EFI-what fuel pressure are you running

[dean]

Hey,

I need to check my fuel pressure. What pressure are you running, where are you measuring it, and at idle I assume? I have a MAP signal going to the FPR but I didn't when I set it originally.

Thank you
Dean

[youngkang]

By no means I am an expert, so take it for what it is.
I set mine at 42psi and disconnected vacuum control.
I had vacuum/boost hose hooked to FPR and had heck of time getting proper fuel map. Once I removed vacuum control, much happier.

FYI, I have MS ecu with batch fire set-up and aeromotive FPR.

[GJF]

Hey Dean,
Most injectors are rated at 43.5psi. With the vac/boost line disconnected you can set it to 42-45 psi. Once the vac/boost line is connected depending on vacuum your motor draws at idle it should pull the pressure down minus the vacuum. So let's say you are running 43 psi and you have 6" of vacuum then you should see 37psi at idle. And then once you romp on it the vacuum drops to "0" and then the fuel pressure will return to 43psi. Then from there for every psi of boost the fuel pressure will rise 1 psi for every psi above 0. So 43psi fuel pressure plus let's say 5 psi of boost you should have 48psi of fuel pressure, 10psi of boost you should see 53psi of fuel pressure.

[GJF]

Oh and you should take your signal under the throttlebody in the middle of the crossover plenum. If you go too close to a side plenum you may get pulses from an individual intake runner. So by staying in the center will give you a steady signal.

[dean]

Thanks for the replys. So is there a difference if I measure on the fuel rail or the FPR? It also seems to me that having a MAP signal on the FPR is a kinda crude way of doing things.

[GJF]

There should be no difference if you take it from the regualtor or the fuel rail. As long it is the same circuit as both. Why would you say crude. With the signal form the plenum the FPR see's any change in the engine. Air density, barametric pressure, air temps which directly affect charge temps directly affect engine performance. With running the plenum signal this helps automatically adjust to the conditions the vehicle is in at the present time. As long as your base map is at the correct AFR's and your timing is happy it will all work in harmony. The MAP sensor helps with all these differences. Therefore the fuel has to match the changes the the MAP see's. If you run with a fixed fuel pressure the ECU has to work the injectors harder and not be able to match performance with the changing outside conditions.

[David]

I'm running 38 psi measured at the FPR with the vacuum hose to a TB port.

[dean]

I measured the fuel pressure and it is just like GJF said it should be in the above post.

It just seemed crude to me. I can change my fuel by .01 millisec. Not that it makes much difference. I know the map reference works but it just seemed more crudereer

Thanks
Dean

[rsrmike]

The reason your fuel pressure should be matched to manifold pressure is to correct for pressure differentials between manifold pressure and atmospheric pressure.

The injectors are tested at 43 psi and their flow rate is calculated against atmospheric pressure.

Spray them into a vacuum (part throttle intake) and their flow rate will increase as there is less resistance out of the injector.

Spray them into a manifold with boost on it and they will not flow the same rate as they are fighting the pressure in the manifold.

The changes in fuel pressure help to equalize this for the most part although there are variables and errors to everything...

You can read more about this in Electromotive's TEC manuals on their website.

[GJF]

Exactly rsrmike!

Now that you have the FPR sensing MAP, unless you were lean before on boost you should now be a little rich on boost. With the MAP signal the FPR compensates for the vacuum and the boost and is variable with the present outside conditions. The ouside amtospheric pressure directly changes the manifold pressure whether it be vacuum or boost.

Having a fixed fuel pressure would need much more injector timing on boost as the boost would be fighting the fuel pressure. And then when the outside conditions change drastically the tune would be off because the tune would be on a good warm sunny day would not run the same on a humid overcast day or at lets say 12pm vs 12 am. The weather conditions are just too drastically different.

By having the FPR sense boost the fuel pressure increases to counteract the boost, and automatically enrichen the fuel. Or when you are at idle and the fuel pressure is lower due to the vacuum and you instantly romp on it. The vacuum drops to "0" and the fuel pressure rises and assist compensating the fuel demand without drastic injector timing changes.

[GJF]

Sorry Dean my explanations kinda bounce from boost reference to outside atmospheric conditions. These are two different issues but they do relate with each other. Like rsrmike said the Tec manual would explain in depth more or your DTA manual should explain the same info as well.

[dean]

Thanks for the info. I understood what you were saying.