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jsveb jsveb is offline
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Join Date: May 2007
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Not trying to be a wiseazz, just throwing in some data.

The most important factor when considering performance is for sure temperature.

When we consider performance we consider density altitude, which is a product of pressure and temperature and pressure.

I will use ISA units:
Standard sealevel pressure is 1013.25 hPa (29.92 Inch)
Standard temp is 15C at sea level.

At 18000feet the pressure is down to half that a sea level.

The standard lapse rate (temp drop per increase in altitude) is 2C per 1000 feet.

When we calculate density altitude ( ie to figure out the current environmental effects on performance) we first need to figure out pressure altitude. Pressure altitude changes by approximately 30 feet per hPa change. At high pressure we have increased performance.

To finally figure out density altitude (DA) we need to consider temp. A change of 1C changes the DA by 120 feet.
Though this is not linear. This is close enough.

Rho is the term referred to as density. When relating this to the temp and press effects. And knowing in ISA we have only 50% at 18000 feet, we can begin to figure out the effect on our engines on a given day.

Let's say we have a hot southern summer at 40C then take the car up north to -10C. This change is 50C which is a change of 6000 feet, which is about a conservative increase in performance of about 15%

I only consider temp in this example since it has Gita way bigger impact. Furthermore press changes are not that big, relatively speaking, even in extreme weather, where we don't drive our cars anyways.

It is important to note that a significant temp decrease does not just bring I creased performance, but it also requires us to adjust our fuel, AFR.

Disclaimer. This is posted on iPhone, so no review of the post. I hope it makes sense.
Old 11-06-2011, 12:47 PM
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