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Message: <blockquote data-quote="shovel" data-source="post: 1545119" data-attributes="member: 29855">Fuel pressure should be about 3 bar, which is about 43psi.    There is very little real difference in fuel delivery between 40 and 45 psi so it does not need to be precise and never will be. Inside your intake manifold is a partial vacuum while the engine is running - not like outer space but less than ambient.  The business end of the fuel injector is in the same air volume as the intake manifold so when your manifold has vacuum the tip of the injector also has vacuum.   At sea level absolute pressure from the atmosphere is about 14.7 psi,  so your fuel pressure with the engine stopped is about ~40psi above that or about 55psi above outer space.  With the engine running at idle with the throttle closed you might have about 20in hg vacuum which equates to about a 10psi difference  (air is pushing on the outside of the manifold with about 10psi,  the air inside the manifold is about 4-5psi above outer space)Since the fuel pressure regulator's job is to maintain about a 40ish psi difference across the injector that means you would measure 40psi compared to atmosphere with the engine off,  or about 30psi compared to atmosphere at idle with the regulator connected to manifold vacuum.   Either way the fuel inside the rail has about a 3 bar difference to the tip of the injector where it shoots out. With the throttle open the engine will speed up and demand more fuel but also there will be less vacuum in the intake manifold,  in a perfect NA engine it would be no vacuum but that is impossible.    More often you might have 1in hg which is about 0.5psi of difference between the atmosphere outside and the inside of the manifold.   So at WOT under load ideally you would be back up to 40psi roughly.    Of course that is not really something you can test while parked unless the car is on a dyno to supply a load to the engine. If fuel pressure drops at WOT it is likely you have a fuel delivery problem like plugged filter or failing pump,  or I know this does not apply to you but if someone has built a very powerful engine and not upgraded their fuel pump and electrical wiring it might just not be able to supply the volume needed.</blockquote>