Bigger injector or more fuel pressure?

[jimz61]

On an EFI 930, what are the positives and negatives to realizing greater HP potential through installing larger injectors (55lb to 72lb) vs. using the same injectors (55lb) and instead increasing fuel pressure? Can you run more than 55lb of fuel pressure through a 55lb injector to achieve greater HP?

[magic930]

Most injectors can't close properly if the pressure is too high. So you might see more fuel delivered but you will have more difficulty controlling it especially at idle or low load.

[jpnovak]

With any EFI setup the Air fuel ratio is determined by the amount of fuel. This is typically increased by giving a longer pulse to keep the injector open. Fuel pressure should not affect the amount of fuel delivery as much. Once the duty cycle of the injector is maxed you need bigger injectors. If you are running lean, you need a longer pulse which may be an adjustment or you may need larger injectors.

[magic930]

Most injectors are rated at 42psi, to find the rate at a new pressure use this formula.

Sqrt(New Pressure/Old Pressure) x (Old Flow Rate) = New Flow Rate

in your case a 55lb injector run at 55psi is the equivalent of a 62lb injector which is a substantial difference. To get it too the equivalent of a 72lb would require a pressure of around 74psi.

[jimz61]

Thanks Magic.
But is there any potential problem getting to the flow rate by increasing pressure instead of installing a bigger injector? If not, why change injectors instead of installing a variable fuel pressure regulator?
Thanks,
Jim

[jimz61]

According to this calculator http://www.rceng.com/technical.htm pressure is a variable, as is static flow. But my intuition (which is usually wrong... LOL!) tells me that there must be a downside to increasing pressure instead of injector size.
-Jim

[magic930]

Yes the downside is the injector won't close all the way since it is working against the pressure. This is a problem especially as it reaches its max duty cycle. An injector has to have some dead time, usually a minimum of .75ms to close properly. As you reach max duty cycle (around 80%) you run out of time to get it to close especially if the pressure is too high. Running an injector past 85% and you run the risk of burning its coil (overheating) or burning the coil drivers out in your ECU.
I believe stock Carrera/Porsche regulators are already at about 55psi.

[beepbeep]

It's generally a bad thing to raise pressure to achieve bigger flow.

First of all, fiddling with pressure is a band-aid. While duty cycle is precise and can be determined in ECU, adjustable pressure regulator is a mumbo-jumbo thing that might work differently on every car. Re-mapping a car with just bigger injectors is often simplified beacuse you know the size of new injectors and can extrapolate duty cycle accordingly.

Also, injector spray pattern might change when running at higher pressure and it might not work with shape of intake.

So by all means, just purchase bigger injectors.

Is it a powerfull engine? Six 55lbs injectors should be OK for quite a lot of power, as they are?

Generally, you want as small injectors as possible w/o having to run more than 80% duty cycle.

[dean]

I have found that increasing fuel pressure doesn't make much difference.