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Old 07-04-2008, 08:29 PM
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building a gas powered air compressor

I would like to put an air compressor on my truck or trailer. It seems like every car I pickup now has locked up brakes or some other problem so they won't roll and they always have flat tires. Money is tight right now so this project has to be cheap. I have an air compressor pump I got free and an engine off a rototiller also free. I know I need an unloader valve and an air tank. This is the one i was thinking of. Is there anything else I need other than plumbing? I don't know much about the pump or motor I have so any help there would be appreciated. I also want to know if this will work and be safe.
Thanks for any help
Brock
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Old 07-04-2008, 09:31 PM
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Yes this most certainly can be done but it does get a bit complicated. You definitely are shopping in the right place for parts and Surplus Center should have everything you will need including the unloader valve (actually it would be used as a dump valve in this case). That appears to be the very common single stage pump used on many different brands and models of electric compressors and if your motor is the 5 HP Briggs I think it is then it should work OK. In addition to the unloader valve you will also need a pressure relief valve, an idle kick-down control and a back-flow valve to prevent the air from flowing back from the tank to the compressor, this back-flow valve is not quite as important on a gas powered unit but you still should use one. Without knowing the displacement or CFM delivery of the pump it would be hard to say just what speed the pump will need to run but for sure plan on keeping it well below 1000 RPM.
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Old 07-05-2008, 02:17 AM
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Thanks for your help. I think I found the parts I need at Surplus Center. If these are the correct parts I will order them with my next check. unloader valve, pressure relief valve, back-flow valve, and on ebay the idle kick-down control. I used their picture from the unloader valve and changed some things. Do I put a T in where the red circle is between the check valve and the tank to get my air?
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Old 07-05-2008, 08:32 AM
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That should do it, Could I suggest this check valve for the tank?

http://www.surpluscenter.com/item.as...ame=powerTrans


I think this one will work better and will be a little easier to plumb in, plus it's cheaper.
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Old 07-05-2008, 09:46 AM
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If you were only needing to inflate tires, there is a much easier way.

Mount an older piston style automobile A/C compressor on the truck motor. All the guys who run the sand at the beach do that.

No tank needed, just an electric pressure shutoff switch from a old compressor, a safety blowoff valve and then an air coupler all mounted to a T fitting. One step up would be to but an idle solenoid to raise the rpms when the pump clutch kicks in.

I still have one truck here with the setup on it and can put up a pic if anyone else is wanting one. I did run a tank on the first truck also...came in handy for other things besides tires.
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Old 07-05-2008, 04:17 PM
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Not to threadjack but has any one ever built one of these for stationary use, ie installed in a home shop. I can see some draw backs but I would think it could be done for a comparable price to a regular electric motor driven unit. Any thoughts??
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Old 07-05-2008, 05:15 PM
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That is a 5hp briggs and stratton, with the diaphram carb. if it turned out inadaquate in HP, those can be tweaked on the cheap..
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Old 07-05-2008, 05:33 PM
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Quote:
Originally Posted by oldschoolrods
Not to threadjack but has any one ever built one of these for stationary use, ie installed in a home shop. I can see some draw backs but I would think it could be done for a comparable price to a regular electric motor driven unit. Any thoughts??

Yes it would absolutely break you up in fuel costs!

They are OK for occasional use or for something like 47 is talking about but not for shop use. To equal a 5 HP (true 5 HP, not "peak") compressor would take at least a 10 HP gas engine and to get any real efficiency out of it you would need a two stage pump so the costs to build it would be more than a good electric compressor long before you would finish it unless you had a lot of the parts lying around already. When you figure just how much time a shop compressor runs and gas costing over 4 bucks a gallon the costs to run it would be staggering.
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Old 07-05-2008, 09:11 PM
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Quote:
Originally Posted by oldred
Yes it would absolutely break you up in fuel costs!

They are OK for occasional use or for something like 47 is talking about but not for shop use. To equal a 5 HP (true 5 HP, not "peak") compressor would take at least a 10 HP gas engine and to get any real efficiency out of it you would need a two stage pump so the costs to build it would be more than a good electric compressor long before you would finish it unless you had a lot of the parts lying around already. When you figure just how much time a shop compressor runs and gas costing over 4 bucks a gallon the costs to run it would be staggering.
Yeah thats a good point, electric = 13 cents per kwh, gas = $4 gallon if you figure 5 HP is rougly 3.7kW the cost of the electric compressor for 1 hour is about 50 cents, gas would be atleast $4 on a good day. so yeah I guess this is not a great alternative.
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Old 07-05-2008, 11:48 PM
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I am doing this because I have been scrapping out cars for people to make a little extra cash since I got laid off. I was looking at buying a gas powered one but I just can't justify $800 for one and building things and learning is more fun. I will be ordering the parts to build mine next week. I will have about $150 into my setup but I would think that even with a new pump and motor it could be built for $400. Thanks Oldred for helping me to get started. I'm sure I will have more questions as I go. I will try to put up pictures when I build it.
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Old 07-10-2008, 05:38 AM
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You can convert a Ford 302 to be an air compressor. It uses the left bank of cylinders as the engine and the right bank as the compressor. These are fairly common in the construction business... running jackhammers, etc. Somebody out there makes a kit.
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Old 07-10-2008, 07:58 AM
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I have a home made (sort of) 5 HP briggs compressor. I bought it from a friend in 1988. He was a contractor and had a 20 gallon 2 HP electric compressor he used.

The electric motor went out and he had a new 5 HP briggs...hmmm...why not?

He swapped the engine on and used a pop-off valve. It works quite well, but cost him too much (even back then) to run all the time. Plus when the valve pops...it is horribly noisy.

I use it in salvage yards and outside around the house. My neighbor roofed his house with it. Dad and I painted his neighbors house with it. We even spot painted a few cars with it. It is very convenient for that sort of thing, but not good for a shop.

I love having it around even with the larger electric compressor we have now. It is handy for a lot of things.
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Old 07-10-2008, 09:08 AM
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Brian, I hope you are not suggesting this as a simple way of building a compressor because not only are you wasting at least half the fuel, probably even more than that, that thing is dangerous as a cocked pistol! The only thing standing between you and a dangerously over pressurized tank is that pop-off valve which is designed ONLY to work occasionally, fortunately when they fail it usually sticks open but if it sticks shut you may be in for a world of hurt! As you can see from the parts 47 has listed it would not cost much at all to buy an unloader valve and throttle kick down and set that thing up right, the small cost for parts would be paid for almost immediately in fuel savings. You could get by with just an unloader but I would not recommend doing that, A POP-OFF VALVE IS NOT AN UNLOADER VALVE and should NEVER be used for one! That pop-off valve is meant only for an emergency in case the unloader system fails or the switch on an electric outfit fails and should NEVER EVER be used for anything else, certainly not to regulate the tank pressure!
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Old 07-11-2008, 12:10 PM
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Somewhere they have stuff-information on how to make a compressor from a VW air cooled engine. 1/2 if it is the compressor.

Also you might want to add a 2nd tank to this. I didn't see how big the tank was. On a cheap 110 volt compressor it seemed to be always running. I added a 2nd tank and it would last longer with the pressure I wanted, but it did take twice as long to get back up to pressure. So it is always a trade off.

Last edited by PapaG; 07-11-2008 at 12:15 PM. Reason: Add some more
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Old 07-11-2008, 05:33 PM
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There really is nothing new about those 50/50 engine/compressor outfits and they have been around just about forever. Back in the fifties the old Ford flathead was popular for this conversion with more than one company building them. I have seen them (commercially built) from Ford 302s, 351s, VWs and Buick V6s, at least one outfit is still building the V6s but these things are not really good for much unless you need a heck of a lot of air. They work OK on a large service truck that might need to run something like 1" drive impact wrenches for extended periods and such as that but otherwise they are more trouble and expense than they are worth. They are for commercial use and will cost a ton of money to operate plus they are extremely noisy so there is really very little demand for them.
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