So I've got this old Vortec FMU laying around... So what Im thinking is that I could essentially use it to make a water injection system that is progressive with boost pressure. The Pressure switch would switch on the water pump at say 5psi at which point it would be operating at a base pressure, but as the boost increases the FMU would increase the water pressure and consequently the amount of water being injected. I assume Im going to need a pretty high pressure pump in order to have good atomization initially and still give it headway to increase with boost. But anyway,give me some input. Hopefully Im not having a Hyper moment here

The downfall for this idea, is that you need high pressure to properly atomize the water. So you would need a FMU that can handle already high pressures of like 100+ psi. What you are proposing is essentially a different implementation of already existing progressive controlled systems. They work by controlling pump speed which in turn controls pump pressure, or by sort of sending the pump a PWM signal to control output. Both are very rough implementations, with uneven atomization throughout the delivery range, and poor control overal of output. It might work though. Give it a try.

Hm, I always assumed other progressive systems worked by pulsing a solenoid at varying frequencies, like its done with nitrous. Which if anybody has ever seen a dyno of such systems you realise how poor power delivery is. Which is why I thought this would be significantly better.
So I guess progressive systems already use progressive pressure.

Are there issues with control using a PWM signal? I would imagine that there is a non-linear relationship between motor speed and pumping.
The RRFPR/FMU would be a very linear relationship.

nope the DO progressive controller alters the voltage of the pump, IIRC

Ok, maybe I'm an idiot and can't see the obvious answer here.

How is an FPR going to alter pressure at the WI pump? I know how the vortec works on a fuel system with a RETURN line. How exactly would you plumb this into the WI plumbing?

you would basically set it up just like a fuel system. It would have a static FPR with the FMU on the return, so you would have a return to the reservoir.

That's correct. However the relationship between pressure and flow volume is non-linear. Doubling pressure does not double flow.

I though for liquids (incompressible) there is a linear relationship between pressure and flow, but I may be wrong, Im not sure what principle explains the relationship between pressure and flow

Joe is absolutely right. Flow does not increase linearly with pressure. For empirical confirmation, look at some of the nozzle charts that show flow versus delta-P. They are not linear.

Here is the equation for incompressible flow through an orifice. Key equation feature, the flow rate is a function of the square root of the pressure differential. Not linear.


More information here. It is a discussion on orifice plates, but the same principle applies.
Orifice plate - Wikipedia, the free encyclopedia

As with any non-linear curve you can make short interval linear extrapolations to approximate the difference in flow. But, the farther you extrapolate, the larger the error.

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ooooh I see, so in other words the electronic way is a better route to a linear increase because you could program the speed of the pump vs. map in order to make a linear relationship, or any other type of curve you want.

I guess ill just try to sell this FMU to some poor soul, haha