2 bar at least. I have an upgraded fuel system with bigger lines and two pumps. I have the HKS twin power also. I haven't boost over 19 psi yet because at the time I was only running one pump. Didn't want to risk blowing the engine just see what the limits of the stock pump is. Now with two walbor 255lph pumps it sounds like a mini water fall with all the fuel returnnig back to the tank. I am going to run one pump full time and the other pump to kick in via a pressure hobbs switch.