Have I set my fuel pressure incorrectly?

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Spiffy
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Have I set my fuel pressure incorrectly?

Post by Spiffy »

I have rc engineering sl-4 440 injectors and they are rated at 440cc @43.5psi(according to their website).
I have my fuel pressure regulator set at 43psi but it was set with the vacuum line connected and I just read somewhere that base fuel pressure is set with vacuum line disconnected and plugged.

I wonder if this is why I'm having really low pulsewidths at idle since pressure is maybe too high?

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Dennis930
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Re: Have I set my fuel pressure incorrectly?

Post by Dennis930 »

It would be OK if you set the pressure without the engine running. However, if you set while the engine is running (not likely) then the base pressure would be higher than 43.5 by what ever KPA your motor runs at while idling.

So, if your base pressure is higher than 43.5, then that would affect the idle pulsewidths making them shorter than if the base pressure is 43.5. Also, it would throw your entire VE table off. If you want to run a higher base pressure, the required fuel needs to be recalculated based of the flow rate of the injectors when operated at the higher pressure.
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Spiffy
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Re: Have I set my fuel pressure incorrectly?

Post by Spiffy »

That's the thing, it was set while idling with vacuum connected. So at idle (about 20-25kpa) my fuel pressure is 43.5psi(300kpa)

I was reading this about the 300zx and its the reason I ask.
https://conceptzperformance.com/blog/de ... ol-system/
From the factory, the fuel pressure in our system is 3.0 bar, or 300 kpa (43.5113213 PSI. Guess why I like metric…). This means that at atmospheric pressure, or 0 PSI of boost, the fuel pressure will be at 3 bar. So the easiest way to set the fuel pressure regulator is to disconnect the vacuum line going to it. Be sure to plug the line on the engine side
Now on my Datsun, you check the fuel pressure while idling with vacuum connected. And it's supposed to be something like 2.3 bar(so about 3 at atm). So that's why when I changed to an adjustable fpr and bigger injectors I adjusted it at idle to 43.5 psi (3bar)like on the rc engineering website. But it probably should have been 3bar with vacuum line disconnected?

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DaveEFI
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Re: Have I set my fuel pressure incorrectly?

Post by DaveEFI »

IMHO it is best to set the pressure with the engine running, vacuum disconnected. As battery volts can effect fuel pressure. So best to set under running conditions - alternator charging.
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Spiffy
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Re: Have I set my fuel pressure incorrectly?

Post by Spiffy »

DaveEFI wrote:IMHO it is best to set the pressure with the engine running, vacuum disconnected. As battery volts can effect fuel pressure. So best to set under running conditions - alternator charging.
What do you think about the pressure? 43 at idle with vacuum disconnected?

I got in touch with rc engineering they said their ratings are at atmospheric pressure

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DaveEFI
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Re: Have I set my fuel pressure incorrectly?

Post by DaveEFI »

You either need to set it at the value given in your injector flow rate chart, or adjust the flow rate for a different fuel pressure. These are normally at the maximum pressure, so vacuum disconnected.
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