Injectors And Flow Testing And Stuff

[rtune]

I was reading about flow testing injectors, and came across two different articles

that mention the higher the fuel pressure in the rail, the higher the (tested) injectors

flow. For example, a 440cc subaru injector that was "de-capped" flowed at 500cc

using the standard pressure equipment, but when the guy doubled the pressure

and tested just one (because the machine could not test all at higher psi),

it flowed at over 800cc!

And then obviously the higher boost pressure the higher fuel rail pressure is

needed to overcome and maintain the same fuel pressure at the injector tip.

So if your injectors are near max duty cycle, why not just increase fuel rail pressure?

and.. how can injector sizing be an exact science if flow rates change according

to boost pressure and/or fuel rail pressure?

[saliya]

So if your injectors are near max duty cycle, why not just increase fuel rail pressure?

Within limits, you can do just that - a lot of people do it in order to avoid changing injectors.

There are a few limitations to consider, but the main ones would be:

* At a higher rail pressure, a given fuel pump will not be able to flow 'as much' fuel.

Which is exactly opposite to what you want (delivery of more fuel).

Fuel pumps have flow ratings at certain pressures; you need to make sure that your pump can supply the required litres per hour at the desired rail pressure.

* A fuel system is designed (OEM) for a certain pressure + a safety margin.

Exceeding the design limits by more than a certain amount can result in all kinds of exciting stuff e.g. hose or connector failure.

Since it's fuel we're talking about, that rapidly becomes a lot more exciting once the fuel hits e.g. a hot motor.

A stock GTR fuel system runs about 43psi + boost (say 56psi max pressure).

I believe there are a number of folks running 29psi-ish boost on stock fuel hoses+rail; meaning about 72psi max pressure.

So anecdotal evidence is the stock fuel system can take at least an extra 36% pressure without failing (I don't know whether anyone's tested to failure).

and.. how can injector sizing be an exact science if flow rates change according

to boost pressure and/or fuel rail pressure?

Manifold pressure (boost) is pushing back against the fuel system pressure when the injector's open.

The fuel system on a GTR increases rail pressure 1:1 with boost so that the net effect of adding boost is zero - increased boost pressure == increased fuel pressure.

This means that the amount of flow is determined by the original base rail pressure (43psi or so).

Injector flow rates are at "a given pressure" (think it's 3bar, or 43.5psi).

So yes, increasing base rail pressure beyond the given standard pressure will allow a given injector to flow "more".

Hope this helps,

Saliya

[rtune]

that helps, thanks.

So if you are over 90% on injectors you can get some extra

headroom by getting an adjustable fuel regulator and increasing

the fuel rail pressure.. That would be give extra headroom..

But that isn't going to turn 555 cc injectors into 740 cc injectors ..