIC
Location: Puget Sound, WA
'00 2.5RS Coupe, AW
'93 GMC Typhoon AWD


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

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].
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