Air compressor pump

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Im trying to build up a air compressor. While back i bought a 6.5 hp gas engine i got fairly cheap (brand new) on a deal. The other month i bought a pump for it. Turns out the pump has a leaking head gasket plus it doesn't put out as much cfm as i would like. Im waiting on the seller to see if i can return it since its brand new.  I wanted to build a solid 15+ cfm air compressor. So im looking for other solutions or another pump. I saw a pump at princessauto for 400$ that puts out 18cfm and seems to be a good solid pump. Only problem is i think its pricy. I would get an electrical one used but i dont have the room inside. I have seen some good deals too on used ones.
Reply:Where'd ya get the unloader valve?My name's not Jim....
Reply:Originally Posted by BoostinjdmWhere'd ya get the unloader valve?
Reply:i hate to tell you but i dont think a 6.5hp gas engine is going to be enough to run a 15+cfm pump you should be looking at something in the 8-10cfm range
Reply:Originally Posted by shanemi hate to tell you but i dont think a 6.5hp gas engine is going to be enough to run a 15+cfm pump you should be looking at something in the 8-10cfm range
Reply:Theoretically the math equates out to about 6.4 CFM/HP where HP = 0.030*P*((R^0.145)-1) where p is atm pressure at 14.7 psi and R is the ratio of compressed air absolute pressure to the pressure of the atmostphere. So theoretically you only need a 2.3 HP engine or motor to get 15 CFM. This also assumes you are using a two stage compressor
Reply:There is a difference between a 6.5HP gas motor and a 6.5HP electric motor, They are "rated" differently and do not compare apples to apples. An electric will also have an easier time handling the torque needed to spin the compressor, they make close to 100% of their torque at startup. With a gas motor you will need a larger motor to spin the compressor than they call for in an electric.
Reply:Originally Posted by ironmangqThere is a difference between a 6.5HP gas motor and a 6.5HP electric motor, They are "rated" differently and do not compare apples to apples. An electric will also have an easier time handling the torque needed to spin the compressor, they make close to 100% of their torque at startup. With a gas motor you will need a larger motor to spin the compressor than they call for in an electric.
Reply:Originally Posted by shanemcorrect  it will take a 8hp engine to pull a 15cfm pump but only a 3.5hp electric motor
Reply:Originally Posted by BCTimberwolfTheoretically the math equates out to about 6.4 CFM/HP where HP = 0.030*P*((R^0.145)-1) where p is atm pressure at 14.7 psi and R is the ratio of compressed air absolute pressure to the pressure of the atmostphere. So theoretically you only need a 2.3 HP engine or motor to get 15 CFM. This also assumes you are using a two stage compressor
Reply:From what i have seen having that CFM being produced in more than one cylinder can be like getting twice the air out of the same HP.  It takes torque to compress the air which is only happening on the up stroke of the piston. If the second cylinder is compressing with the other is doing a down stroke. Turns out that pump i was looking at getting is imported from china. I guess they usto come from US manufacture.
Reply:Originally Posted by rlitmanI have no idea how a 3.5HP electric motor can pump 15CFM.  Everything I'm seeing online that's 3.5HP is running more like 12CFM.