[michaeld]

I am planning to buy an air compressor (my first) and I'm not quite sure what tank size/hp/cfm/etc unit to get.

I've never used a compressor before; frankly, I'm not really all that sure how much I would even use one. So I really don't want to go with the "buy the biggest sucker you can possibly afford"-advice. I'd be afraid it would end up being what the wife would label "that great big giant thingy wasting space that you spent all that money on."

All I really wanted was to have compressed air. And then, when I started to look at them, the little dinky things were STILL $70-80 bucks. So I figured, "Why not get something bigger and better for not a whole lot more?"

I notice there seems to be some pretty significant step-ups in price between compressors.

I've got my eye on a Central Pneumatic 21 gal, 3 hp oil lubricated unit that has 4.9 cfm @90 psi that I could buy for $127.50 at Harbor Freight.

Another "hot Harbor Freight buy" is a 4 hp 10 gal 2 capacitor motor model I could get for $100.

As to that step-up, the next performance upgrade is a 4 hp 29 gal unit for $349. And it's stationary.

I figure I'd use a torque wrench, perhaps occassionally the ratchet. And definitely the compressed air! But I have no idea what else I'd do with the thing at this point.

I'm a "typical household maintenance" and "typical automotive repair" guy. I'm not building my master-blaster big-block; just maintaining the engine I've got. Most of my maintenance is tune-ups plus taking off bad parts and putting on new ones. What I'm really looking for at this point is a decent compressor that would meet typical household needs adequately.

BTW, the whole house is 110, even the washer/dryer and oven. I saw a step down converter at http://www.dvdoverseas.com/store/catalog6_0.html which says,
"Step Up-Down Heavy Duty Transformers Type-2
These transformers are for continuous use with both electronic and electrical products. Step up-down heavy duty transformers convert AC electricity either way from 220 volt to 110 volt (step down) or 110 volt to 220 volt (step-up) for use with appliances anywhere in the world." If I got a 220 v model, I'm not sure whether that would allow me to benefit from the extra "oompha" of a 220 volt unit or not.

So what advice can you guys who are familiar with air compressors give me?

[Colin Frolick]

i've been shopping for a compressor lately too.

i have a ~1.5 hp, 25 gallon, 110v, oil-less model I got at home depot. It was about $300, including a decent set of air tools. I can't wait for it to break so i'd "need" to get a new one. It's noisy and slow to pump up. I'm actually surprised it hasn't broken. However, it has been adequate for what I've done so far... wrenching, filling tires, nail guns, and shooting drywall texture. I lately tried to use a DA sander with it, and it's completely inadequate for that.

I'm going to paint my car, so I've been looking at big air compressors to run sanders, cutters, an hvlp gun, maybe a soda blaster. Soon I'm going to buy from www.eatoncompressor.com

As far as the power goes... i'm no electrician, but one of my uncles is, so i think most all houses have 220v to the breaker box. It's the breaker you install that grabs either 1 wire or 2. My garage doesn't have 220, yet, but I'm pretty confident I'll get around to wiring it up.

[Cliff R]

Run from the self contained units with the sealed engines (hermetic?). We buy the 60 gallon cast iron units, 220 volt with separate electric motor. Most of them are 120-140psi. The one we have in the shop now is from TSC, about 12 years old, no troubles to date.

The hermetic ones are slow, noisey and short lived......Cliff

[Brewster]

It all depends on what your going to use it for and how often?

Spend the money on a two stage recip and figure about 4 cfm per h.p. you never have enough air plus there repairable.
Also note moisture levels in your area and buy a automatic drain for the bottom of the receiver at an extra 100-125 bucks.
The tools that eat up cfm are sand blasters, air hammers, impacts, but remember that high cfm is also what you need for blowing off parts near and far.
Good luck with the most important part of your garage(beside the car),
Brew

[67drake]

I have a 30 Gal. 2 stage 220 volt,don't remember the CFM offhand.There is no way I would consider anything smaller. I use mine about 2 times a week avarage. I know you said you only can go 110,but I would get the biggest I could,or consider a bigger 220 motor,and get 220 service.As said above you can never have too much air.
If you get too small,trust me,you'll kick yourself later,IMO.I was just running a die grinder last night grinding off some screw threads that were staked,and my compressor could barely keep up. My sandblaster,I have to blast for a minute,wait 30 seconds,blast for a minute,wait 30 seconds,ect..

[george kujanski]

I have a 3 HP, 220V unit, 30 gal. and have had it for 25 years or so. As mentioned tools like air sanders, sand blasters, etc. really need more compressor than that, but it's been good enuf for all my other uses.

A 110-220 transformer is not going to be cheap, so if there's any way you can get a 220v supply, do that and get a 220 volt compressor. otherwise get the largest 110 unit you can.

A smaller unit will take longer to pump up and tank capacity really only comes into play when you need volume, like the mentioned sanders and sand blasters.

Good advice is to get the largest one you can power/afford.

George

[69lm69gp]

I would first check to into having a 220V outlet installed, that would open up your choices.

I just upgraded my compressor from a Craftsman 5HP 33 Gal oilless unit. It got the job done but not very well. It had a hard time keeping up things like my die grinder, inflating flat tires, and my spot blaster. Small compressors have to work harder and this translates into moisture in the air not being able to be removed effectivly because the compressed air never has a chance to cool down. This caused problems with my spot blaster as the moisture would make the sand clump up and stop flowing and also cause the metal I was blasting to rust almost instantly.

My advice to you is ask yourself what you see yourself doing in the future, find out what tools you may buy and then figure out how much CFM your most demanding tool needs. Compare compressors by CFM not HP or tank size, but keep in mind CFM rating are asually overstated. The best quote I have read about buying a compressor is "no one ever regretted getting too big a compressor".

[Blue'69]

I'm just about to upgrade from a Craftsman 4HP 25gal. oiless model as well. Really looking hard at buying a 4.5 running HP 80gal. model from Lowe's for $649.00. Should be more than enough to run air sanders, HVLP sprayers, and other high demand air tools.
I'm working on wiring my garage with a 220V receptacle both for the Air compressor and another for welding when I get around to buying one of those. Smaller compressor is about 10 years old with no problems other than it can't keep up with larger cfm tools.
Good Luck.

[Stuart]

I don't know where you live or how old your house is, but it's very unusual to have only a 120vac electrical service...they are almost universally 120/240vac these days.

However, for what it sounds like your usage will be, I don't think you'll need a compressor that's large enough to require 240vac. Get one that will work on a 120vac 15 amp circuit so you can plug it in anywhere you may want to use it.

[Stuart]

Originally Posted by Blue'69

I'm just about to upgrade from a Craftsman 4HP 25gal. oiless model as well. Really looking hard at buying a 4.5 running HP 80gal. model from Lowe's for $649.00. Should be more than enough to run air sanders, HVLP sprayers, and other high demand air tools.

Remember to look at the cfm rating of the compressor and make sure it meets the requirements of your air tools, not the size of the tank.

Also, you may as well disregard the horsepower ratings of most consumer grade air compressors as they're made up by the manufacturers...terms like running HP are meaningless, it's kind of like how all cheap car stereos have 500 watts of instant peak power. Go by the actual horsepower stamped on the engine nameplate if it has one, or by the amp load listed in the owners manual.

[michaeld]

Thanks, as I've obtained some useful info from the posts.

I agree; I would not buy an oilless compressor (I even hate it that cars today are non-lubricatable in the chassis). I like to be able to add oil, and make sure the level is right.

I've never used a compressor before, never really thought I needed one (other than having the ability to run compressed air to blow dust/junk off before engine work. So I just don't know how much I'd use the thing.

I noted that running a polisher used about 6+ cfm, and thus far, that's been the only "high cfm" tool I can really see myself using. I have an electric sander, and am considering buying an electric polisher for the car. I can't see myself spending nearly a grand to buy a compressor so I can use a polisher, when I could get a nice electric one for under $40.

But at the same time, I think MAYBE I'll use a compressor a LOT; maybe it'll be like the light bulb - once you've got one, you can't possibly go back to normal.

My understanding is that a 220 v compressor is able to maximize power in a way that 110-120 units can't match. The transformer I listed was only $25, so not a lot of money; but I'm not sure if it would give me 220 volt power, or just allow me to plug in at the relatively puny 110 v power level. It's not that I can't have 220; it's just that I'd have to spend additional money to do a conversion. Currently all my plugs are standard 110.

Of the two compressors I mentioned at Harbor Freight, I didn't cite the specs of the 10 gal 4 hp model.

It features a heavy duty, easy start 2 capacitor motor with thermal overload protection and auto shutoff at 115 psi and restart at 85 psi.

It is a higher cfm unit than the 3 hp 21 gal model, with: 4.5 cfm @ 115 psi; 5.6 cfm @ 90 psi; 6.5 @ 70 psi; and 7.2 @ 40 psi.

The air torque wrench I already have has 4 power settings with a max setting of 230 ft/lbs that operates at 4.5 cfm @ 90 psi.

As I understand it, the smaller tank unit would tend to run all the time, whereas a larger tank would cycle on and off more.

Stuart says that the cfm is much more important than the size of the tank. So of the two, should I opt for the 4 hp 10 gal model (of the two)?

If I KNEW I'd use a compressor a lot, I'd go big. I like big. A lot. But I just don't want to spend a ton of money for something that probably wouldn't get a lot of use. What I want to do is get the best deal I can on a reasonable unit that would do most everything I would want a compressor for (and I'm not quite sure what "most everything I would want a compressor for") is yet.

[Dan968]

I have 2 air compressors
1) 2hp motor with seperate pump and 20 gal tank. Its made by Ingersol Rand. This compressor has wheels and can be rolled around. It runs on 110v 15amp circuit, but runs best on a 20 amp circuit. I have used air wrenchs with no problem on a 20' hose. Its great for filling tires and mechanic type jobs. Plus its cheaper to run.
2) 5hp running motor with cast iron pump and 60 gallon tank, Husky brand. I was told it has a 100% duty cycle. It runs all the body shop tools, it does run all the time with a DA and would probably run out of air if I was painting with a HVLP gun.
I know someone already brought up the voltage, but must houses have two sides of the fuse or breaker panel that provides 220v.

If you have no plans for Autobody tools I would think a 110v volt compressor would work fine. You might check farm supply stores for prices.

[michaeld]

Dan,
Actually your "two compressors" idea sounds pretty good. I figured if I discovered I really needed a bigger compressor, I could either sell the smaller one or have a pair.

With that in mind...

Which compressor sounds like the better "smaller compressor": the 3 hp 21 gallon model or the 4 hp 10 gallon model? I have the cfm specs of both listed in my above posts.

The 4 hp model is a 2 cycle with auto shut off, and is advertised as having clear readings of psi and oil level. I'm not sure how the 21 gal model compares in those regards. There's only a $25 difference in price.

The 4 hp model has higher cfm (5.6 cfm @ 90 psi vs. 4.9 cfm); the 21 gal model has double the air capacity.

[69lm69gp]

Originally Posted by michaeld

Which compressor sounds like the better "smaller compressor": the 3 hp 21 gallon model or the 4 hp 10 gallon model? I have the cfm specs of both listed in my above posts.

The 4 hp model is a 2 cycle with auto shut off, and is advertised as having clear readings of psi and oil level. I'm not sure how the 21 gal model compares in those regards. There's only a $25 difference in price.

The 4 hp model has higher cfm (5.6 cfm @ 90 psi vs. 4.9 cfm); the 21 gal model has double the air capacity.

Niether as none of the compressors you listed can provide the 6 cfm the polisher you stated that you see yourself using. Keep in mind that a 6 cfm compressor running a 6 cfm tool will have to run continuously to operate the tool once you have used the the air in the tank. Also keep in mind that the cfm on those compressors are more that likly overstated.

Check this thread out and in particular the posts by oldred.

http://www.hotrodders.com/forum/comp...air+compressor

[Dan968]

I would go with the larger tank. Here is what I have experienced, when using mechanic tools like an air wrench it is short burst of air thats needed not long continous amounts. The tank cfm is only limited by the air pressure in the tank and the size of your air hose and fittings.
The pump cfm ratings are measured with the pump running continous. Granted it will fill the tank quicker but a larger tank will not cycle the pump as much.
The other thing to consider is your line voltage. If you want to roll it around and plug into any outlet, make sure it works with a 110v 15 amp circuit.

[michaeld]

69lm69gp,
Actually, I tried to express in my post that at least for now, the use of such a mega-high-cfm tool wouldn't be my criterion. I have never used a polisher (other than human labor), but plan to buy an electric one. I've looked over the 6+ cfm tools, and can't see myself using them enough to justify a $600+ purchase - at least I can't now.

I haven't visited your link yet, but will when I log off here. I DO appreciate your advice; and you're right: if I really want to run 6 cfm tools, I've got to buy a 6 cfm compressor.

Actually, this leads to a concern I've been having: namely, I keep reading that horsepower is almost ALWAYS exaggerated, and I wonder if this means that cfm ends up being exaggerated as well?

For instance, using 110-120v, I've read that at best they are only actually capable of producing no more than 2 hp MAX. You're just not able to obtain sufficient amerage from the circuit. When they list the cfm, are they doing so in an "ideal condition" lab test, or are they plugging it into a standard household outlet?

After reading up on it, I've pretty much decided to go with a 110 unit. I know I'm not getting as much "oompha" as I could, but rewiring panels seems like more of a hassle than I am prepared to deal with. Also, I'd kind of like to be able to use a compressor for a couple of "backyard" projects, which would mean TWO 220 circuits.

Dan's advice is quite meaningful to me: the larger volume tank (in this case a little more than twice as large) is more significant than the larger cfm (5.6 v. 4.9 cfm @ 90 psi).

Does that view represent a consensus?

My thinking at this point is to get a semi-decent entry level compressor to begin with, and then get a larger unit if I discover I really want/need one. But I still want to get the "smartest" entry level unit. At entry level, you can't have it all: bigger tank or bigger flow.

[scotzz]

cfm and tank size both matter and should be matched.............for instance, a 60 gal tank with low cfm may take a little longer to kick on but then it takes longer to fill up..................the same pump would probably be better suited to a smaller 30 gal tank as it doesn't have to work as hard to keep up with air demand nor tank fill...........overall, from what I have been researching the past month, I would place more value in cfm than tank size..............especially if you have space limitations...........I am looking at the Harbor Freight 4HP max (3 rated) 10 gal tank because it has the highest cfm of all of the small units I have seen..........don't even consider an oiless model............they are low cfm, noisy and will not last as long as cast-iron pump model............

[michaeld]

Hey, Scotzz,

You ought to be interested in this thread, then, because I am asking for a comparison between the compressor you are looking at (the Harbor Freight 4 hp 10 gal unit) and a 3 hp 21 gal unit also at Harbor Freight.

Of the two, which is most practical? Double the tank volume or an additional .7 cfm at 90 psi?

Interestingly, when I clicked on the manual in the Harbor freight link, the 21 gal model is listed as "3 HP working / 5 HP peak" and the 10 gal model as "4 HP peak / 3 HP rated." Peak HP only measures the moment of startup, and doesn't mean diddly. But if "working" means the same thing as "rated" than we're at parity. The difference is a slight increase in cfm, or a bigger increase in tank volume.

I'm coming home with one or the other tomorrow...

[Colin Frolick]

i've read that generally an electric motor should draw ~5 amps / hp. Reading the amp rating on the motor is the best estimate of its "true" hp.

[Dan968]

FYI
1 HP = 746 WATTS
AMPS = WATTS DIVIDEd BY VOLTAGE
So in a perfect world, 3hp powered by 110v = 20.34 amps. This does not take into account startup requirment.
It would be misleading not to be 3hp but I would question if the motor really is , the owners manual that was on their website showed a picture of a 110v 15amp outlet.
In my experience with a 2hp compressor, a 3hp would have trouble starting on a 15 amp circuit and in theory it would not work.
The motor should have a current rating on it.