Here's that thread... as pressure goes up, volume goes down...

https://www.dieseltruckresource.com/...53&postcount=4

 

I believe you are interpreting things incorrectly. That chart shows pressure vs volume in relation to the ability of the pumps efficiency. Using the shim kit is not changing the volume or pressure that the pump is producing. You are not changing voltage or line size pre pump or between the pump and the cp3. What you are changing is the the amount of fuel that is allowed to return to the tank. That does not directly affect the work the pump is actually doing and the pressure and volume coming out of the pump have actually not changed at all. If you shim the return valve, you are creating more pressure at the gauge by actually increasing the volume of fuel at the gauge and thus to the cp3.

 

Pressure as a function of restrction, yes. Not pressure as a function of pump output. Dropping the system pressure is dropping the flow volume and your mods are overrunning the delivery ability. Not gonna flaunt the laws of physics and make it run better.

Without running 40-50 psi, more as you start adding demand, to the CP3 the pump is cavitating and messing up the quality of the high presure fuel delivery. The flow is not smooth enough and there are too many places for the air to collect and cause issues. the only way to cure that problem is up the pressure to keep things full of fuel.

Another one of the major downfalls of the Walbro pump setup is not removing the entrained air. Pressure is the only tool you to overcome it.

 

Pretty impressive how fast that boost climbs



maybe when the FP gets rectified another BBQ & Coronas in the pool LOL

 

Technically, pressure increases alone won't make power, because the CP3 is designed to pull fuel directly from the tank (i.e. 0psi from LP).

However, since the fuel supply line is fixed (in terms of I.D.) - a pressure increase will effect an increase in flow velocity, which results in a higher volume of fuel fed to the injection pump over time... so - all else being equal - higher LP psi = more lbs/second of #2 to the CP3.

Of course, "fuel quality" (cool, clean & de-aerated) also benefits from an overabundant supply to the IP.

 

I think you also need to up the pressure to atleast 20psi at idle. My single 392 is set to 20 like it came and when dyno'ing or pulling/racing it drops to 17 and that is it. I know I am using alot more fuel than you are. The only times is drops like that is taking off hard or accelerating when tank is near empty due to fuel slosh.

 

I'll disagree with this statement.

The velocity of fuel supply the engine is a function of the fuel flow rate. The total fuel flow rate must be accounted for. Let's say you have a nominal 30gph to the engine. That means you must account for all 30gph between the fuel burned, the injector drain return, the CP3 return, etc.

If you increase the fuel delivered to the engine, it will show up somewhere-- either you're burning more or draining more back to the tank.

My understanding is that the CP3 has a fixed volume of fuel it will admit. Because the fuel is incompressible, the flow rate cannot change without changing the volume of fuel demanded.

Velocity= flow rate/cross sectional area

Since I can't change the flow rate, and we assume the cross sectional area is fixed, the velocity is constant.

If I try to push more fuel to the engine by increase the FP, the engine will push back by a force equal to that. They end up canceling each other out.

The higher fuel pressure will tend to reduce cavitational tendencies, but it cannot increase the quantity of fuel delivered to the engine.

Not that I can see, anyway.


Maybe if someone explained it better to me it would make sense. But as far as I can see, fuel consumed by the engine is only a function of injector cup flow, injector duty cycle, and rail pressure. Anything other than this just increases the amount of fuel returned to the tank.

 

Umm... unless the return to the tank can flow enough so that there is not adequate flow to the cp3. The only way to increase the pressure to the cp3 is to decrease the volume returning to the tank. Since nothing else in the equation has changed, the only way the gauge reads a higher pressure going to the cp3 is because there is more volume to it. Therefore, because of the configuration of the GDP kit, more pressure showing on the gauge is in fact more volume to the cp3.

 

More pressure will more fully fill the plunger chambers and thus raise volumetric efficiency of the CP3 pump. Higher VE will net more flow through the common rail system.

 

In a modified applicatgion that statement is incorrect. More pressure will make more power up to a point the low pressure fuel delivery cannot keep up with demand. Without a lift pump the CP-3 cannot suck enough fuel to supply a stock engine.

Depends on what part of the pump you are referencing as far as the fixed volume is. There is a point at which it is too hard to push enough fuel thru the low pressure side to supply the hihg pressure demand, but, that happens somewhere north of 700 hp.

The flow rate from the low pressure side is variable depending on rpm. The excess is dumped back to the tank which is somewhere half the fuel up to a point. Increase the pressure and the rate and velocity increase. Its not a constant.

Now, once demand from the high pressure side exceeds the supply then the pressure drops and you get rail pressure drop or crappy fuel content delivered to the injectors which shows up as a loss of power and high EGT's.

The CP-3 can pump a lot more fuel if it is pressurized high enough o the low pressure side.

 

That's what I get for trying to give a simplified explanation of pressure vs. volume in regards to the CP3 fuel supply... you guys try to turn it into a fluidics thread!

 

I'll see if I can pull up a tech report on the CP3 at work today..

jh

 

A "pressure" reading is a simple indication if you have enough "volume". Thats as simple as it gets.

 

My walbro started to loose a crap load of pressure after the Flux 3.5's so I switched to a big Aeromotive marine diesel pump....35PSI and never moves a bit!!

 

Wow... what fireworks...

I'm thinking about moving to an air dog 150 anyhow... the walbro has been good, but...

I'll keep it around for emerengcies.

I still have not swapped out my filters to see if that is the case. FP is great until WOT... I have had these problems before with filters that were reaching the end of their lives.