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Get an old 2 stroke diesel, maybe a Detroit Diesel 3-71, or 3-53; they make other sizes also. You simply reve the fuel injectors and block the holes, then drive the engine with another engine (100+ hp), and collect the air from the exhaust manifold. You will need a 6-71 blower. The blower will need an aluminum adapter to fit the smaller engine. Given good measurements, you could esign the adapter and a machine shop could cut it for you.  

I’ve just found this list and am in the process of converting a little 4 cyl into an air compressor for use with a jackhammer.

I used to drill wells used many types of rigs. The Schramm T64 was one of my favorites. It used a straight 6 to produce 230 to 250 cfm at 180 to 230 psi(diven by an 318 Detroit Diesel).

Most drillers are using screw compressors now, less noise and better fuel economy.

I am going to run my compressor with a farm tractor (pto). I may someday use another small 4 cylinder to power the compressor and mount both on a trailer. For now the goal is to just get about 50 cfm at 90 psi.

I will disable the exhaust valves so they remain closed at all times (does away with air loss because of over lap). Remove spark plugs and pipe together with check valve between each cylinder.

I can also plumb it to give me a high pressure by diverting 3 cylinders into the 4th ( if the engine can take the pressure) just a matter of additional check valve and switching valve.

Trying to run the engine and compress air by removeing a cylinder from it’s power cycle is very damaging to the engine.

You must balance what your doing so you would have to take at least 2 cylinders and most engines will not run well under these conditions.

Look into DynaCam http://www.dynacam.com. It is a 12 cylinder engine with 6 double ended pistons, barrel rotary engine. It has been use for this purpose but cost 50 thousand dollars.  

When I first played with autos (1940s) converting two (never just one) cylinder to air compressor was common. Montgomery Ward sold kits for certain engines, such as the A-bone and 60hp Ford V8.

It was common on construction jobs to see trucks with the hood up and an airhose coming out.

I have an idea you can find a book which tells you how. Try www.Lindsaybks.com.

The engine must be kept balanced (which is why you don't convert just one cylinder) and since it was done frequently with fours in the twenties and thirties, using a V6 or V8 should be a comparative snap.

Don't drop the idea--although I have to agree that even high volume compressors have become so cheap that I would be inclined to get one of those and put your engine to it instead of a 20hp gas two cylinder.  

I have read alot of negative replies about how dumb this is and how it won't work.  I am looking to do this, and my grandfather build a compressor out of an engine back in the 50's or 60's, and stated that it was fairly common practice.  I don't think this would be very practical to replace a small electric compressor around the house, but I am looking to use it for sandblasting.  The way it was done is as follows:  You need a V8 engine with a dual plane intake manifold.  You cut off the fuel from one side of the carburator, which causes the engine to run on 4 cylinders, the inner two on one side and outer two on the other.  You then put very weak valve springs on the pumping cylinders (the ones with the fuel cut off) which would make them act as reed valves, and remove the exhaust rocker arms and pushrods from those cylinders.  I will probably have to figure out a way to keep the oil flow correct in the lifter galleys, as those lifters will likelely need to be removed.  this could be solved by having those exhaust cam lobes turned down so the valvetrain could be left in place but the exhuast valves wouldn't open (they didn't have to worry much about this in the 50's or 60's, everything was solid lifter and didn't have galleys).  Then you would need 4 one way check valves and plumbing to screw into the spark plug holes.  These would be plumbed so that the engine could only pull air in throught he intake valves and exhaust the air throught the spark plug plumbing and into an air tank.  Then there will need to be some way of controling the motor speed, I am thinking a solenoid controled by a regular air compressor switch, with a bleed for the air generated at idle.  The way I figure it, you could build a nice setup  for less than 1000.  The real advantage of this type of compressor is that it will deliver a huge flow.  Even figuring on 25% volumetric efficiency, a compressor like this build on a 350 cid engine will deliver 75 CFM at 3000 RPM, great for sandblasting.  You are limmited to the compression ratio of the engine as far as how much pressure it will pump, but most engines will make 90 PSI.  There will likely be some engine oil present in the air that will need to be separated out.  I actualy stumbled onto this thread because I am looking for some affordable one way check valves, everything else I havn't had much trouble finding.  Hope this helps clear some of this up.  

I don't even know if that is possible, but if it is, you will lose almost 0.5L of engine, because you would sacrifice one cylinder for air compression purpose. I don't think it would be benifitial. If that is possible you really should consider costs and hazards to this modification. It would be beter to get just a very good supercharger for your car. It wouldn't be so hard to modify your engine to that and it would be safer for your engine too.