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PostPosted: January 23, 2017, 11:12 pm 
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Joined: July 24, 2008, 9:18 pm
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My question is in reference to turbo engines and 1:1 rising rate Fuel Pressure Regulators.

There are heaps of online calculators to help calculate the size of injector needed for the predicted horse power. Using the fuel pressure, desired HP, BSFC, and how many injectors, (soke ask for more info) you can get a pretty good estimate of what size injector you need. Almost all injector flow rates are based off a 43.5psi fuel pressure.

Well, as you increase the fuel pressure, the rate (or size) of the injector goes up as well.

For example an injector at 43.5psi fuel pressure is rated at 550cc. When the fuel pressure is raised to 53.5psi, it is now rated at 609.95cc. So, when you have a 1:1 rising rate Fuel Pressure Regulator, and add that 10psi of pressure from your choice of forced induction, your injector is now rated higher than that rated information.

So, my question is. When shopping for injectors, or figuring if my current injectors are capable of handling new upgrades. Is it better to calculate your fuel needs using the corrected rate, or base rate the manufacturer lists? Corrected rate meaning your intended boost and raised fuel pressure in effect. I believe in not oversizing injectors. Smaller injectors have better spray patterns and are much easier to get good clean idling and cruise while tuning. Bigger is not always better in this case...

-Yes there are variables like fuel choices, and the addiction of turning the boost up. Then up some more.... UT let's pretend this is not an issue for sake of conversation. I'm really trying to figure out if what I'm thinking is correct, or if I'm just scrambling my head.

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PostPosted: January 24, 2017, 9:06 am 
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Joined: February 8, 2014, 10:47 pm
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Location: Cornholio OR "Where the magic happens"
Use the base rate because,

Injector fuel pressure is raised in boost to correct the differential pressure across the injector.

Otherwise the injectors will get smaller as boost increases.

Look at it this way, it you had 40 psi of boost,
And 40 psi of fuel pressure, how much fuel would be injected when the injector opens?

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PostPosted: January 24, 2017, 9:37 am 
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I understand the pressure equalizing. But the injectors still raise the amount they flow when the fuel pressure increases. That's the part that gets me.

I'm really interested because I'm converting from pump gas to E85. As a rule of thumb, you generally need 30% more fuel. If I add 15psi to my fuel pressure, which is how much boost I run. Then my injectors still have enough room to run them safely. But if I figure them at my base fuel pressure, then they are on the limit.

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PostPosted: January 24, 2017, 10:09 am 
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Location: Cornholio OR "Where the magic happens"
"the injectors still raise the amount they flow when the fuel pressure increases"

Injectors dont change flow rate when the differential pressure stays the same.

The injector is a valve between 2 different pressure zones, that valve sees the pressure difference between the two zones

That is what that 3 bar test pressure is all about, 43.5psi of differential pressure.

Here's another look

You have 50psi of boost
It is going to take 93.5psi of fuel pressure to get 43.5psi of flow (differential pressure)

And another look

If you have 50psi of boost
and 30psi of fuel pressure no fuel will be injected and likely boost will push into the fuel system through the injector. (if it could)
This is because of the differential pressure -20psi, 50psi of boost against 30psi of fuel, boost wins.... The injector will not open.

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PostPosted: January 24, 2017, 11:06 am 
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Joined: April 23, 2006, 8:26 pm
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Location: SoCal
As others have said, the sole purpose of running the MAP line to the fuel pressure regulator is so that the injector always sees the same pressure drop from the fuel inlet to the outlet - the flow is always exactly the same regardless of boost. It's the ECU that handles the changing fuel requirements with load. I run E85 too; my ECU adjusts the injector on-time to deliver roughly 30% more flow. To run E85 you'll very likely need larger injectors.

Though slightly off-topic, keep in mind flow is not proportional to fuel pressure, it's to the square root. Doubling fuel pressure only produces 1.4 times as much fuel.

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PostPosted: January 24, 2017, 11:33 am 
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Ah ha... Thank you.

I get it now. I can always count on good information here.

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PostPosted: January 24, 2017, 7:01 pm 
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Location: Cornholio OR "Where the magic happens"
I would also add that doubling the fuel pressure from the rated pressure (not differential),
will likely cause injector operating issues.

So correct injector sizing is important, you cant arbitrarily raise the differential pressure to get the flow you want/need.
That would be forcing it beyond it's design parameters and operation would become unpredictable.
Injector operation HAS to be predictable for the ECU to properly regulated fuel delivery.
Injector sizing is changed to match flow needs (loosely based on HP levels). That is why there are so many different sizes of injectors.
Also for the street you should stick to 80% DC to minimize stress and maximize predictability.

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