Fuel Pressure & injector question
 

For my RX8 engine, I'm using the yellow injectors that have been modded and tested at 820cc/min @ 43.5psi.
RX8 & WRX Injector Modification

Everything I read says the fuel pressure rail should be around 55-65psi (is that right, it sounds high for NA?), so is it correct to say that I have to calculate my new cc/min if I adjust my fuel pressure reg to 60psi? (I have an aftermarket adjustable fuel pressure unit)

On the MegaSquirt forum, I got this equation. Just want to make sure i understand this correctly.

Q2 = SQRT(P2/P1)*Q1

Where:
P1 = Original Pressure
P2 = New Pressure
Q1 = Original Flow Rate


EDIT: I have a return fuel system, so that may explain it. A returnless fuel system is around 55-65psi, where a return fuel system can be lower. I guess I'll leave it at 43.5psi.

Your formula is right. Flow rate varies with fuel pressure and that is the correct way to get a good approximation.

It is usually easier for a return system to compensate for flow variation, because the pre regulator pressure is higher than the actual rail pressure. Therefore, when the flow demand increases (during acceleration), the regulator simply lets in more fuel. The reaction time is minimal and the rail pressure will barely drop. With a returnless system, when the flow demand increases, there will be a momentary lapse of time where your rail pressure will drop, because the increase in flow has to come from the pump itself, which also means the fuel lines have to fill up with the extra fuel. By setting the fuel pressure to a higher value, the pressure drop will be less significant to the driver, because you will have sufficient fuel available right away to compensate for the increase in flow demand. Therefore, even though there is a drop in pressure at the rail, your engine management system will be able to properly compensate for it. If you were to run a lower pressure, you would have more difficulty doing so.

This being said, you could run more than 43.5 psi on a return system and get more flow out of your injectors. You would have to make sure that your pump can supply enough pressure though.

Thanks for the explanation. That makes sense. I'm running an external Walbro 392, lots of room for flow at higher psi. I'll have to play around once I get the engine running to see what the duty cycle is and to see if I need to increase the pressure rail.

Thanks again.

Sure thing. Glad I could help.

So I've jumped back to this for a day and re-read my numbers.

Injector-Rehab sheet shows the modified Yellow injectors tested to 820cc/min @ 2.25bar (32.6psi), not 3bar (43.5psi) like I originally thought.

That would mean at 4 bar (58psi), the injectors would flow 1094cc/min. This brings the required fuel down to 4.0ms, which is getting really low duty cycle for idle. Maybe I should put in stock yellow / red for idle, then stage the modified yellow's after xxxx rpm.