Air compressor CFM and PSI question
#121987
09/16/08 11:37 PM
09/16/08 11:37 PM
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Joined: May 2004
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Sooner state
Slim Smitty
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I'm looking at compressors and spray gun cfm/psi ratings and I'm not quite sure how to determine what compressor rating works for what equipment ratings because the compressors and equipment both give different cfm/psi ratings.
I've looked at two Eaton compressors. The three horse version has a 14 cfm @ 90 psi rating and the 5 horse version has a 19.5 cfm @ 100 psi rating.
Then I looked at some HVLP spray guns that are rated at 13 cfm @ 30 psi.
How do you compare 90 psi and a 30 psi ratings? I've heard you can 1-2 cfm from a 90 psi rating in order to get a cfm rating for 40 psi as long as the compressor motor is good.
Is this true?
Anyone have a method for comparing these different cfm/psi ratings? It seems the compressors generally give 90 and up psi ratings while the equipment gives 30 and 40 psi ratings.
- Confusing.
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65 Coronet
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Re: Air compressor CFM and PSI question
[Re: Slim Smitty]
#121988
09/17/08 01:09 AM
09/17/08 01:09 AM
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Joined: Feb 2003
Posts: 21,318
Manitoba, Canada
DaytonaTurbo
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Manitoba, Canada
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From what I've seen, the lower the PSI, the higher the CFM output of a compressor pump. So a 13cfm@90psi compressor would put out more than that at 30-40, how much I'm not sure.
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Re: Air compressor CFM and PSI question
[Re: Slim Smitty]
#121992
09/17/08 11:12 AM
09/17/08 11:12 AM
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Joined: Nov 2005
Posts: 15,118
85086
moparpollack
Lil Herman
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Lil Herman
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Posts: 15,118
85086
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If it helps I've been priming my dart with heavy epoxy primer with a 22 gallon compressor. I turned the output pressure down to 44lbs and spreay with 30lbs at the regulator. A small compressor will do fine with a HVLP gun.
56 Plaza 63 D100 step side 67 Coronet, 68 Roadrunner, 69 Super Bees, 69 Coronet 500 convertible, 70 Roadrunner Post, 79 D150 360, and a severe case of Mopar a,d,d
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Re: Air compressor CFM and PSI question
[Re: moparpollack]
#121993
09/17/08 07:15 PM
09/17/08 07:15 PM
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Anonymous
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Anonymous
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The reason you are fighting ratings is that the "cheap" volume store units are rated at lower pressure--!!! and sometimes at suction, or atmospheric !!!--- for "better looking" ratings
Please don't buy any "oiless" or "direct drive" compressors. Find an industrial store in your area that supplies good, brand name compressors. I'm not speaking here of Lowe's, etc
One reason you need to go to a trusted, brand name industrial supplier, is that regardless of suction, rated at 40, rated at 90, you need to remember:
Sometimes even these figures are a great big lie, and, you can't even go by horsepower, because SOME sellers are spitting HP ratings BASED ON POWER DRAW rather than actual developed HP.
In short, you need to look at compressors ONLY rated at 90-100PSI. Good quality compressors ARE NEVER rated at 30-40 or suction. EDIT--What I mean is, good compressors are never rated ONLY at 40psi, but will at least have a 90psi rating
Last edited by 440sixpack; 09/17/08 07:19 PM.
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Re: Air compressor CFM and PSI question
#121994
09/17/08 08:43 PM
09/17/08 08:43 PM
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Joined: Mar 2005
Posts: 1,545
Seattle, WA
375inStroke
Special needs person
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Special needs person
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Posts: 1,545
Seattle, WA
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As said before, the numbers are inflated. If you convert how much power can be drawn out of the wall, a 15 amp circuit can only supply 2.3HP, and a 20 amp circuit can only supply 3HP. That's 100% efficiency. Take into account the losses in the electric motor, conversion into heat, the friction in the pump, the pumping losses, and you get much lower power, so HP cannot be used. Also, the CFM at a certain pressure are at 100% duty cycle. The compressor usually must be used at a lower duty cycle, like 50% on and off, so that CFM must be cut in half if used for any length of time.
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Re: Air compressor CFM and PSI question
#121995
09/17/08 08:53 PM
09/17/08 08:53 PM
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Joined: Feb 2005
Posts: 28,312
Cincinnati, Ohio
Challenger 1
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Quote:
The reason you are fighting ratings is that the "cheap" volume store units are rated at lower pressure--!!! and sometimes at suction, or atmospheric !!!--- for "better looking" ratings
Please don't buy any "oiless" or "direct drive" compressors. Find an industrial store in your area that supplies good, brand name compressors. I'm not speaking here of Lowe's, etc
One reason you need to go to a trusted, brand name industrial supplier, is that regardless of suction, rated at 40, rated at 90, you need to remember:
Sometimes even these figures are a great big lie, and, you can't even go by horsepower, because SOME sellers are spitting HP ratings BASED ON POWER DRAW rather than actual developed HP.
In short, you need to look at compressors ONLY rated at 90-100PSI. Good quality compressors ARE NEVER rated at 30-40 or suction. EDIT--What I mean is, good compressors are never rated ONLY at 40psi, but will at least have a 90psi rating
He said he was looking Eaton compressors which if I'm thinking right, they make industrial compressors here in Ohio. So he's good there, I recomend them.
 with everything you said.
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Re: Air compressor CFM and PSI question
[Re: moparpollack]
#121996
09/17/08 08:55 PM
09/17/08 08:55 PM
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Joined: Feb 2005
Posts: 28,312
Cincinnati, Ohio
Challenger 1
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Quote:
A small compressor will do fine with a HVLP gun.
Boy I'm not sure about that? I don't think it would keep up when I'm doing a overall paint job. My 5 horse 2 stage compressor runs almost non stop when I'm painting. 
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Re: Air compressor CFM and PSI question
[Re: Challenger 1]
#121997
09/18/08 05:05 AM
09/18/08 05:05 AM
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Joined: May 2004
Posts: 503
Sooner state
Slim Smitty
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Thanks for all the comments. I spoke with the guy at Eaton and he said there is no real formula to determine cfm at different pressures. He just said all compressors will produce more cfm at lower psi. His best guess was that his 14@90 (that's the more accurate free air rating) compressor will pump out 16@40.About ratings, you guys are right. Lowe's/Home Depot compressor ratings are all BS. They typically advertise horsepower just at start up, which is much higher than running horsepower. And their cfm ratings are not a true 'free air' rating. Which again, just inflates the numbers. After talking with Eaton, it seems they know what's up. They advertise their systems using 'free air' and 'true horsepower' ratings. About that comment that a small compressor can run an HVLP gun. What compressor are you using? I have two smaller 30 gall compressors with dinky 1.5 horse motors (not a true horse rating, so it's even less) that I got at Lowe's and they can't keep up at all with my gun. Only a few seconds and then the motor kicks back in. Always behind. Those small ones, at least what I'm using, are worthless. That's why I'm upgrading.
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65 Coronet
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Re: Air compressor CFM and PSI question
[Re: birdboy]
#121999
09/18/08 09:40 PM
09/18/08 09:40 PM
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Joined: Feb 2005
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Cincinnati, Ohio
Challenger 1
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Quote:
not tryn to hijack the post, but i been looking at a 5 horse 2 stage compressor from home depot $800-$900. would you pass on it? is it junk?i dont need the cadillac of units. thanks.
I would be very hesitant buying something like that from Home Dump, I mean Depot. 
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Re: Air compressor CFM and PSI question
[Re: Challenger 1]
#122002
11/17/08 02:44 PM
11/17/08 02:44 PM
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Joined: Feb 2004
Posts: 936
OLD318
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Quote:
Quote:
A small compressor will do fine with a HVLP gun.
However well intentioned..
That statement is completely false!
I mean no offense...
HVLP means High Volume Low pressure...
High volume - High dollars...
A small compressor will not put out the volume
required for an HVLP gun... ANY HVLP gun..
High Volume means at least 19cfm at 40 psi...
For example:
A devilbiss finishline gun requires 19cfm
at 23 psi... which will produce 10psi at the air cap outlet... If your compressor produce 19Cfm
at 40psi it will also produce 19 at 23psi (probably more) ...
(You can get that gun which is a great starter
all purpose HVLP gun for around $125.00) an
excellent value for the money BTW...
Regardless, Don't be fooled,
forget sears, Home depot Lowe's etc...
For the absolute best compressor for autobody work
go to www.tptools.com and buy a champion...
5HP 60 gallon or 80 gallon ..it will cost you
$1500 to $1800 bucks... YOU"LL LOVE IT...
They are very quiet, will last for years
and will run everything you've got for a home
garage/bodyshop and not cycle endlessly and burn out!!! (ask me how I know!)
You are competely wasting your money on $600-$750
compressors they are not suited for body work.
You will buy 2 of the cheap ones and burn them both out restoring/ or doing body work on a car
(...ask me how I know!...)
auto mechanics yes..
Nuts and bolts require High pressure low volume
(cheap compressors will work fine for this)
body work needs the reverse
High volume low pressure...
(cheap compressors are a waste of money)
If you absolutely can't afford the right compressor you can still paint with the
high pressure one (cheaper compressor)
but you will have to use an old-style
conventional paint gun..
(which are no longer used in body shops BTW)..
But you can still use it if you have to...
Best of luck to you!
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