Weather, Humidity, Air Pressure And Engine Power For Rolling Road Days

[spect]

There was some argument about some cars underperforming at our recent rolling road day and Martyn tried to explain about it. Not sure if everyone understood why their cars may have been down on power that day. Mine was mapped at 27 degrees Celcius but the rolling road day was 13 degrees and I lost just over 13 bhp. I thought an explanation might help some of you undersatnd this so I researched it on the net and found the following: So some of you may get even better readings on a warmer day with different humidity and air pressure. Our temp was around 13 - 14 degrees and the pressure was 1014mb sorry but cannot remember the humidity. (29.92" of mercury is 1013mb if my memory serves me right)

WEATHER EFFECTS:

First of all, figure 60deg F, zero humidity, and a 29.92" barometer as 100% air density - AKA the "Standard Day”

Now that we have the Standard Day figures, we can get to how each of the three measurements (temperature, humidity and barometer) affect air density. Actually, there is a moderately involved formula for calculating air density, but I'll skip that and get to how you can closely estimate pluses and minuses.

1. Barometer readings have a linear effect on air density. That is, if the barometer goes up or down 1% from the standard reading, so will air density. Therefore, a barometer of 30.22" gets you to 101% air density, 29.62" nets to 99%, and so on.

2. For each 5o change in temperature (away from 60o), you get about a 1% change in air density. Therefore, an 80o day means 96% air density, and a 40o day means 104% air density. Note: This is *not* linear, since the calculation has a base of absolute zero, but it's close enough so we don't have to worry about it.

3. At 60o, jumping from zero to 50% humidity will cost you roughly 1% in air density. However, that same 50% humidity at 90o will cost you more than 2% in air density. The reason for this is that it requires a *lot* more water vapor to get you to 50% humidity at 90o than it does at 60o, since air can hold more water as it's heated - like sugar in your coffee or tea. Hot tea can hold a *bunch* of sugar, but iced tea can't hold too much. "Relative humidity" is expressed as a percentage of water in the air compared to how much it *could* hold at a given temperature.

All 3 figures need to be juggled. As an example, a 50o day with 50% humidity and a 29.62" barometer nets you to just about 100% air density, since the 2% you pick up in temperature is offset by the losses you incur for the barometer drop of 1% and the humidity loss of 1%.

Now, how does air density affect power?

This is also something that is moderately involved, but figure around 75-80% power delta compared to the air density delta. That is, an 8% drop in air density will likely cost you somewhere around 6% in power.

Actually, barometer changes give you something right around a 1.2-1% swing in power, temperature changes give you something right around a .7-1% swing, and humidity is near 1-1%. In practice, temperature is likely to be the biggest variable, hence the 75-80% "rule of thumb".

In general, temperature has twice the effect on the car than humidity does. If the humidity goes up 2 points, it affects the car the same as if the temperature goes up one degree. And vice-versa if the air temp goes down. This is a rule of thumb that holds fairly well for all cars.

Anyone still awake? I hope that helps some of you understand what you saw on the day!!

[beefoss]

Thanks for that night time reading Terry. It was like my air conditioning course all over again.

Now go and put your slippers on, have a cup of Horlicks and read up on mapping cars for cheap haha. Well done though.

[Tractor Trev]

Nicley put Terry, what on earth does it mean lol anyway it did the opposite for me as i increased by 15 bhp! And no you cant have it back lol

[spect]

Increased by 15 bhp from your last rolling road? What was the temperature, pressure and humidity at that one? Or are you say increased 15bhp from satndard? If so you could actually have another 15 bhp on that if it had been checked on the day mine was remapped

[Tractor Trev]

That was my first rolling road, my car should have been 227 standard but for some reason it increased by 15 to 242 :-)

[spect]

So on a nice day it could be nearly 260 then!! Maybe it has already been fiddled with.

[Tractor Trev]

Might have been, but it still sounds like a bloody smart car! I think i need to cancel xmas lol

[spect]

Get a nice new back box. It will make a big difference and not cost too much. I have a ninja 2 from scoobyworld