Just as the title asks... what happens?

Strictly speaking of a VP44 (whether it be a HO, SO, or hotrod VP44)

My assumption is that with bigger injectors you will have more fuel flow from idle on up to the point where the pump is puting out its full potential and then stay there on up to max rpm. So a dyno graph would rise quicker and then flatline (to some extent)?

 

The way the fuel injection works in the 24 valve ISB is the ECM looks at all the sensors and based on their info decides on how many milliseconds the injection event has to be. The ECM has no idea if you are running stock injectors or showerheads. If the ECM wants to have injection event 10 milliseconds long it tells the VP to inject for that time. The VP will build pressure in its high pressure stage and the injectors will pop off at around 7000 psi and will continue to inject for 10 mS. With stock injectors the VP will have extra fuel in the three high pressure cylinders and this fuel will dump before the cylinders are empty. Now if you are running shower head injectors the same pop off pressure is used to get the injection event started but the three high pressure cylinders will run out of fuel before the 10 mS injection is completed. This is why an SO VP will make more HP than a HO VP if running large injectors (shower heads). The HO VP has smaller high-pressure cylinders and cannot inject as much fuel.
Bigger injectors may not add HP.

 

Great explaination!

Please let me ask a little more. How does each, a Smarty, tapped box and an un-tapped box effect the ECM/VP communications? How about the boost fooler?

 

How do you tell which of the three the VP44 on your truck is?

 

if you install a hotrod vp44 on your truck will it last longer then a regular vp44?

 

Bent valves, nice explanation...

So as you go up in injector sizes, you reach that point of full fueling sooner in the rpm band. (Strictly talking about large injectors, 200hp and on up..)?

Would this also increase the overall timing of the injection event? Since the injector pintle would be open for a shorter duration (due to the larger holes draining all the fuel faster)?

 

In regards to the pop off pressure. I installed stage 3 jammer tips in some nozzle bodies I had. The shop that did the pop testing said the spec was like 4000 psi. Can anyone verify that this is right or wrong. 7000 psi seems too high for a DI mechanical nozzle. If 7000psi is correct then there is a chance mine have been pop tested incorrectly which could be why I have such a crappy rough idle. Almost a miss. Poor atomization!!!!!!!!

 

Hmmmmm, mine popped at about 3500.....what does that mean?

 

7000 psi, I must have had a senior moment. I believe the injector pop off pressure is close to 4000 psi.

The wiretap boxes hold the dump solenoid closed longer to get more fuel injected. The VP44 builds high pressure in the three cylinders and starts the injection event. The fuel control module located on top of the VP receives commands from the ECM on how long the injection should last. It stops injecting fuel by dumping the high-pressure fuel back inside the VP body. I have read another theory that the wiretap boxes initiate a second injection event but I have never confirmed that.

The boxes that do not tap the dump solenoid wire get more fuel by overriding the ECM command for injection duration causing the fuel control module to add a longer injection event. This module in stock fueling never receives a command to fully fuel. The add-on boxes max out what the VP can do without tapping the wire.

And yes, bigger injectors will effectively advance timing by getting more fuel in sooner.

I have no idea, which VP will last the longest. I am encouraged that the more recent rebuilds seem to be holding up. I had to replace my early ’98 VP after the electronics became intermittent, around 100K miles. I had the wire tapped and ran a lift pump pressure of 5 psi under all conditions. Never had a P0216 code.

 

Keep in mind that the VP can build pressure faster than the injectors can relieve it by popping open. So even if they pop at 4ksi, peak injection pressure will be much higher.

Now when you install larger injectors, the VPs ability to get ahead of the injectors is reduced. In short, the less injector restriction there is, the less peak pressure the VP can build against them after they pop open.

So what it boils down to is a question of flows-- VP flow vs injector flow. Once the injectors get big enough (flow wise), the VP can't build sufficient pressure to atomize the fuel efficiently, and hence you see no power gains even at higher fueling levels.

Think of the HO VPs. It's fairly well established that you won't see any improvement in dyno HP installing an injector larger than 160hp or so rating. You can swap in 220hp injectors that will actually flow a good amount more fuel-- but peak injection pressures will have fallen far enough to where atomization is poor.

This is why you can often see HP gains with oversized HO sticks with nitrous, and not with just #2. The nitrous somewhat compensates for the lack of poor atomization.

By contrast, the .5mm larger barrels in a SO vp pump provide enough flow capacity that even a 200hp will still have enough peak pressure to atomize pretty well.


With this perspective, you can see why the CR trucks have so much incredible potential, and we're already seeing amazing numbers from them.

Justin

 

Thanks for all the info...

Now where can I find a good Nitrous system...