What's new
By:
Filters

No vehicle painting section, I'll ask my dumb questions here, lots of them, mainly compressor related

makemeknowit

makemeknowit

Kwisatz Haderach
Joined
May 19, 2020
Member Number
72
Messages
278
Loc
Memphis, TN
Be forewarned, I'm likely overthinking/overcomplicating this.

So I want to paint my own car. Started by picking up a used Sanborn Black Max. 80 Gallon, 2 stage. Thing is, someone replaced the electric motor with a 6hp Subaru SP170 gas engine. Best info I have found says in stock form, the electric would have had a 5.75" pulley and the pump a 13.75". Subi has a 2.5, pump has a 16.5. This has me scratching my head as far as cfm output. I plan to replace the pilot unloader valve with one that will throttle the subi down once max psi is reached.

Here come the dumb questions.......So you start with the 80 gallon full and your paint gun requires x cfm at x psi. I'm new to this. I'm curious as to depletion rate of the 80 gallons. Everything I read says a 60 gallon is sufficient for painting an entire car so this 80 should be even moreso. CFM output of the compressor means nothing, in my mind, when starting to spray because you are pulling from reserve, ie the pump isn't recovering because it doesn't have to.....yet. I get that you regulate rhe air right out of the tank and again at the quick connect on the gun. My confusion is the x cfm at x psi the gun requires. When pulling from reserve, pump not running, and you've got the air regulated to x psi at the gun, how is the gun cfm required being regulated? I'm guessing hose size, 1/4, 3/8, 1/2 is affecting cfm?

I'm assuming the pump will go into recovery mode while painting even with 80 gallons of reserve? I'm adding an aftercooler to reduce air temp before it enters tank as well as oil/water separator and filter.

Sorry for long rambling stupidity.
 
Cheapish test, you can fill your paint gun res with wax and grease remover and test spray panel. Run it like you're painting the car and see if the compressor keeps up.
 
JNHEscher said:
Cheapish test, you can fill your paint gun res with wax and grease remover and test spray panel. Run it like you're painting the car and see if the compressor keeps up.
Click to expand...
My cousin suggested that very thing. He is going to bring his gun, just a hf cheapo, and will test
 
irate4x4.com

The PAINT thread!

Whole vehicle, quarter panels, cages, fenders... whatever! Let's share some knowledge for those interested (This includes ME, because I am certainly no pro!). I have painted to date about 25 vehicles with a variety of paints (though I am really partial to single stage urethanes/ enamels for...
irate4x4.com irate4x4.com
 
What is/was the original CFM (and at what pressure) rating of the compressor before being creatively improved-upon?
What CFM (and at what pressure) does your paint gun need?

Gun CFM is a function of nozzle size, which also has a big bearing on volume of paint per time. If you want to measure the gun spray nozzle, or if you have a number for it, post it up. Or if you have a spec sheet that it needs X CFM, even better.

Hose size needs to be big enough to supply the CFM the gun needs. Bigger won't hurt (other than that it's heavier, more expensive, and less maneuverable). Smaller may hurt, but it ultimately is a function of your gun. Basically, the gun is the gospel "thou shalt" item, everything else just needs to keep up.

Gallons of compressor tank is just a buffer for how long your tank can make up for your compressor being inadequate to do the job on its own, assuming you are willing to work intermittently. Get me a starting point (I'm a compressor guy for a living, for bigger stuff than you're talking about, but the math still maths) in CFM at PSI on the gun (and ideally, what the compressor was originally rated for) and we can probably get pretty close. Secondarily, I'd like to know what the original motor RPM or the original compressor head RPM was at that, but we can guess at that and probably be about-right or off by half.

IMO, gallons is a sales point to sell smaller (cheaper) compressors as looking bigger than they are, CFM is what you really need to do the job. Most big compressors (my job is almost 100% towables, mostly diesel powered) have almost no tanks, just plenty of CFM to keep up.
 
Last edited:
18cfm@100, 16@175 in stock config. I haven't purchased a gun yet. May utize my cousin's hf cheapo (6cfm@40) for prime and get a better (12.4@22) for base/clear. Sticker on comp says 1150 rpm, that might be original electric speed. Pump speed unknown. Car is fully disassembled/gutted and will be painted as such since I'm doing a color change and want it done right.
 
Last edited:
80 gallons @ 130psi should get you somewhere around 6 minutes of 10cfm spray time before you hit 30psi

Some marketing genius started selling compressors by tank size, which is dumb. But if you buy a compressor package its pretty safe to say there isn't a 1cfm pump on top of an 80 gallon tank. I expect that's the only reason you see people say 'a 60 gallon compressor is enough'

You can time how long it takes to fill the tank and calculate average cfm
 
If the 1150 was pump RPM, and you had a 2.4:1 underdrive (your pulley numbers), you had a 2750 RPM electric motor on it before.
The googler says 4k RPM on the Subaru for max power, you have a 6.6:1 underdrive now, which would put the pump at 606 RPM at engine 4k.
That's about 1.9:1 less than what you started with, so your CFM output would probably be about 9.5 at 100.
Since the gas engine won't actually stop, you'll need to have a bleed-off for when it get to pressure, or it'll just keep going till it pops the pop-off or blows something up, if your unloader does that already, great.
If you have 9.5 CFM at 100psi going through the gun and not regulating down before that, you can run a spray orifice of 0.075" (just over 1/16") with that. If you're regulating down on the way into the gun, I'd need to know what you're regulating down to, to have more info for you there.

Coming back to the depletion rate on your tank, if you have an 80 gallon tank, and a cubic foot is 7.48 gallons, you have a 10.7 CF tank. If you're running at 9.5 CFM, you're going to deplete that tank to zero in just over a minute (pretending that it stays at 100 PSI the whole time until it's depleted, which isn't the case). In reality, you're going to have things start to deterioriate consistency-wise probably 20-25% under spec pressure, so grossly oversimplifying, you have about 15 seconds of gun-running compressor-off spray time before things get wonky. On the other hand, if you're spraying at 12 CFM, but only have a 9.5 CFM compressor, you can run about 45 seconds in a blast, take a 15 second break, and the tank will buffer that overdemand on the compressor and let the compressor catch up. Similar ratio with paint swipes, if you run a swipe down a quarterpanel that takes 3 seconds, you let off the trigger for 1 second to reposition for the next swipe, you're good. If your ratio is worse, like you have an 18 CFM gun and a 9 CFM compressor, you need to have 50% breaktime in your run.

On the hose size, if you have a 50 foot 3/8" hose running 20 CFM, you'll see a pressure drop of 10 PSI with a 100 PSI source. So 100 in the tank = 90 at the gun on the other end of that hose. Run 1/4" hose for the same operation, and you're out past where my calculator works properly, it says 79 PSI (if it returns more than 14 PSI, your setup is bad). If we drop it to 10 CFM, 3/8" hose is at 2.9 PSI drop, 1/4" is still bad at 22 PSI drop.

This is all assuming you're running the gun at 100 PSI; I doubt that's the case, and regulating it down generally makes things better, but need to know pressure at consumption point (what the gun wants) to be more accurate there.
 
They are different connectors for automobile spray painting. The bores are larger. Called high-flow.
 
Last time I sprayed a bunch with a harbor freight gun my 5 hp 80 gallon compressor would run maybe once or twice per cup. I generally set my pressure as high as I can on the compressor regulator when I'm painting and then regulate it down at the gun. I also run a little inline dryer at at the gun in addition to the water trap at the compressor.

Cfm at the gun is regulated by the orifice size and pressure. More pressure, more CFM, bigger orifice, more CFM.
 
You can also cheapo-flow-meter your compressor if you want to be more knowledgeable about what it has for performance.
Basically, if you have a dump to atmosphere of known size, and a steady pressure feeding that, you can calculate the flowrate.
So if you dump your tank through a 1/16" hole, and it holds a steady 100 PSI, you are moving 6.5 CFM.
If you have a 1/8" hole, holding steady 100 PSI, that's 25.9 CFM.
Or going the other way, if you believe you have an 18 CFM compressor at 100 PSI, it should maintain 100 PSI dumping through a 0.104" hole.
If the pressure holds steady at lower or higher, that indicates higher or lower CFM flow; if you give me the hole size and the pressure it maintains running, I'll run the calc on it and give you the flow result. Engineering toolbox IIRC has the same calcs online if you want to go that way.
 
How accurate is the "start with tank empty and time how long to reach 175 psi" test for determining cfm?
 
You are overthinking this a little bit. Your gun doesn't care what cfm your compressor is, what size the tank is, or what pressure is in the tank. All it wants is 25-45 psi of clean dry air.

Your tank will hold 125+/- psi of air at 80 gallons is quite the volume. So with an additional pressure regulator set at 30psi you will have a long runtime before your compressor kicks on. As long as your pressure in your tank doesn't fall below 100 psi and your hoses aren't a mile long you will have adequate cfm at your gun.

As long as your painting and compressor is cycling, then it is keeping up. If it doesn't then just stop for a few moments for it to catch up.
 
You must log in or register to reply here.
Top Back Refresh