Vapor Pressure
 

Can someone explain vapor pressure? What effect does it have on throttle stop timer settings?

Re: Vapor Pressure
 

Vapor pressure, humidity, and water grains are an expression of the amount of moisture in the air. More moisture = less fuel vapor in the intake charge = less HP. You can different arguments about which one is better.
Dan
SC4698

Re: Vapor Pressure
 

Not sure I can explain it however, I believe it is best to watch it when the humidity is high.
I my case it affects the car when it is high and there is a breakover point at which it really affects the et.
It is a expression of humidity which is also affected by the atmospheric pressure. They are tied together when it comes to the HP the engine can make.
Just watching humidity is not enough as the adjusted altitude is factored in. It is really something to watch when racing at altitude tracks. It can fool you.
Just My 2 Cents
Rick Cates

Re: Vapor Pressure
 

Most of us use CC7 for throttle stop timer predictions I assume. I do and all those calculations are done there. I've watched VP for a while but, grains of water what I looked at more along with humidity.


I bracket raced yesterday and used a Jeg's logbook for the first time. It had a spot for VP so I filled it. I tried looking it up online but, really didn't understand any of the explanations I found.

Re: Vapor Pressure
 

Vapor Pressure is a measure of the actual water content in the air. Water Grains are also a measure of the actual water content in the air. The SAE chose Vapor Pressure for their Dynamometer Correction Factor, and that's why I use it exclusively.

Relative Humidity is derived from both moisture content and temperature and is used to indicate how a human feels in certain atmospheric conditions. It is of no use for predicting the effect of atmospheric conditions on a race car.
Relative Humidity: Noun, the moisture on your ball sack when you are banging your sister in law.

See you at the races,
Wayne Kerr

Re: Vapor Pressure
 

Thanks Wayne. I understand now. One more little question.


VP of .475 would be .475 of what unit?

Re: Vapor Pressure
 

Nomad,
Without getting into the science of gas laws and partial pressures, think of the V.P. number as an effect that subtracts from the barometer. An increase in V.P. means the air is carrying more water vapor I.E: slower E.T. So, a vapor pressure change of plus .25 (.40 to .65) would have the same effect as the barometer dropping from 29.75 to 29.50.
I have found the effect of V.P. on car performance to be linear and consistent on gasoline burning engines both normally aspirated and supercharged.
Now for the cherry on top: .06-.07 change in V.P.=.01

See you at the races,
Wayne Kerr

Re: Vapor Pressure
 

Got it. This goes in my race notebook.


Thanks Wayne.