Vapor Pressure

nomad · 11-27-2016, 10:14 AM

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

voltdr · 11-27-2016, 11:50 AM

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

63corvette · 11-27-2016, 01:08 PM

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

nomad · 11-27-2016, 04:57 PM

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.

Wayne Kerr · 11-29-2016, 04:11 PM

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

nomad · 11-29-2016, 11:53 PM

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

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

11-27-2016, 10:14 AM	#1
nomad Junior Member Join Date: Oct 2013 Posts: 47 Likes: 5 Liked 7 Times in 4 Posts	Vapor Pressure Can someone explain vapor pressure? What effect does it have on throttle stop timer settings?

11-27-2016, 11:50 AM	#2
voltdr Senior Member Join Date: Jan 2008 Location: Texas Posts: 705 Likes: 77 Liked 97 Times in 50 Posts	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

11-27-2016, 01:08 PM	#3
63corvette Senior Member Join Date: Mar 2007 Location: Canyon, Tx Posts: 551 Likes: 127 Liked 353 Times in 102 Posts	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

11-27-2016, 04:57 PM	#4
nomad Junior Member Join Date: Oct 2013 Posts: 47 Likes: 5 Liked 7 Times in 4 Posts	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.

11-29-2016, 04:11 PM	#5
Wayne Kerr Member Join Date: Nov 2003 Location: Out to Lunch Posts: 191 Likes: 2 Liked 26 Times in 11 Posts	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

11-29-2016, 11:53 PM	#6
nomad Junior Member Join Date: Oct 2013 Posts: 47 Likes: 5 Liked 7 Times in 4 Posts	Re: Vapor Pressure Thanks Wayne. I understand now. One more little question. VP of .475 would be .475 of what unit?