Humidity

[Michael T. Kasimirsky]

On Tue, 23 Sep 1997, Tom Cloud wrote:

> I really didn't want to get into this postulation session, but
> I'll bet any motorcycle rider out there will tell you that you
> **definitely** have more power on hoomid days -- and it doesn't
> matter what the temp !!  I think the reason it's more obvious on
> the motorcycle is it's fewer cylinders -- dunno.

I race an FZR400 motorcycle and in my experience, humidity does not 
help.  I run FCR flat slide carbs, which are pretty sensitive to jetting 
changes, and I log barometer, relative humidity, and temperature for 
every lap I run.  The best power comes from cool temperatures and high 
barometric pressure, since this allows the largest main jets.  

As someone else pointed out, relative humidity is just that, relative.  It
is, by definition, the amount of water vapor in the air compared to the 
amount of vapor that the air can hold, expressed as a percentage.  The 
max. amount of water vapor can be found from a steam table listing 
saturation pressures.  I use this data, combined with the temp, RH, and 
baro measurements, to calculate absolute dry air density, which I use to 
set my jetting.

Now humidity may help with cooling, since water has a larger heat 
capacity than dry air, but combustion-wise, it's not doing you any 
favors. 

Michael T. Kasimirsky       ---->              mtk at tmc.astm.cmri.cmu.edu
Days:    Staff Engineer                     or mk4u at andrew.cmu.edu
         ASTM Test Monitoring Center           Phi Gamma Delta,
Nights:  1992 Suzuki GSX-R750 Pilot            NRA Life Member, AMA Member
         1990 Yamaha FZR400 Superbike Racer    DoD #1848
         1991 Suzuki GSF400 Bandit Mechanic    WERA Novice #753