Questions about Air Compressors, CFM and Impact Wrenches

[MazdaSpeeder]

I'm looking at a great impact wrench that would be perfect for working on my rust-riddled P5. The problem is, I'm trying to figure out the best compressor to go with it. It has a working torque of 320 ft/lbs and an average air use of 4.9 CFM. Here's my conundrum...I can't break the bank on a compressor, and getting one that is a 240v is also not an option. So that leaves me with 120v options that really don't make much more than about 2.5hp, and the best I have seen as far as airflow is 4.5 CFM at 90 PSI.

If I bought a compressor like the one above, would it be enough to give me the 100-200 ft/lbs I would need to do work on my car? I realize that compressors (especially cheap ones) can have inaccurate hp and CFM ratings. I really just need enough torque to break loose suspension bolts, exhaust bolts, maybe the occasional wheel hub nut. Would a cheap compressor flowing 4ish CFM at 90 PSI give me that?

 

[pw~mp5]

The air compressor you described should not have an issue running the 320 ft/lb impact wrench for home shop use. You will receive the full torque of the wrench as long as the pressure in the tank is above 90psi. Below 90psi the torque of the wrench will drop off. A larger compressor would give you the ability to run at full capacity "continuously" (most compressors need to have some time off to cool). The CFM is more important with sprayers, blaster, sanders and other tools that have a higher duty cycle than wrenches/ratchets.

 

[MazdaSpeeder]

So, even though the compressor at 90 PSI will flow .5 CFM less than the average need for the tool, I should get enough torque for operation at the level I need it. That's what I thought, but I don't want to invest in a system and then find it's not what I need.

 

[pw~mp5]

The 4.5 CFM is the output of the compressor pump, not the tank. As long as the the pressure in the tank is above 90psi the CFM only limited by the diameter of the outlet, attached line conditioners/regulators and the tool consuming the air. The 4.5CFM @ 90psi is what the compressor pump can maintain while it is running. If you let the pressure build until the pump shuts off you should have 120-140psi depending on the compressor. You can then run any CFM tool until the pressure drops below the tools recommend pressure, 90psi for your wrench. You should be able to remove or tighten several bolts before the pressure drops below 90 and the pump has to refill the tank.

Worst case scenario you could remove/tighten a few bolts then you have to wait for the compressor to rebuild the pressure before you could remove/tighten more. The size of the tank would determine how many bolts you could work between waiting on the compressor to catch up. With a larger compressor, you would never have to wait once the pressure reached 90psi.

 

[pw~mp5]

Another option is an electric impact wrench, less expensive than a compressor/air impact wrench setup and easily portable.

http://www.dewalt.com/tool-categories/cordless-impact-driverswrenches-high-torque-wrenches.aspx
http://www.dewalt.com/tool-categories/fastening-impact-wrenches.aspx

 

[MazdaSpeeder]

I looked at those, but the big draw of this impact is that it is a dual hammer design and only 4 inches long, so I can fit it in all kinds of tight spaces. I have found this to be an issue in the past when working on my car, so I wanted to go pneumatic for size.