New Cams Fuel Pressure

[bNasty]

What should my new fuel pressure be after installing the cams. Ive left it stock (fpr) 43.5 psi but its strikes me that it should be different now that im getting only 10 inch of mercury of vaccum, should i leave the vaccum line in and just adjust it to 33 psi?

 

[steve]

No you shouldn't change it. The ECU expects the injectors to deliver at that pressure. The FPR takes care of the changes in manifold pressure/vacuum so that the injectors deliver the same rate that the ECU expects.

 

[jrohner]

I don't understand why you'd think different cams would require a change in fuel pressure. The reason there is a vacuum line going to the FPR is because a change in manifold pressure needs to be compensated for in fuel pressure at the injectors, otherwise you'd be richer in vacuum and leaner in boost. Changing the fuel pressure changes fuelling. If you use closed loop like stock, changing it will mess up your fuel trims; if you use full-time open loop, changing it will change your AFRs.

 

[bNasty]

I understand, I was just curious becuase now that i have less vaccum it seems like my fuel pressure would be less when i plugged the vaccum line back in?

 

[steve]

If your fuel pressure didn't drop when you reconnected the vacuum line after setting it to 43.5 psi you have a problem with the regulator or the vacuum line is plugged. It should have dropped about 5 psi if you have 10 in hg of vacuum.

 

[jrohner]

You would have MORE fuel pressure at less vacuum. 10inHg vacuum is -5psi so your fuel pressure should be LOWERED 5psi -- 43.5-5=38.5. 18inHg is -9psi, which would be 34.5psi. When your boost gauge reads 0 cruising, you're at your base fuel pressure. Anything in vacuum lowers it, boost raises it; it's all in a 1:1 ratio. This is all done because the injectors have to overcome the pressure in the intake to give you the right amount of fuel "everywhere on the boost gauge".