Fuel pressure and injector size

[Wicked One]

I just installed a fuel pressure gauge and it is reading 39#'s at idle. Now when I upgrade from the stock 19# to 39# injectors, will this change?

If I am getting 39#'s now, why would I change the injectors.


Just got me thinking.

 

[AbdullaGT500]

It's not idle that you need to worry about.

 

[JeremyH]

The fuel pressure gauge just measures fuel pressure at the rail, has nothing to do with you injector size. BTW the fuel pressure at idle should be around 27-30psi. edit

 

[jchambers1]

BTW the fuel pressure at idle should be around 25psi.

Fuel pressure at idle or any time under manifold vacuum should be roughly 39psi - (vac/2) where vac is in in-hg. This is assuming your tuner has the base pressure set at 39 psi. Under boost should be 39 + boost.

 

[Wicked One]

Unless my gauge is screwed, I am getting 39.

makes sense about now, the rail as opposed to the injector. Thanks.

 

[JeremyH]

Fuel pressure at idle or any time under manifold vacuum should be roughly 39psi - (vac/2) where vac is in in-hg. This is assuming your tuner has the base pressure set at 39 psi. Under boost should be 39 + boost.

Nope. Your confusing delta pressure with fuel rail pressure.

With the car maintaining a delta pressure of 39psi, at idle with a vac of 22-23 in/hg (-12psi) the car should idle around 28-30psi, thus maintaing a delta pressure of 39psi. Just as boost drives the rail pressure higher, vacuum takes it lower.

Btw 25psi as i stated above is a little low and you would need a strong vac, my car idles around 28-29 psi.

 

[JeremyH]

Unless my gauge is screwed, I am getting 39.

makes sense about now, the rail as opposed to the injector. Thanks.

Yeah your pressure should not be that high at idle. What gauge?

 

[jchambers1]

Nope. Your confusing delta pressure with fuel rail pressure.

With the car maintaining a delta pressure of 39psi, at idle with a vac of 22-23 in/hg (-12psi) the car should idle around 28-30psi, thus maintaing a delta pressure of 39psi. Just as boost drives the rail pressure higher, vacuum takes it lower.

Btw 25psi as i stated above is a little low and you would need a strong vac, my car idles around 28-29 psi.

I was speaking of gauge pressure that you'd see on your in-car gauge. Sorry for the confusion. Just trying to point out that to state "should be" can be somewhat misleading. A car making 10 in-hg at idle is going to show differently on an in-car gauge than one making 20-22 in-hg. Your on the money with your car at 28-29 with the vac you're pulling.

 

[Wicked One]

I am using an autometer elect Fuel pressure gauge.

I will hook up my boost/vacuum line tomorrow and see what that reads.

Thanks guys, Was not concerned, but needed some info so I know what the hell is going on.

 

[Vapour Trails]

The only way you would get 39 psi is if your engine was creating zero vacuum at idle, and that can't be.

Something is not right. You should be roughly 10 psi lower.

How is the gauge connected to the fuel rail.