Ron,
Here are a few choices for AR Pumps for a HP Washer using your 20 HP
Engine, you should be able to run around 8 GPM @ 2500, or 9 GPM @
1600 - 2000 PSI.
Model GPM PSI
XWA8G35 8.0 3,500
XWA9G16 9.0 1,600
XWA9G24 9.0 2,400
For a higher volume building wash and street sweeper application you can
get away with a 5.5 HP Honda and a Flomax pump, like we build. 20- 40 GPM
@ 40 - 90 psi.