Entering fuel injector sizing

[MolsonB]

This may seem like a silly question,

My fuel injectors have been tested and their cc/min listed based on 43.5 fuel pressure. The engine is a rotary RX8 (NA) that looks like between 55-65psi is the fuel rail range to go for. Just wondering what values we plug in to MS? Does the flow stay the same, at a higher fuel pressure?

The regulator is Holley 12-846
https://www.holley.com/products/fuel_sy ... rts/12-846

[CRSTune]

Flow increases with higher differential pressure. An injector rated at 500cc at 43.5psi for example will flow more fuel at 58psi.

Flow does not increase when rail pressure increases due to increased manifold pressure (vacuum referenced).

You need the flow rate of the injector at the operating pressure of the engine. If you do not have that, some fancy math will give you a good approximation:

Q2 = SQRT(P2/P1)*Q1

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

So, my example injector at 500cc at 43.5psi would flow about 577cc at 58psi.

55-65psi sounds like a 4bar/58psi rating (most OEM's these days on returnless systems operate at that pressure give or take a few psi).

[MolsonB]

Ah thank you. A light bulb went off, I never thought of returnless vs return fuel systems requiring a different pressure. That makes sense now that you said it, why a returnless needs the higher pressure. The stock car is 'returnless' (I got the psi range from the service books), but I've converted to a return style setup. I'll dial my FPR back to 43.5psi then.
Thank you for the equation though, I have it saved.

[CRSTune]

OEM's are going for better emissions these days so they need to maximize the amount of fuel that actually makes it into the cylinders when injected. The higher fuel pressure usually improves fuel atomization, thus improving emissions, driveability, response, etc. You could run a return style fuel system at 58psi if you wanted. Just know that higher fuel pressure reduces fuel flow from the pump, in case that's a limitation for you. Also, older style injectors (EV1 instead of EV6 or EV14 for example) may have problems operating at higher pressures.

[DaveEFI]

This may seem like a silly question,

My fuel injectors have been tested and their cc/min listed based on 43.5 fuel pressure. The engine is a rotary RX8 (NA) that looks like between 55-65psi is the fuel rail range to go for. Just wondering what values we plug in to MS? Does the flow stay the same, at a higher fuel pressure?

The regulator is Holley 12-846
https://www.holley.com/products/fuel_sy ... rts/12-846

If you know the flow rate at a given pressure, there is a calculator to give the rate at a new pressure here:-

http://users.erols.com/srweiss/index.html

[MolsonB]

So I've jumped back to this for a day and re-read my numbers.
I have Mazda RX8 yellow injectors that are rated at 420cc/min (not sure what psi they advertise that flow at)
Sent them away to Injector-Rehab to be modified, which shows the modified Yellow injectors now test to 820cc/min @ 2.25bar (32.6psi).

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 injectors for idle, then stage the modified yellow's after xxxx rpm.

[billr]

Did you flow those injectors at 58 psi to get that 1094 figure? I think that equation given earlier is a good approximation, but can't account for all minor factors. Besides, you would still need to properly determine dead-time, and getting flow rate is trivial once you are set up to check DT.

[MolsonB]

Did you flow those injectors at 58 psi to get that 1094 figure? I think that equation given earlier is a good approximation, but can't account for all minor factors. Besides, you would still need to properly determine dead-time, and getting flow rate is trivial once you are set up to check DT.

No they were only flow tested @ 2.25bar (32.6psi). The equation is all I got to guess the new flow @ the new psi. I'm guessing you can get close but the VE tables will sort out the fine tuning? Maybe I'll look for a different shop that can flow @ 3bar instead.

I've found a few posts on how to find the dead-time. Good to know! Thanks for the tip.