Is a 5-7psi fuel pressure drop normal?

[RedLT4]

I'm trying to pin down whether my fuel pressure is staying where it should be. I've installed a sending unit to monitor the pressure while logging. i suspected I had an issue when trying to get my LTFTs closer. It seemed the lower my gas gauge, the leaner it would get. It's a new 255lph pump and the regulator is built into the filter(corvette single feed line).

It's running just over 3/4 of a tank now, so, I should be able to see if this pressure drop gets worse later with less gas. But for now, I'm seeing anytime the injectors open up a little more, I'm dropping pressure. Some searching I've done says this is normal, others say no drop is acceptable? I'm leaning toward no drop is better, but wondering what others have to say?

[kingtal0n]

As long as there is nothing wrong with the setup, i.e. no clogs, no excessive heating, no voltage drop, proper filtration well maintained, high quality pump working as intended,

As long as everything is 'good' then the pump is just an electrical device which can only do whatever it was designed to do. In other words, there is no harm if the pump is providing 55psi one second and 47psi the next, then 35psi, then back to 55psi, because it is designed to work well at those values and no harm will come to it.

This answers the question of whether or not the pressure drop is an issue with the physical well being of your pump and fuel system is concerned. It isn't or should not be where concerning majority of typical DC fuel pumps used in the conditions from the first sentence.

Now, as to whether or not the pressure drop is a health/safety concern to the engine is another matter entirely. If you notice a drop in fuel pressure with an increased demand for fuel flow from injectors, this can significantly skew the tuning for those pulsewidths by forcing larger duty cycles to keep up with a dropping fuel pressure.
Since there is nothing technically wrong with allowing a pump to drop in this manner (as discussed above it won't harm the pump) you can theoretically tune around this situation as long as the injector has a duty cycle that is reasonable.

The reasons that this is a bad idea are many, but the main concern is the variation in output of the fuel pump as conditions change. For example when the fuel is cooler and the height of fluid is higher the pump and electrical circuit might work slightly more efficiently and provide slightly more fuel, thus pressure drop would not be as dramatic. If you tune the engine in that condition, and then the fuel heats up and the fuel pump electronics are now becoming much warmer, the pump output could drop, perhaps dramatically.

So the question becomes one of relevance, or statistics. You have to examine each case carefully. Lets use two examples to justify our observations:
1. 6L engine using 28psi of boost, twin walbro 450's on E50 to E85, enormous injectors (1500 to 2000cc), say around 840rwhp pushing up into 900rwhp mark, the fuel pressure begins to drop from 60psi to 52psi and keeps dropping. Injector duty skews from 60% to 70% to make up the difference, its climbing.
Does it need a third pump? Assuming we stop at 900rwhp where the pressure only drops around 15psi.

I'd naturally be cautious about leaving it like that, even on a closed loop controller, but given the reliability and robust nature of wideband systems in today's modern ecu controller era I don't really see much of a problem with this situation. The pumps are running out but they can operate like that just fine, it won't hurt them. There is plenty of injector duty for the closed loop to grab if necessary, the fuel pressure could drop another 10psi and it would be fine. As long as the fuel temps are kept low, no voltage drop, no clogs, etc... the fuel system is safe. It's all about safety factor, how much farther can it go before it breaks?
Even as an open loop it could be done easily because the safety margin of alcohol being over 50% and the fact you can 'throw more alcohol' at it than necessary is a factor. Certainly on gasoline this sort of thing would be impossible But overall I wouldn't be afraid to drive it like this with the pumps running out despite the incredible power output of the configuration and potential for disaster that it seems to entice by running fuel pumps to 100% output, because DC pumps mostly run at 100%. They either pump the fuel into the rails or back to the tank because of the DC current, it isn't like a new PWM system, the pump can't slow down when demand is low.

Anyways
2. 6L engine using no boost. factor computer. Single 255 walbro, small injector (36lb/hr?)
at say 400rwhp the 60psi fuel pressure dropping to 53psi by 6,000rpm, injector duty is 85% and the wideband is leaning out 12.8 when 12.2 is desired.
This one needs a better pump, and likely a less restrictive fuel system (perhaps some better lines or a larger filter will help). First, the injectors are much too small to grab any significant fuel from duty cycle reliably. Even if we had headorom, theres too much resolution with small injectors, the difference in fuel flow would vary greatly. In other words, if the injectors are enormous then a very small duty cycle adjustment yields a lot of fuel, there isn't much resolution, but that is a desirable trait during situations close to wide open throttle. The only time you want a bunch of resolution is when the engine is cruising or idling so you can nit pick and fine tune those. At wot it just needs to get fed, the exact 0.X value is not as critical. So you can add 'a rough, small amount of fuel' to a very large injector and have a ton of extra fuel go in with plenty of head room in case, say the fuel pump is seen to drop pressure. But you can't do that with a small injector, it isn't safe. It will always be under or over fueled because of all the variations unaccounted for.
Next, the pump is too small and the fuel is too inconsistent. Gasoline isn't as forgiving as some other fuels when it heats up and goes slightly lean. 12.8's and 13.2's can be an issue if your n/a engine is truly grabbing some VE. Finally, there is no serious complication with upgrading to say a 320 from the 255. It isn't like adding a third pump to the already complex twin pump system, where there would be a real 'job' involved or serious work. Taking out the 255 and using a 320 in it's place seems like a simple and logical step to prevent the disastrous consequences on gasoline of a pump dropping 5 or 10psi if the engine is leaning out as a result and there isn't enough injector size to reliably account for that drop in pressure.

I hope this was clear

[kingtal0n]

To put it another way. If the pump pressure is dropping but the duty cycle is still very reasonable (say 75%) and the wideband on the dynometer is staying nice and rich to redline, no leaning out.

Then its fine to leave like that imo nothin to worry about


On the other hand if the wideband is leaning out even just a little bit, any indication whatsoever there is a lack of sufficient fuel, something needs to be done