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Why do you need to set a afpr with the vacuumline disconnected?

Nobody who can explain?

Is this relevant to anything?

Yes, it's relevant. Because many people don't know why you should set an aftermarkt afpr with vacuum disconnected.

Me neither.

To prevent making errors by not installing something the proper way.

Google search, 10 seconds:

Q: How do I correctly set the fuel pressure on my new adjustable fuel pressure regulator?

A: The correct way to adjust your fuel pressure is with the vaccum line of the fuel regulator disconnected. The instructions with many adjustable fuel regulators don't tell you to do this, but it is essential! You must remove and plug the vaccum line going to the regulator or you will be getting false readings, similar to adjusting the timing on vacuum advance distributor. What you are attempting to do is set the maximum fuel pressure. At idle, fuel pressure is reduced due to high vacuum, so the presence of the vacuum line is detremental to fuel pressure. Repeat the process until you have the desired reading.

Additionally a couple veterans of MX3 told me we can just leave our stock FPR in place, remove the vac line then add the AFPR ahead of it. I guess if it fails you just hook up your stock vac line and your back in business. Cheers

Oke, thanks

But leave the original fpr? The afpr takes care of the pressure left on the fuelrails. If you'd leave the orginal one in place it will go through that one first before it reaches the aftermarket one.
This would eliminate the whole principle of installing an aftermarket afpr