Ok, there is a lot of bad info being pushed around with no data. Seems to happen all the time where people spout out about how great their setup is without running or testing each setup back to back. But let’s take a step back and look at the science and actual data. Let’s also put all of our Dicks down and stop arguing about 30% vs 80%. Let’s just talk smaller nozzles vs bigger nozzles in ANY diesel application. There are pluses and minuses to each. If there were no pluses and minuses to each we would all be running the smallest or biggest nozzle possible. AKA stock or 400%. But there are pluses and minuses which I will go over in simple terms with facts/data… not feelings and misinformation. These are not my theories, they are well documented science that you can research for yourself and they are not 7.3 specific. They are Diesel specific.
This is also a lot of info so if you are serious about understanding, read to the end, look at the pics, then read again because it all comes full circle and you kind of have to understand the ending to better understand the beginning.
Here are some terms that will be used but I am going to abbreviate to keep this shorter.
PW is referring to Injector Pulse Width which refers to “how long” the injector fires for.
SOI/Timing – Start of injection which is what we can control. We can command the start of injection and only theorize the start of ignition.
Tq – foot lbs of torque.
HP – horsepower
ICP – injection control pressure
TDC – top dead center referring to crank/rod/piston
Alright, just so we have the basics covered it is important to realize the HP is just a derivative of torque over RPMs. So horsepower is calculated through a formula using rpms and torque. So you are an idiot if you say “horsepower sells cars and torque wins races" because they are both derived from the same number. But most people refer to torque as “low end power, and horsepower as High end power. But more torque at higher rpms is how you get more horsepower. Another way to think of it is torque is how much force/work is being done and HP is how many times you can do it. For this reason ALL vehicles will make the exact same amount of torque/horsepower at 5250rpms EVERYTIME.
Lets clear something else up really quick, smaller nozzles do not make more torque from excess timing, that is 100% false. They make more torque from extra pulse width? WHAT??? That does not make sense? WELL! Let me explain. There is a lot to go over before this will make sense.
These are rough numbers and drawings to teach a theory, so don’t worry so much about the exact number and focus more on the science.
Lets move past the simple fact that smaller nozzles atomize better, lets dig deeper.
For any given scenario you will need a certain amount of hp to do that work. Let’s say that you will need Y amount of fuel to make that hp and do that work. Lets call it 250hp
So it takes Y amount of fuel to make 250hp.
Y = PW x ICP x NOZZLE SIZE Those are the only things that will change the amount of fuel in the cylinder and the relationship between them is what causes smaller nozzles to make more tq (aka hp down low).
So if you keep Y constant to make that 250hp and change nozzle size… then you will also have to change PW and ICP to accommodate. If you increase the nozzle size, then to keep the same amount of HP/Fuel you will have to either drop ICP (which causes you to lose efficiency and torque) or you will have to lower PW (which we will now show how PW can make such a big difference in HP/TQ). It is like that silly game of (reliable vs cheap vs fast, you can’t have all 3 perfect). So less PW is bad in certain scenarios?
I know what you are thinking. I just blew your mind. How can a longer PW be better in any way? That would not make sense to some people but those who have studied will know/understand.
See Exhibit A: This might be obvious to some but it is important to show how these engines make power. It is by the force exhibited downwards AFTER but before 180 degrees (most don’t go that far). You control timing/SOI in hopes that the start of ignition will hit the “sweet spot” and exert more force on the downward stroke of the crank. But what is that “sweet spot?”.
Think of this like a bicycle, it is the exact same theory except your leg is the driving force. What would happen if your pedal was facing STRAIGHT UP and you kicked down as hard as you could, NOTHING except maybe a broken leg. You need the pedal to start the downward stroke before applying force. Anyone that has ever popped a wheely on a bike knows that sweet spot, which is also demonstrated in the drawing. Apply that same amount of force as before when the pedal was facing straight up, but slightly change “crank angle” and now you are popping wheelies all day long. Simple way of explaining the “sweet spot”. Same as a bike, if you continue to apply force on the crank after top dead center all the way to 180 degrees you will increase torque.
See Exhibit B For those who like graphs, here is basically the same info above, but put into a graph. Something this graph points out that was not explained before is that any burning that happens BEFORE TDC is negative work… actually causing a loss in HP and can even lead to broken parts. So it is a fine balance of timing/SOI vs Start of ignition vs PW vs Nozzle size to hit that “sweet spot”. Too much timing and you are doing too much negative work. Not enough timing/SOI and the delay between SOI and start of ignition might also make you miss the sweet spot.
Now that we understand the very basics of diesel engines and how tq/hp is made. How does a larger vs smaller nozzles affect this?
See Exhibit C: Once again, don’t focus on exact nozzle size. Just theory of bigger vs smaller. This illustration is showing how two varying nozzles sizes will affect PW by keeping fueling and timing and ICP the same. The larger nozzle will have a shorter PW and the smaller nozzle will have a longer PW. You can see that by shortening the PW too much you will start to miss that “sweet spot” that we are looking for. The smaller nozzle at the same timing/fueling/ICP will hit much more of the sweet spot, thus resulting in hitting more of that “sweet spot”. Under light throttle conditions the smaller nozzles will be able to take better advantage of the sweet spot and produce more low end tq. The bigger nozzles will have to change something to hit the same amount of power, lower icp to extend PW, dump more fuel, etc. Tuners can do their best to dial this in but at this point the smaller nozzle is BETTER IN EVERY WAY.
As I said before, there are pluses and minuses to everything. So why not just run the smallest nozzle possible in every scenario with the longest PW??? At a certain point you will run out of Air for the fuel to burn, so spraying excess fuel just makes more smoke with minimal HP gains. There will be a much more efficient burn closer to TDC when the pressure is higher and you have more air. A better air/fuel ratio will also result in a cleaner/cooler burn with less smoke/egts. But fact of the matter is that as long as you keep spraying fuel you will keep making power… but there are diminishing returns with excess smoke/egts. You can also start spraying outside the fuel bowl which is also not good.
See Exhibit D: Now let’s bump up the fuel, power and PW. This is when the bigger nozzles really start to shine. The bigger nozzle is hitting much more of the sweet spot to make more power/tq. Now the smaller nozzle is starting to spray far past the “sweet spot”. This will result in the smaller nozzles having higher EGTs and smoke? So these two setups can make the same power, but the smaller nozzles will have the downside of more smoke and egts. How much is too much? That really depends. As long as you are not spraying outside the fuel bowl and you keep egts in check enough not to melt down anything… then you can run as much PW as you want. But finding that happy medium is the key. We have found that when racing stoplight to stoplight or in the ¼ miles, EGTS DON’T MATTER. You cannot make enough heat to melt a piston in 10-15 seconds. Even at 1500 egts. Where egts matter is when you sustain higher than 1250-1300 for long periods of time. Think of it like an ice cube sitting in the shade in 40 degree weather? It is not going to melt and turn to water in 10-15 seconds. But go put that same ice cube in the sun on hot asphalt in 110 degree weather and it will melt fast. The melting point of our aluminum pistons is 1200-1300 degrees. The higher you go above 1200-1300 egts the shorter amount of time you can spend there.
Here is a general rule of thumb (this is not exact science)
1200-1300 all day every day and will not melt anything as long as EOT and ECT stay in check
1300-1400 30-60 seconds
1400-1500 less than 30 seconds
1500 + less than 10 seconds
See Exhibit E: So lets exaggerate this to the point that neither nozzle size makes sense. Lets run the biggest nozzle you can imagine along with the smallest nozzle possible, much smaller than stock. Now the smaller nozzle is spraying for so long it is actually spraying while the piston is going back up the other side. The bigger nozzle is SO BIG, that to get enough timing to make sure to still be able to spray the fuel near top dead center so you have enough heat/pressure for a good ignition you are nowhere near the sweat spot where tq/hp is derived from. This would be like dumping a gallon of gas on a candle instead of misting gas with a spray nozzle.
Something else to take into account that throws a monkey wrench in everything is RPMS. Keeping the exact same amount of fueling at lower rpms you have much more time to for the smaller nozzle to extend that spray pattern right into the sweet spot of the crank angle. But as rpms go up the same affect happens as with “adding more fuel”. The smaller nozzle will no longer be able to dump the fuel fast enough without spraying too far past the sweet spot and the bigger nozzle will take better advantage.
So, to wrap up the debate. Bigger is not always better, but neither is smaller. They both have their advantages and disadvantages. Anyone telling you that bigger is always better or smaller is always better does not understand how these things work. Same goes for turbos, tires, gear sizes, bore size, etc., there is always give and take when it comes to these things.
As you go up in rpms and spray more fuel the bigger nozzles will be more advantageous. As you go lower in the rpms and spray less fuel the smaller nozzles will be more advantageous. Finding the sweet spot for your setup and how you use your truck is going to be key.
Feel free to his us up if you want to learn more or want help figuring out what setup would work best for your truck.