Does anybody know the formula for how many cubic feet of air it takes?

av8or

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The rest of the question is ............. to fill a 35/12.5r17 tire from 12 psi to 32 psi? compressor manufacturers talk about cfm and I’m just trying to figure out how that can be figured in time to fill different tires sizes and/or multiple tires.

Obviously I need help in the smarts department, so please be nice.
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Left Field

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Its kind of hard to take CFM directly and figure out how long it takes to fill a tire as the CFM rating drops off as the pressure increases. I recently boutght a VIAIR compressor, it has fill rate specifications that are helpful and could probably be reasonably scaled for comparable compressors:

Screenshot from 2020-12-11 15-41-36.png


For example, the ARB High Output 12V compressor puts out 2.34CFM at 29 psi where this model VIAIR (400H) is [email protected] The 29 & 30 psi rating points are ballpark close enough for comparison, so if this VIAIR model will fill a 35x12.5 tire from 15-30 psi in 165 seconds, then the ARB should be the CFM ratio faster:

1.86 CFM / 2.34 CFM x 165 sec. = 131 seconds = 2 min. 11 sec.

This math should get as close as you can reasonably expect. Even with a given compressor I'd guess the results could vary by 20% or possibly more. The fill rate can change significantly on whether your engine is running or not (12.x volts vs. 13.x volts), the with wiring length & gauge and even the type of air chuck. It will also vary to a lesser extent with air temperature and altitude (which you probably know all about) and will likely be a little slower as the compressor heats up going from tire to tire.
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Robertcladner

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I have the VIAIR 400P mounted under the hood with about a 6 gallon tank. It works great. I imagine the portable model is just as good.
2.VIAIR-400P.png
 
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Beancooker

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I have the VIAIR 400P mounted under the hood with about a 6 gallon tank. It works great. I imagine the portable model is just as good.
2.VIAIR-400P.png
Do you happen to have pics of where and how you mounted this under the hood? I have a 400p automatic, and was looking at doing the same thing, but the engine bay is packed full.
 

HungryHound

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I have the VIAIR 400P mounted under the hood with about a 6 gallon tank. It works great. I imagine the portable model is just as good.
2.VIAIR-400P.png
I used my 400P for the first time yesterday. Very happy with it and glad I got the automatic feature instead of saving the $50 as I had debated. The case has a side pocket that is perfect for a set of deflators. My only gripe is the pressure gauge which goes way too high for practical tire inflating purposes. I'd rather have a 0 to 50 PSI gauge and have smaller increments than what they include. I think the markings are at 5 PSI so you'll need a digital gauge to get all 4 tires to match closely (I don't have my fancy JLURE yet to beep at me when the tire pressure is exact). I have multiple Jeeps, so portable was the only way I was willing to go.
 

Beancooker

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Agreed on the inflator gauge. It’s worthless aside of getting higher than needed to release air to get the exact pressure needed. I bought this and it works really well. I am tempted to replace the Viair gauge with this, on the inflator pistol. I do like having the pressure gauge separate though. That’s the only reason I haven’t swapped them out yet.
I also like this because it’s wicked accurate, and I can read it without cheater glasses.

https://www.amazon.com/gp/product/B0010E1UGI/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1
 
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