When calculating for new injectors?
 

When calculating for new injectors using RCinjectors calculator or other online calculators to get accurate info do I need subtract expected boost psig from expected rail psig?
IE If I have a 1.6L and wanted to make 200hp flywheel with a fuel pressure @ the rail @ idle of 37psig and I knew I need ~12psig to do this, in my mind I have 37psi rail - 12psi boost = ~25 psi effective rail pressure resulting in a much larger injector.......is this correct, or have I over thought this?
37psig I would need ~440-460cc/min
25psig I would need ~550-570cc/min

Please advise me on the correct way to do this?
Thanks;
Nevadasmith

If it makes a difference I will have an AEM F/I Controller and I'm in the process of trying to decide on a sleeper project car I have 3 to choose from, all were less than $100 with titles
all have 110k-160k miles run and drive. The other 2 will be sold 2 fund the project so like $700-800 budget.
95 Ford Aspire 4 door 1.3L
00 Geo Metro 2 door 1.0L 3cyl
02 Daewoo Lanos 2 door 1.6L

It will not be a daily driver weekend car only and only needs to live 20k miles with boost

I have an eaton M-62 super/an RBH5 turbo/ and garette (t-bird .60/.63) and some misc intercoolers off other projects....to play with.

Please don't tell my to go buy a Honda or Mustang I am fully aware that they would be faster than any thing I have boosted with out boost. This is just for fun with junk turbos and junk cars. Something to play with when i get home from work if I scatter the motor or trans I will keep the turbo goodies and sell the car for more in scrap than I paid for it.
Thanks
Nevadasmith

injector size all depends on how many injectors, fuel pressure and how much hp your planning on producing. injector flow rates are based on 43 psi fuel pressure.

a 4 cylinder engine with 43 psi fuel pressure and a 200 max hp goal on a boosted engine will require 35 lbs injectors

So I found the answer on a mustang web site and yes you do subtract boost pressure from fuel pressure to calculate fuel pressure when figuring for new injectors!!!! and I have been posting my questions there ever since !!!!!

You might want to unsubscribe from that site. The driving force for flow across the injector is a pressure differential, (rail pressure - manifold pressure). Most factory fuel injected engines use a 1:1 fuel pressure regulator: for every psi of boost you run, the regulator will raise the fuel pressure by 1psi. This keeps the driving force for flow independent of manifold pressure.

The answer is no, you don't subtract boost pressure to size the injectors. You can test this by simply pulling off the vacuum hose going to your regulator and watching the fuel pressure while the car is idling. If your engine pulls 20 inHG vacuum at idle and you suddenly pull the vacuum hose off the fpr sees atmospheric pressure. A change of 20 inHG is about a 9.8psi change, your fuel pressure should rise by 9-10psi.

I thought about what you are proposing when I was sizing injectors. It makes sense that manifold pressure would affect the flow across the injector if the rail pressure was constant. Luckily, the FPR is designed to make the fuel pressure and manifold pressure rise at the same rate.

***Also, it probably won't make too much of a difference in your calculations, but you base fuel pressure is not your fuel pressure you normally see at idle. Its your fuel pressure when you pull the vacuum hose off of the regulator.

base fuel pressure is with the fuel pressure regulator vacuum hose conencted.

most factory fuel pressure regulators are not 1:1 boost reference. a car mfg is not going to put a 1:1 boost reference fuel pressure regulator on a naturally aspirated vehicle. they would be spending extra money for nothing.
a factory fuel pressure regulator will increase the fuel pressure up to 0psi. once the stock fuel pressure sees 0.1 psi and over the fuel pressure regulator no longer increases rail pressure.
a factory fuel pressure regulator with the vacuum line connected will run at ~36-45psi(some vehicles are higher) depending on the vehicle . as you mash the gas pedal you intermittently lose vacuum. this causes the fuel rail pressure to rise to its peak setting(around 45-60psi) adding more fuel so the vehicle doesnt lean stall when you hit the throttle.

I've personally tested both honda and nissan factory fuel pressure regulators using a simple hand vacuum/pressure pump. I've only tested them up to about 10psig and they both increased the fuel pressure by 10psi over the no vacuum fuel pressure. I'm sure at some point the regulator will saturate and no longer increase the fuel pressure. I don't know what point this will occur, but it is not 0psi like you propose. Try it out yourself.

### You never talk about pressure on one side of an orifice when calculating flow across an orifice, you talk about the pressure difference across the orifice ### If you don't understand or believe the above statement crack open a fluid dynamics book.

When companies flow test injectors on a bench at 43.5psig of rail pressure, they do not pull a vacuum on discharging side of the injectors. So the differential pressure is 43.5psig - 0psig (atmospheric) = 43.5psi. When you want to size injectors you need to compare your differential pressure (base pressure) to 43.5psi. What is it when your car is at idle? Its the sum of the fuel pressure gauge reading and the vacuum in the manifold, which equals the the fuel pressure gauge reading when you pull the vacuum line of the regulator.

When you blip the throttle the regulator does raise the rail pressure, but it does not increase the differential pressure across the injector. It keeps it the same. Factory fuel pressure regulators are 1:1 because they want the differential pressure to be constant and independent of manifold pressure.

What your service manual calls base pressure is irrelevant when calculating aftermarket injector sizes. What the injector sites mean by base pressure is the pressure differential across the injector, which incidentally is the rail pressure when the vacuum hose is disconnected.