Why does my car have green topped valve covers?

[RichardsFRS]

Originally Posted by strat61caster

Tire pressure changes can dramatically alter the handling of a vehicle, with standard atmospheric air the temperature increases lets say 3 psi after an autox run or lapping session. By using dry air (typically accomplished with nitrogen) you can reduce the pressure change, lets just make stuff up and say it reduces the increase by 1 psi.

To the average person, doesn't make a lick of difference.

To the hardcore competitive guy that means he starts his day +1 psi over standard air, he's 1 psi closer to his optimal temperature, the tires will react more predictably sooner in the run, if there's an unanticipated handling problem (under/oversteer) that would heat up the fronts or the rears disproportionately or unexpectedly with nitrogen/dry air the pressure change will be lessened reducing the impact of the problem vs. atmospheric air.

I remember doing some testing about a decade ago, these were go kart tires and my dad had access to a house sized nitrogen tank at work, nobody would miss a bit here or there, one of the perks you could say. We saw maybe 0.5 psi difference at the absolute most and our methods weren't very scientific. We deemed it wasn't worth the hassle and after a season of messing with it dropped it and put our efforts into more valuable testing time. The local championship and track records came long after we stopped bothering.

I fear I may have misinterpreted your post, oh well, already wrote it.

I wouldn't hesitate to use the desiccator I linked a few pages back but at this point it's not on my radar for minimizing variables, if I were to use it I'd have to spend a day re-confirming my air pressures...






Edit: Easy DIY test, take one tire filled with nitrogen (courtesy of your local dealership, costco, american tire, NASA, whatever) take one tire filled with atmospheric air (stop off at a gas station on the way home drain a tire flat, fill it up with the air compressor there), let them sit in the garage overnight, equalize the pressure in the morning, let 'em sit in the sun for a few hours and measure the pressures again. You could be fancy and measure every half hour and make some nice Excel graphs, of course make sure they get equal sunlight and aren't close enough to influence each other significantly.

Might do this if I get my next set of tires at costco, lets see if they offer any trackable compounds aside from RE71R and MPSS...

Mine are bright like that. They are smaller chrome ones with a green dot on the top of the cap.

[RichardsFRS]

They look like these
http://www.ebay.com/itm/4-CHROME-TPM...-/141028650401

Only I was missing two of them so I ordered two from toyota and my wife picked them up. These idiots sent me two normal plastic caps with a 15$ invoice

Taking those back this weekend. 15 bucks for two caps, he must have been on crack
I'm just gona get some like I posted here

[strat61caster]

Originally Posted by RichardsFRS

Mine are bright like that. They are smaller chrome ones with a green dot on the top of the cap.

Personally I prefer plastic, typically they're lighter and there's zero chance of them chemically interacting with the valve stem. But right now I'm running without anything so :shrug:

[Tcoat]

Originally Posted by RichardsFRS

They look like these
http://www.ebay.com/itm/4-CHROME-TPM...-/141028650401

Only I was missing two of them so I ordered two from toyota and my wife picked them up. These idiots sent me two normal plastic caps with a 15$ invoice

Taking those back this weekend. 15 bucks for two caps, he must have been on crack
I'm just gona get some like I posted here

Find a parked car with caps you like and...

[mdm]

Originally Posted by strat61caster

I do have some reservations about your math as I am not saying there is a 150% difference, rather my example is ~3% difference between the two scenarios (~30+3 vs ~30+2)

I was referring to the ratio of pressure gain for nitrogen to pressure gain for (moist) air. Basically, I am trying to figure out if we have certain ΔT, what is ΔP_air/ΔP_nitrogen. So 3/2. Not saying though that my entire math (or physics) is correct, criticisms and corrections are very welcome!

Originally Posted by strat61caster

Like I said, I'll actually run an experiment if I ever get my tires filled with nitrogen.

Great!

[mdm]

Originally Posted by justatroll

For a sealed system (tire), n (number of moles) and R (ideal gas constant) will not vary AT ALL.
They are constants and by definition will not change during this experiment.

To clarify: they will obviously not change for a sealed tire, but what I mean is that will be different for tire 1 filled with gas 1 (say nitrogen) vs. identical tire 2 filled to the same pressure (or volume) with gas 2 (e.g. humid air)


And it is the nR that determines how the pressure in a tire filled with a particular gas (with it's particular n and especially particular R) will respond to temperature changes.



In other words: My thought experiment is that I have two tires at the same temperature and pressure, one filled with gas 1 and the other one with gas 2. Then I heat up both tires to a higher temperature, still identical between the tires. The pressure will rise in both tires, but probably by a different amount. This pressure increase is determined by nR of the particular gas.

My point is that even though R of water vapor is higher than R of dry air or nitrogen, there is so little vapor in the air even at saturation (100% relative humidity) at high temperatures, that the effect on pressure change in response to temperature change will be minimal.

[humfrz]

Originally Posted by Tcoat

Find a parked car with caps you like and...

................


humfrz

[justatroll]

Originally Posted by mdm

My point is that even though R of water vapor is higher than R of dry air or nitrogen,.

R is a universal constant.
It does not change no matter what.
The only difference you will find for the value R is based on what units you are using, NOT what gas you are using.


It would be like saying that "G" varies based on what material is falling.


Maybe I am the one that is confused....


http://www.katmarsoftware.com/gconvals.htm

[Tcoat]

Originally Posted by Tcoat

Find a parked car with caps you like and...

Originally Posted by humfrz

................


humfrz

ask them politely where they got them from.

[mdm]

Originally Posted by justatroll

R is a universal constant.
It does not change no matter what.
The only difference you will find for the value R is based on what units you are using, NOT what gas you are using.


It would be like saying that "G" varies based on what material is falling.


Maybe I am the one that is confused....

My understanding (possibly wrong) is that the universal gas constant describes an ideal gas, but there is also individual gas costnat (http://www.engineeringtoolbox.com/in...ant-d_588.html) which describes how an actual gas would behave. But I might be wrong when using it in the same equation.




Now, if we assumed your point of view that we should use the universal R no matter what gas we are considering, and that n is very similar between the gases (because for most gases the volume of one mole under standard conditions is close to "ideal" 22.4 litres), that would mean that the differences between temperature-related pressure changes between gases are far smaller than I calculated. Because it was the difference in R_individual that was responsible for the bulk of the effect.


(Also, I realized that I used the volume of one mole (22.4 to 24.5 L/mol) in place of n (the amount in moles), obviously I should have used the reciprocal of the volume, but it has little effect on the result as again it is R that largely determines the effect.

[strat61caster]

Originally Posted by mdm

I was referring to the ratio of pressure gain for nitrogen to pressure gain for (moist) air. Basically, I am trying to figure out if we have certain ΔT, what is ΔP_air/ΔP_nitrogen. So 3/2. Not saying though that my entire math (or physics) is correct, criticisms and corrections are very welcome!

Well that's the trick isn't it, your math actually says that you'll see a ~3% difference in pressure for a given amount of air.

If that given amount of atmospheric air is at 30 psi at X temp then nitrogen would be approximately 3% less pressure so that's a ~1 psi difference, you've totally validated my estimates!

Math is fun,

[mdm]

Originally Posted by strat61caster

Well that's the trick isn't it, your math actually says that you'll see a ~3% difference in pressure for a given amount of air.

If that given amount of atmospheric air is at 30 psi at X temp then nitrogen would be approximately 3% less pressure so that's a ~1 psi difference, you've totally validated my estimates!

Math is fun,

It appears to me that we agree in principle, and where we appear to disagree, it's not an actual disagreement, because we are talking about different things, so obviously we see different numbers. I'll get back to this later, and I'll also try to get my Rs correct.

[humfrz]

Originally Posted by Tcoat

ask them politely where they got them from.

Oh, sorry ....... I thought you were advocating ..... borrowing them ..... my bad ......


humfrz

[Tcoat]

Originally Posted by humfrz

Oh, sorry ....... I thought you were advocating ..... borrowing them ..... my bad ......


humfrz

Well now depending on the neighbourhood...