[michaeld]

The 3 HP 21 Gal model was listed at 14.3 Amp under load, requiring a dedicated 20 Amp circuit at startup. I don't have that info for the 4 HP 10 gal model.

My real question at this point is, is a little more cfm better (5.6 cfm @ 90 psi vs. 4.9 cfm @ 90 psi) OR is a lot more tank volume (21 Gal vs. 10 Gal) better? I'm choosing between a little extra cfm, or a tank that's twice the size.

[BILTIT]

CFM is the most important thing when buying a compressor. Tank size is usually matched to the cfm except with cheaper manufacturers who put a large tank on a small compressor to fool you into thinking its something its not. Figure out the most cfm you will ever use and go one size bigger.

[chuckmnv]

And, MichaelD -
You don't need a second 220 volt outlet. All you need is more air hose. As a matter of fact, that's kinda what the manufacturers recommend, as opposed to using an extension cord. Hope this makes sense to you.
cm

[67drake]

Don't go too crazy with the hose either though.You will see a pressure loss the longer your hose is.

[michaeld]

Well, shoot.

I've got two votes for the slightly higher cfm, and two votes for the larger volume tank (one on the thread, one from my brother, an engineer [though not an auto engineer who has restored cars for 20 years as a hobby].

I'll try to find "the compressor guru" at Harbor Freight and talk to him before I buy. There's one other thing: the 10 gal but higher cfm unit weighs like 75 lbs, and the 21 gal but slightly lower cfm unit weighs 150 lbs. All things being equal, the sheer size of the thing might make my decision for me.

I'll post back as to which one I got, and what I've learned, for the sake of posterity (i.e. the next guy in my shoes) or further discussion.

[scotzz]

Originally Posted by 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...

Michaeld.................I'm leaning toward the 10 gal model because it has a higher cfm rating and I'm looking to get all I can right now...........I need it mostly for touch up painting and to run a small blaster.............maybe even some air tools............for these applications I don't think the 21 gal is better. Another reason I am leaning toward the 10 gal is because eventually I'll probably get a much larger unit and the 10 gal is small enough to keep for filling up tires and some other uses and is pretty portable. I am also getting mine tomorrow......with some gravity feed guns............

[michaeld]

Scotzz,
Good luck on your purchase. It sounds like you've got a plan.

I got the 21 gal model. I talked with a few of the Harbor Freight guys (certainly they aren't all "experts" there, but one of the fellers I talked with has been with H.F. for quite a while and knows his way around tools pretty well. Anyway, the skinny that led me to get the 3 horse 21 gallon model can be summed up in a few points:

1) Durability. For one thing, the 21 gal model has been around for a while, and has been a good solid unit. With a larger tank, the unit has more volume, and the motor won't be continuously running, as it would be with the 10 gal model. That 10 gal model - according to my friend - isn't nearly as "tried and true."

2) Tradeoff in volume's favor. The 5.6 vs. 4.9 cfm @ 90 psi represents a 14% increase, whereas the 21 gal vs. 10 gal volume represents a 110% increase. While cfm rules, it can't totally dominate with disregard for every other feature. I was hoping that some of the wise compressor gurus might give me further insight into this aspect. Had the cfm been significantly higher, I would have gone with the smaller tank.

Interestingly the motor looked identical in size and appearance on both units (and maybe it WAS identical; both are actually RATED at 3 HP, and both units are made by the same manufacturer). One of the Harbor Freight guys noticed that the copper air connections were smaller on the 10 gal model; is it possible that they obtain higher flow per minute by forcing the air through a smaller hose? If so, then that seems like a gimmik.

Scotzz has a point about the small size of the 10 gal model; it almost seemed like a kid's toy next to the 21 gal model. But that 21 gal model weighs 150 lbs, and is NOT very portable. Having a small unit has its advantages.

If you're looking for a big, powerful compressor, there's a lot of info available. But if you want a smaller unit, you're pretty much on your own, I found. I hope this thread is useful for the next guy who wants a smaller air compressor.

[69lm69gp]

michaeld,

It doesn't sound like you read the thread from Hotrodders that I linked. I hope you are not disapointed.

scotzz,

If you try to use a blaster with either of those two compressors you will be disappointed. The compressor I replaced was much larger that those and I had a hard time blasting with it. Do some research on Hotrodders before you buy.

[Formulas]

I am on my third air compressor
First i bought a craftsman 110v 4HP 7cfm@90 that didnt keep up with much and ran alot then soon spit oil due to being run to death.
Then i bought a 220v stand up tank oil less 5HP @ 11cfm@90 that was much better but was flying apart blowing rods as they have a limited life span.
Then i was determined to buy only one more compressor in my life as it was getting old at this point, I bought a campbell hausfeld 7 peak HP 17cfm@90 60 gal upright tank cast iron 4 yes 4 cylinder pump they slowed the pump speed down to make it last longer as well or it would have more cfm. it was around 800.00
But anyways my advice would be buy bigger than you think you need so you dont end up spending twice as much in the long run by things falling apart from overuse do to being to small.

The min i would advise anyone to buy would be a oil lubed upright tank with at least 10cfm@90psi it wouldnt keep up totally with a DA and you would have to give it breaks to build up pressure for sand blasting BUT at least you could do those tasks with it.

[Dan968]

MichaelD
Glad to hear you found something in your price and usage range.

Just curious, does the motor have a amps rating or wattage rating?

Dan

[scotzz]

michaeld................I'm not sure which one to go with.............for sure the 10 gal does not have an undersized pump but looking at the cfm of the 21 that may not be true........I am willing to bet that the motors/pump are the same and that is why the cfm is lower for the 21 gal.............I'll makemy final decision when I go check them out tomorrow...........

[scotzz]

Originally Posted by 69lm69gp

michaeld,

It doesn't sound like you read the thread from Hotrodders that I linked. I hope you are not disapointed.

scotzz,

If you try to use a blaster with either of those two compressors you will be disappointed. The compressor I replaced was much larger that those and I had a hard time blasting with it. Do some research on Hotrodders before you buy.

69lm...............I read all the really good threads on Hotrodders and that is why I am waiting to purchase what I really will need at a later date......which I don't know exactly what that is at this point...............as far as blasting........I am just going to blast small areas like the front framerails in the engine compartment.....not big panels............and the cfm of these HF compressors match the blaster requirements(just about)................people say you cant do this or cant do that but I just sprayed the trunk weatherstrip channel with a PPG epoxy primer using an aerosol spray kit I bought from Eastwood for $15.............this is not epoxy primer in a spray can........it is an aerosol can with separate reservoir for paint and an adaptor between the aerosol bottle and reservoir.............it came out fine but a lot of the snot noses would just laugh if I would have asked first if I could do this ..........since I am not blasting/sanding/spraying an entire car I know don't need a $1200 Eaton compressor..............

[mike nixon]

www.eatoncompressor.com even if you don't buy from him read what he as to say re comparing compressors. the man is sharp and builds on of the best compressors on the market.

mike

[michaeld]

Originally Posted by Dan968

MichaelD
Glad to hear you found something in your price and usage range.

Just curious, does the motor have a amps rating or wattage rating?

Dan,

I actually haven't unpacked the crate yet, but according to the manual the motor is 120V 60Hz 14.3Amp (load).

I learned something else when I was at Harbor Freight the other day that Scotzz might find interesting. I discovered that there are polishers and there are polishers. There are smaller polishers that my new compressor will be able to handle - 4.5 cfm @ 90 psi - and then there are big "manly" polishers that would be too much for my little guy to power. I bet that there are sand blasters that are the same way: there are probably small blasters that have a smaller, more limited radius, and then there are the big boys that can do a much larger area.

If this is the case, then it is simply a matter of matching your tool to your compressor, rather than it being the case that you can't polish (or paint, or blast) with a 5 cfm compressor.

[scotzz]

Originally Posted by michaeld

Dan,

I actually haven't unpacked the crate yet, but according to the manual the motor is 120V 60Hz 14.3Amp (load).

I learned something else when I was at Harbor Freight the other day that Scotzz might find interesting. I discovered that there are polishers and there are polishers. There are smaller polishers that my new compressor will be able to handle - 4.5 cfm @ 90 psi - and then there are big "manly" polishers that would be too much for my little guy to power. I bet that there are sand blasters that are the same way: there are probably small blasters that have a smaller, more limited radius, and then there are the big boys that can do a much larger area.

If this is the case, then it is simply a matter of matching your tool to your compressor, rather than it being the case that you can't polish (or paint, or blast) with a 5 cfm compressor.

michaeld............according to the research I have done that is basically correct but it doesn't factor in "run time"...........what I have read is that if your compressor puts out 5 cfm at 90 psi and your tool requires that amount then theoretically your compressor may be running continuous to keep up.........not a good thing for extended periods of time...........also there would be no "safety factor" and if your compressor does not put out what it is rated at then you could be disappointed..........this is why the larger more powerful compressors are better for the applications we are using them for.............the blaster I am looking at requires I think 6 cfm at 60 psi and the HF compressor should be able to handle it...........assuming of course that the manufacturers ratings are accurate.................the eaton site does have a lot of good info and I may up purchasing one eventually...........anyway.........I am out to purchase today

[69lm69gp]

scotzz,

The thing about blasting and painting is not so much having a compressor that can keep up, but rather having dry air, as I mentioned in my first post in this thread (#7). A compressor that runs continuously or near continuously never gives the air time to cool off ,thus never allowing the moisture to condense so the moisture separator can do its job, this moist air can make freshly blasted metal rust before your eyes. I know because I have seen it happen and I was only blasting my air cleaner lid with a spot blaster. Now maybe if you live in a dry climate this is not a problem but where we live here in the Northeast with our humid summers it is a big problem. BTW I am just trying to give you the benefit of my experience, I don't believe I said it couldn't be done.

[scotzz]

Originally Posted by 69lm69gp

scotzz,

The thing about blasting and painting is not so much having a compressor that can keep up, but rather having dry air, as I mentioned in my first post in this thread (#7). A compressor that runs continuously or near continuously never gives the air time to cool off ,thus never allowing the moisture to condense so the moisture separator can do its job, this moist air can make freshly blasted metal rust before your eyes. I know because I have seen it happen and I was only blasting my air cleaner lid with a spot blaster. Now maybe if you live in a dry climate this is not a problem but where we live here in the Northeast with our humid summers it is a big problem. BTW I am just trying to give you the benefit of my experience, I don't believe I said it couldn't be done.

69lm................that's interesting information but the only way to run a lot of these air powered tools and sprayguns is to have a fairly good size and powerful compressor.................seems that most folks are using/recommending minimum 60 to 80 gal units and I just don't have the volume of work on this car that warrants that kind of investment right now..............my primary use this year is only to preserve my car by fixing very small areas of surface rust like aroung the rear window and the bottom of one fender...........after doing lots of research on the "rust encapsulators" I found that the best way to fix rust is to remove and use an epoxy primer............I've not seen epoxy primers in aerosol so I going to use a touch-up gun and then top coat with the original color laquer which I can get from PPG.............the paint on most of the car is pretty good but the roof is going to have to resprayed at some point and maybe I'll have it done professionally...........depends how I make out with the small compressor and touch-up gun as to whether Ill spring for a good compressor..........my comment regarding "cant be done" was not aimed at you or anyone here (I belong to several boards) so please don't take it that way and I do appreciate your advice and experience...........I'm going to try the blasting because.........I am blasting off crud and rust on a frame and I can't come out worse that what I am starting with...........lol


michaelb............I wound up buying the 10 gal unit.........it was $10 cheaper than they advertised so it came out to be $119...........and I bought a touch up gun that uses 3 cfm................let me know how you make out with the 21 gal unit

[michaeld]

Scottzz,

I'll try to let you know.

The 10 gal model had one obvious benefit compared to the 21 gal model: it was small. Space is a premium in my garage. I hoped that the 21 gal's extra size being vertical would alleviate that. But there is still the 150 lbs vs. 75 lbs of weight.

In addition to the sale price, I had a 15% off coupon which brought the 21 gal model down to $126. There was more minutia to buy than I realized: I bought a 50' hose and an air regulator/oil lubricator unit for 18.99 each (on sale) along with the extra plumbing connections I'd need to install it. My plan is to use smaller tools, and to always remember that patience is a virtue. My fear at this point is NOT that the compressor won't be powerful enough for me; but rather that I simply won't use the darn thing. Basically, I figure I'm out a lot less to buy a 21 gal unit I won't use than I would have been to buy a 80 gal macho unit I won't use.

From what I've read, if you have the capability to keep a compressor on "continuous run," you should do so if the motor kicks on more than 20 times per hour.

In terms of 69lm's point, I live in about as dry a climate as one could come across. So hopefully I shouldn't have to deal with the same set of issues he's describing. Humidity DOES play a big part in a motor's performance: when I was in Panama when in the Army, we actually had to get out of the Blackhawk helicopters a couple times and help lift them to get them off the ground when the humidity got real bad. Never had to do anything like that in "drier" climates.

If someone comes across this thread, and knows something about the "junior" compressors (such as the models scotzz and I have been discussing), drop a line and give us your expertise on analyzing which set of features would likely be best (i.e. 110% more tank volume, or 14% more cfm). I still don't feel like I've heard that discussion from any of the posters to the thread thus far, and am interested.

[Dan968]

Be careful of the lubricator, your air line will get a coating of oil in it and spray oil when you want dry air. Like even filling a tire with air. I oil my tools manually.
Dan

[Greg Reid]

Originally Posted by Dan968

Be careful of the lubricator, your air line will get a coating of oil in it and spray oil when you want dry air. Like even filling a tire with air. I oil my tools manually.
Dan

You're right about that I believe. I never use one of those line lubricators. That's for guys running impact wrenches and ratchets all day...Never a paint gun or sandblaster. My opinion only.