Weather and Performance

[Guest prescience]

We all (probably) know that lower temperatures mean more power because the air is more dense.

 

Here is a calculator which enables this to be quantified

http://wahiduddin.net/calc/calc_hp_dp_metric.htm

 

It comes in various forms as can be seen but the above is a convenient one since the required data can be easily sourced from here

http://weather.noaa.gov/weather/GB_cc.html

for example

http://weather.noaa.gov/weather/current/EGLL.html

 

It is interesting to note that the variation between a 90F day and a 40F day (with same air pressure) equates to 5 or 6% which represents SIRO 17BHp variation for a stock motor.

 

Do avoid the common mistake that more moist air is also more dense, it is NOT, it is less dense (cf increase the dew point and the air density drops) water molecules (as a gas) 'weigh' less than either oxygen or nitrogen.

[Removed]

I knew it made some difference, but that is quite a lot!

[lomoto]

I can remember when I was setting up TZ Yamaha race bikes you could go up 20-30 sizes on main jets ,just due to ambient temp.. amazing difference also on damp days it altered the settings required

[Guest prescience]

Another thing is with regard to the Cooling Water Injection that say high power turbo cars employ - primarily to lessen the tendency for engines to knock.

 

However, on the inlet side this would seem

a) to work against you because the water-filled air will be less dense, hence less power

to work for you because the charge temperature will be lower.

 

The equations are pretty insensitive to the water content (cf modifying the dew point) and rather more sensitive to charge temperature 'suggesting' that it works more for you than against you (on the inlet side)

 

[As I say I know this is not the 'primary' purpose of water injection which is described here where my point above is noted as a lower-order effect http://en.wikipedia.org/wiki/Water_inje ... engines%29 ]