Why do most Throttle Body EFI systems use 58psi fuel pressure but are return type systems.?

In the Injector Flow course, you mention that the fuel pressure is 43.5psi (3 BAR) for a typical fuel system using a pressure regulator that is manifold pressure referenced, but a returnless system the pressure goes up to 58psi (4 BAR). However, I've used Edelbrock, FiTech, and Holley systems that all require return type fuel systems but all use 58psi. Do you know why that is?

4 bar is chosen so that when manifold pressure is low (high-rpm, closed throttle), the differential fuel pressure as seen by the injector, will then be just above 3 bar. It has nothing to do with returnless design, it's about the air pressure reference for the pressure regulator.

Yes, they're nominal values - some manufacturers may use 2.5, or 2.7, 3.5bar, or whatever. It may be a de facto standard that's developing, if all the USoA sourced companies are using 4bar?

Dead-head/returnless or return type may just come down to the manufacturer's preference and cost - there are advantages and dis-advantages for each type. In practice, it doesn't matter for tuning - except maybe fuel temperature in the rail(s) and return would keep cooler fuel circulating.

I just want to clarify something.

If Steve is referring to a return fuel system with base (differential) fuel pressure of 4 bar, the differential pressure across the injector should remain 4 bar at low or high engine speed or load, if the system has a manifold reference fuel pressure regulator. The same goes for a returnless fuel system if the FPR is manifold referenced, but modern returnless systems do often operate without manifold reference.

Whether return style or returnless, if a system has no manifold reference at the FPR, you get static fuel pressure, which results in dynamic differential fuel pressure at the injector, as intake manifold pressure varies which engine conditions.

Back to the original question, it's a great question.

I do find that some injectors appear to deliver fuel in a more optimized fashion with a bit more than 3 bar base fuel pressure, so perhaps that was a factor.

On the other hand, it may have come down to their marketing and sales teams more than engineering. For example, maybe they want to use injector X due to its cost and availability, but want to advertise the system as capable of flowing enough for Y horsepower, and at 3 bar those injectors don't flow quite enough to support that power level, so they went with 4 bar to support a bit more fuel flow and horsepower.

Often OEM return less fuel systems, especially on turbocharged engines, will work with a Fuel Pressure Control strategy in the ECU. They will be monitoring the rail Fuel Pressure along with the Manifold pressure, and changing the flow rate of the fuel pump to maintain the target differential fuel pressure across the injectors. Some NA engines will not use a fuel pressure sensor, but will have instead modeled the required pump control values in the ECU and vary the pump output based off the model and the MAP sensor reading.

DI return less systems can be more complicated if they are coupled with a port injection system, as the bleed pressure from the DI pump is fed back into the fuel rail, this can result in a transitory pressure pulse in the fuel rail of multiple MPa that has to be handled correctly. Some ECU control strategies control this well, others, not so good. It requires either a good model of the behavior when the DI relief valve operates, or a high speed pressure sensor that can detected the pressure changes if the PI is injecting when the relief valve opens.