At idle it's about 40 psi. Under light throttle it will actually decrease to about 36 psi, and when I move the throttle to WOT it comes up to 44.

What would possible be the cause of the pressure drop at partial throttle driving???

 

The vacuum tube going from the intake to the FPR reduces absolute fuel pressure under low load conditions so you always have the same differential in PSI between the intake and the fuel rail.

At WOT manifold vacuum = near zero, meaning manifold pressure is the same as atmospheric pressure. At part throttle though it could be as low as half that...which means the total fuel pressure seen at the tip would be greater than what you read in the gauge. I know Injuneer has explained this better but basically what you are describing is normal.

 

I talked to Jay Fisher today and he explained to me that as MAP decreases, Fuel Pressure should increase. He explained that as the throttle blades open, decreasing the MAP, the fuel pressure should increase, not decrease.

Another question would be, what is the threshold fuel pressure for the injectors to open up all the way?? If there's not enough pressure they'll only crack open, and instead of a stream you'll get a mist. Jay explained it in this manner calling it "pissing". I'm curious to know what the threshold pressure for them to correctly operate is. I'm running SVO 30# by the way.

 

You may have misunderstood Jay.... he should know the correct info. MAP is "manifold absolute pressure". As the throttle blades open, MAP increases. When MAP increases, rail pressure should increase.

MAP is the pressure in the intake manifold, referenced to absolute pressure, or in effect, referenced to a true vacuum. Looking at it another way, MAP is sort of the "opposite" of vacuum. As the throttle blades open, MAP increases, vacuum decreases.

If you are at sea level, the normal (absolute) barometric pressure is about 30"Hg, and that is the only thing that pushes air into the cylinder as the piston drops. You want as much as possible of that pressure to exist in the intake manifold, to fill the cylinders.

At idle, the throttle blades are closed, and that reduces the amount of pressure available in the intake. In a car with a stock cam, that MAP has been reduced to about 10"Hg. And vacuum is about 20"Hg. See the relationship....

BARometric pressure - MAP = vacuum

With your cam, you might see only 15"Hg MAP (or vacuum) at idle.

So... at idle, fuel rail pressure should be equal to 43.5psi minus the vacuum... or about 35psi. When you open the throttle all the way, pressure should return to the 43.5psi setting. Since the factory tolerance on fuel pressure is +/-2.5psi, you might see 38psi at idle and 46psi at WOT.

In between idle and WOT, the rail pressure will fluctuate with load. When you are coasting down with the throttle closed, you might see only 6"Hg MAP, so the fuel rail pressure could dip closer to 30psi.

I don't quite understand the comments about the injectors "pissing". The injector coil pulls the injector open, overcoming the pressure in the rail. It is going to go all the way open as long as the pressure in the rail isn't ridiculously high (high would be something over 60psi with stock RP's and over 100psi with SVO's, for example). And a lower rail pressure only makes it easier for the electric coil to pull the injector open.

And.... injector flow, and atomization is controlled by DIFFERENTIAL, NOt by rail pressure. That is the function of the vacuum compensation line - to control DIFFERENTIAL pressure. NOT the rail pressure. Differential pressure is the rail pressure, minus the intake manifold pressure. When the MAP is low, the rail pressure must be low, to keep the differential pressure across the injector at the 43.5psi that it is flow rated at.

It does seem odd that you get a small decrease in fuel pressure as you put the engine under load by starting to open the throttle. That would indicate that maybe you aren't properly tuned for max possible vacuum at idle. After reading your other post about "what is normal MAP", is appears to be that you already suspect you have a problem.

 

Man, good informative explanation. Thanks for that, it explained a lot.

That said, in backup to the possibility of a MAP problem, I'm showing anywhere from 18 to 20" HG map at idle.

At idle I'm pulling about 40-42 PSI of fuel pressure from the schrader valve on the rail.

I thought this was right, but according to what you just informed me of it isn't??

 

Well, I'm not not sure which car you are even talking about, and in the other post on MAP you usaid you couldn't remember ..... but in any case, niether of the cams you are using should put you at 20"Hg at idle, IF they are tuned correctly. My 230/242 114LSA runs 15"Hg at 800rpm.... although that is in a 381, which will dampen the effects of the overlap.

I may be wrong... maybe 20"Hg (= 10"Hg vacuum) is the best you can get with a 224/230 in a 350ci engine. Maybe someone else who has this cam can provide some input.

 

Well, I just was made aware that Datamaster shows what the map stores as the baro, which is where I got that number from.

I'm speaking specifically of my 224/230 car btw.