This is a topic that I have battled with since I began tuning 7.3 liter PowerStrokes. At some point, someone posted online that you can have large 100 % - 200 % nozzles while maintaining the ability to tow heavy simply by having your tuner “detune” the truck. What I am frequently encountering, people believe that “detune” means we can adjust the rate of flow & atomization of fuel down to that of a stock nozzle. I can’t. No tuner can. If they tell you that they can, they are simply full of ****.
The easiest way I can explain this within an example is to compare a stage two - 180cc / 100 % against the characteristics of a Stage 1 – 180cc / 0 %.
The injector pulsewidth (PW) is a measurement for how long the injector is spraying fuel into the cylinder. Our goal as tuners is to match the desired pulsewidth duration to the depletion rate of the injector. Depletion rate is the amount of time it takes to empty the entire displacement of the injector.
A Stage 2 – 180cc / 100 % injector, it is capable of spraying all 180cc of fuel in an injection window of just 2.6 ms PW.
A Stage 1 – 180cc / 0 % injector, it is capable of injecting all 180 cc of fuel at 3.5 ms PW.
As a general statement, the amount of power your truck is capable for has to do with the amount of fuel injected. So if you are able to inject 180cc of fuel, you will have around 380 - 420 rwhp out of a tune which fully depletes the injector. I know someone will say “but a Stage 2 builds more power”, and they would not be incorrect. But what people do not pay attention to is the amount of torque you sacrifice to build that little bit of extra power.
For those that do not know where hp / torque comes from in relation to the injection cycle, I will try to briefly explain it so we can move on to the next point.
As the piston raises, the fuel will start injecting pre-top dead center (TDC). This is your measurement for timing advance. As the piston crosses the point for TDC, the explosion of energy that bursts the piston downward is where your horse power comes from.
The force that continues push the injector downward is what creates your torque.
You will only gain 10 – 15 rwhp when going to a stage two injector. This is because the higher injector flow rate of the injector is able to inject a greater amount of fuel before the piston is at top dead center giving the piston a greater push on the decent of the stroke. The point that is often overlooked is the continual push of the piston downward which creates your torque. Since the Stage 2 injector will deplete at 2.6 ms PW and .8 ms of that injection window was pre-TDC, you only have 1.8 ms of driving force to continue to push the piston down with a stage 2.
Now compare that to a stage one injector that has a depletion rate of 3.5 ms PW. You now increase the duration of the driving force to generate torque up to 2.7 ms (holding timing advance constant at .8 ms). This is one of the reasons that you will find stage one injectors pushing around 50 lb ft of torque greater than a stage two injector.
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When we attempt to “detune” an injector, the main thing we are able to do as tuners is to reduce the amount of fuel injected into the cylinder. We do this by trimming the pulsewidth down to a shorter injection window. The thing that we face is the larger nozzle does not atomize fuel as well as a factory nozzle will causing the injection of larger droplets of fuel. Fuel burn rate is directly related to the amount of surface area where the particles of fuel contact air. Smaller droplets of fuel (smaller nozzles) atomize the fuel better causing a higher surface area for the fuel to react to air. This causes a more efficient burn. More efficient burn = lower exhaust gas temperatures (EGT).
The rule of thumb that I use when recommending injector size to people is strictly based off the amount of weight they plan to tow with the truck or if they do not tow, what their horsepower goal is. Since I can not find a good conclusive list for injector size, I feel this is as good a place as any. This is just a guideline. There are many factors that come into consideration such as turbo size, compound turbos, nitrous, etc. This list does not apply to those guys but if they are building that type of power, they already know this information.
