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If you are planning on using it heavy, I'd stay away from the underhood compressors. They are a pain to install and a bigger pain to work on in addition to jamming up an aready jammed up engine compartment. Had lots of problems with factory mounting brakets breaking as well. Depending on the type, size, style and connection to the engine, they can also be hard on other accessory driven items such as water pumps, alt's, PS pumps, ect. PTO driven underbody mounts are the most effective heavy duty use ones as long as you don't mind running up the trucks engine hours. You can get these in both direct drive and belt drive. There are many different styles of these for different applications. Belt driven ones can be run from a sheave on the PTO shaft or by a hydraulic motor driven by a hyd. PTO pump. Some allow for a direct drive hyd. motor connection as well. Hyd. driven is the best way to go if you have low underbody clearance which would cause problems using a shaft driven unit. Shaft driven are the least complex and problematic simply because they are less complex. It's not going to save you any money in the set-up, most underbody compressors and good quality underhood compressors run as much as a self contained engine driven one. As far as RPM's, you have to set this for the pump itself. Obviously with a hydraulic or belt drive, you can adjust for PTO speed by changing oil flow/pressure with a regulator or valve and with belts you can change sheave sizes to get within the limits. Most compressor pumps run 650 to 950 rpm, with some exceptions for special application pumps. PTO come in a various ratios but you need to know all the info on the tranny and the required speed of the pump in order to get the correct PTO. If you need any specific info on a particular set-up, email me. I've been installing and repairing all of these for many years on all sorts of trucks. The amount of information and variables is far too great to try and sum up with a general post like this. Be glad to help you out and keep you from making mistakes that will end up costing you serious money to fix. I've delt with at least 250 different compressors and set-ups over the years.
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