Question
I have 85psi and 20 GPM.  Can I run 10 Rainbird 2.5 inch popup heads with that?

Answer
Hey Jeff, I hope this helps. Each head you use needs a nozzle for it. A nozzle is simply a piece that screws on the top of each head, and sprays a certain pattern (90 degree spray, 180 degree spray, 360 degree spray, etc.) and also how far it will throw (8 feet, 12 feet, 15 feet, etc.). Each nozzle will put out a different GPM. You will need to look at a nozzle chart from Rainbird, to determine how many gallons per minute (GPM) each nozzle puts out. For instance, lets say you have a nozzle that is going to spray a 180 degree pattern, and throws 12 feet, it will be a 12H nozzle, 12 for the distance and H for half (180 degrees). Lets say you check the chart and at 85 psi it uses 2GPM. If you put on 10 heads with a 12H nozzle, each one is going to use 2GPM so this zone would use 20GPM (10 heads at 2 GPM = 10x2=20GPM). So in this example you could put 10 heads on one zone (valve). Lets say you have a nozzle that throws a 360 degree pattern, and throws 15 feet. We check the chart and see that a 15F (15 foot spray and 360 degree pattern) puts out 4GPM. This means that we could only put 5- 15F heads on each valve (5x4GPM=20GPM). As far as your question goes, yes you could put 10 heads on each zone, depending on what nozzle (GPM per each nozzle) that you use. Good luck and let me know if you have anymore questions, thanks and have a good day.