Fuel Proper Way to Check Fuel pressure

[93gtmustang]

So I saw this thread from 2020.

Fuel Pressure Regulator Help

Alright guys I've been searching the internet for two days now with no luck, so to keep me from banging my head on my desk im hoping someone here can chime in and help me out. So I have a 91 fox with ERG delete- BBK 70mm throttle body- Trickflow Track Heat Intake - RPM Performer Heads-...

stangnet.com

I have a Holley adjustable fuel pressure regulator. I've always checked / adjusted my fuel pressure with the vacuum off the regulator. I saw in this thread to take the vacuum line off and plug it. Just wondering what the proper way is? You would think taking the vacuum line off the regulator, and then plugging it was the same as leaving it on the regulator in the first place?
Thanks for any help!

 

[Bullitt347]

You plug the line after removing it from the regulator so you don't have a vacuum leak while the engine is running.
If the vacuum line is not on the regulator, then it can not move the diaphragm inside the regulator that causes the pressure to change.
The result is a reading of the base fuel pressure that the regulator is set at.

 

[93gtmustang]

So I just checked the fuel pressure to give some numbers to the forum for reference.
Car running.
38 psi vacuum on the fuel pressure regulator
42 psi vacuum off the fuel pressure regulator not plugged
42 psi vacuum off the fuel pressure regulator and plugged

 

[AeroCoupe]

Your pressure should be 39 psi with the vacuum unplugged unless the motor is needing a little extra fuel. Typically it’s not advised to bump the fuel pressure as the tables in the ECU are based on 39 psi. If it has a chip and been tuned then that’s different.

 

[93gtmustang]

Yup 39 psi is stock setting. I moved it up to 42 psi years ago because the car felt like it was falling on its face after the 1/8 mile at the drag strip. 42 psi seems to work for that. It's a trailered drag car only now. It ;s not tuned , no chip. Plugs read good.

 

[93gtmustang]

Your pressure should be 39 psi with the vacuum unplugged unless the motor is needing a little extra fuel. Typically it’s not advised to bump the fuel pressure as the tables in the ECU are based on 39 psi. If it has a chip and been tuned then that’s different.

I'm going over the car today for next weekend at the track. I'm going to set the fuel pressure to 39psi and see how it goes. It makes sense. If the ECU tables are based on 39 psi, why set it at a different number.