HEllo!!
This is my best air compressor the instructions may be a 
bit blurry but ut was the best i could get with my video 
camera.
THe last picture shows how to make a 4 pump compressor
with a medium motor. the long axles are 12L.
to use an xl motor for more power, just add it like in the 
instructions
see the 2 pump version in action at:
http://www.youtube.com/watch?v=N6eCelmeYqI
see the 4 pump version at:
http://www.youtube.com/watch?v=k9-P60KWxTE