Engine/Transmission (1998.5 - 2002) Alright, exactly how do bigger injectors make power?

[mikel]

I think I've got a pretty good grasp on how the diesel fuel injection works, but what I don't get is how a power gain is achieved through changing the injectors.



Somebody correct me if I am wrong, but the vp-44 'sends' the same amount of fuel to any of these injectors, right? Stock or Ultra super mega fire-hose injector makes no difference to the pump, but the power gains are definitely there.



I can understand that bigger holes would let more fuel through, but I just don't see how there would be any more fuel there to let through on an otherwise unmodified truck.



I can also see how the bigger holes would let whatever fuel actually was there through in a shorter amount of time, effectively advancing the injection timing, but can this in and of itself be responsible for the big gains that some vendors are getting from their injectors?



Just trying to understand.



Thanks,

Mike

 

[John Berger]

As I understand it, bigger injectors mean more fuel at delivery. While flow is constant, there is more fuel "in" the injector at the time it is introduced for combustion. Somebody else can probably explain this better than me.

 

[zman]

I'm no expert, but...



The VP44 pressurizes the line between the pump and the injector. When the pressure reaches a certain level, the injector opens manually. Bigger injectors usually have either more holes and/or bigger holes in the tip. This allows more of the pressurized fuel to flow into the cylinder before the VP44 reduces the pressure beyond the point that the injector closes. I think that is how it works...

 

[BV]

If I'm not mistaken the v pump is only responsible for the timing and pressure and not a metered amount so with bigger injectors in place, the pump opens them at the right time and keeps the pressure up which in turn will pump more fuel into the cylinder.

 

[zman]

As far as the V pump is concerned, the metering of the fuel is a factor of time. I'm pretty sure the pressure created by the pump is a constant. The ECM tells the VP44 how long to pressurize the line. Bigger injectors just allow more fuel to flow into the cylinder during the time the line is pressurized. Fueling boxes DO cause the VP44 to increase the length of time that the line remains pressurized though.

 

[mikel]

Ok so if I understand what zman says the pump will not provide each injector with a specific quantity of fuel, rather it will provide pressurized fuel to the injector for a specific period of time. Building on this the larger holes in the performance injectors *will* inject more fuel since more fuel will flow through them in the period of time that pressure is applied to them.



Furthermore from my first post, the vp-44 does *not* 'send' the same amount of fuel to whatever injector, but it would send pressurized to any injector for the same amount of time.



Have I got it?



Thanks,

Mike

 

[zman]

I may have under-simplified things a bit. There is true metering of fuel going on (i. e. the pump is supplying a specific amount of fuel), but it is based on injector size, pressure, and length of time the line is pressurized. If idle speed is set at 800 and you add bigger injectors, the idle speed doesn't increase. Rather, the ECM reacts by reducing the length of time that the line is pressurized which reduces the amount of fuel injected into the cylinder - so that the engine continues to idle at 800. So the V pump is metering the amount of fuel - but using time to do it.



The only other thing I would add is that the length of time the line is pressurized varies, and is dependent on engine speed and rate of acceleration (and the fueling box - if so equipped).



I'm not sure about this part, but I'd assume that in addition to the length of time the line is pressurized, there is also another variable that looks at the length of time before the next injector line is pressurized - which also varies based on engine speed and acceleration.



Steve

 

[HoleshotHolset]

Great info - keep the posts coming!



Matt

 

[mikel]

Originally posted by zman

If idle speed is set at 800 and you add bigger injectors, the idle speed doesn't increase. Rather, the ECM reacts by reducing the length of time that the line is pressurized which reduces the amount of fuel injected into the cylinder - so that the engine continues to idle at 800. So the V pump is metering the amount of fuel - but using time to do it.

But if I understand what you are saying, the idle speed *should* increase due to more fuel flow in the same period of time however, the computer detects this increase in idle speed and compensates for it.

The only other thing I would add is that the length of time the line is pressurized varies, and is dependent on engine speed and rate of acceleration (and the fueling box - if so equipped).



I'm not sure about this part, but I'd assume that in addition to the length of time the line is pressurized, there is also another variable that looks at the length of time before the next injector line is pressurized - which also varies based on engine speed and acceleration.



Steve

So on a non-computerized (12-valve) engine does the idle speed increase if bigger injectors are installed?



Once again, if I understand what's going on here the bigger injectors all work by letting more of the pressurized fuel supplied by the vp-44 be injected into the engine during whatever period of time that fuel is available, and it's simply (at the level of this conversation) an issue of fuel volume flow through the injector. I suspect that this "additional" fuel comes from the excess that would normally be returned to the tank, correct?



Thanks again,

Mike

 

[HoleshotHolset]

On the mechanical injection pumps... . the internal governor takes care of idle speed with a series of springs, etc. So the mechanical pump will compensate via rack travel to a certain extent too..... but if REALLY big injectors are added, your idle will change.



Matt

 

[andrewkoperdak]

Your VP44 is not providing a "quantity" of fuel... it is providing a fuel period and pressure. To help deliver more fuel and a better atomized spray, larger injectors usually have more holes drilled in the injector tip. This is another reason you getmore fuel to flow into the combustion chamber per time interval. The VP44 is a computer controlled pump so as you increase load it changes timing intervals to permit more or less fuel into the combustion chamber. So larger injectors let more fuel in per time interval than stock injectors. Look at it this way. If you have a 44 oz cup of coffee and tip it over for 1 second you will spill more coffee than if you tip a 20 oz cup of coffee for the same time interval. The injectors work similarly.

R,

Andy

 

[zman]

Mike - The "additional" fuel isn't coming from what would be returned to the tank. The fuel that is in the line between the V pump and the injector stays in there until it eventually gets delivered into the cylinder. There is sufficient pressure and fuel volume in the injector line to allow additional fuel to flow through the bigger injectors without causing significant pressure drops at the injector tip (which would cause the injector to close prematurely).



Andy - I wasn't saying that the V pump did the metering. I'm just saying that there is metering going on. The metering is done by way of the the ECM telling the V pump what to do and when to do it given a constant known fuel pressure. The ECM tells the pump when to charge the line and for how long based on inputs from the various engine sensors.



Steve