one more question. I have a 1 in pipe from the well at 38 to 58 psi. The volume is about 12 GPM. My question is there any benafit to running 1 1/4 PVC from the valve to the heads (MP3000)? Is there a down side ?
On the plus, side, larger pipes have less pressure loss. You might find that using 1" pipe everywhere is good enough. If so, then it's cheaper and simpler to just do everything with 1" pipe. But if pressure losses are such that you don't have enough pressure left at the heads, then you have to find ways to reduce your pressure losses, and larger pipes are one of the ways.
On the down side, larger pipe is usually more expensive. From an operation stand point, there's almost never a negative to using larger pipe. But it obviously can hurt more in the wallet.
To keep your velocity below 5 feet per second, 12 GPM is the maximum you want to push thru. Assuming sch 40 PVC, at that rate the pressure loss is 3.35 PSI per every 100 feet. (as opposed to a loss of only 0.88 PSI/100 feet with 1 1/4 inch). So if you have very long runs, or are operating at a pressure near the heads lowest limit, no need to upsize.
Another disadvantage comes to mind....more water to drain afte the system is off.
There really is no difference of pressure loss with oinly 12 gpm. It it very minor and not worth the money unless you are going over 50 feet to get where you have to go.