Proper fuel volume is required for the necessary fuel pressure to feed the injectors, but it is not the fuel pressure from the fuel pump that atomizes the injected fuel. It is the High Pressure Oil (ICP) acting on the injector plunger at the time of SOI that atomizes the fuel. If the injectors have 7:1 intensifier pistons then the fuel is being injected at a pressure equal to aproximately 7 times the HPO pressure.

Edit: This is in reference to a HEUI system used in the 7.3.

 

I'm going to give this "quiz" two more days to see what others think.

 

1. Never knew how much power I had but I'll trust you on this one.....
2. Bigger is better......
3. Tubes and stuff like that...
4. More gas being used, I see the fuel gauge go down when I do this........
5. I have no idea I'll shoot you a message...
6. Again no idea I'll have to shoot you another message...

Just kidding Cody, trying to add a little humor.

 

I have already written up a lengthy reply. I'm just kind of wondering if I should wait the aforementioned two days......

 

You'll have to shoot me a PM in this forum because I don't "do" PM's. Everything is open for discussion!

 

Just kidding about the pm. I think I have posted a visitor message to you before about a thread and you replied. I'll leave the tuning, roofing, drywall, concrete,...etc to the pros. I always like reading your posts but can't always understand all of it. Reps to you if I can.

 

Thanks.

I actually turned off my PMs a few months ago because I was being asked questions that I thought everyone would benefit from with a simple answer.

 

I don't think I can take the suspense of sitting on this any longer!!!!!


AAAAHAHAHHHHHHHH.

Head hurts now. If you had any idea how excited I get when I get to transfer my thoughts to paper or screen, you'd laugh your ***** off. I get shaky and tense up....it used to make my wife shake her head in disbelief.

 

Then don't wait....let 'er rip Cody!

 

I was hoping that more people would jump in and put in their $.02. Not that I'm trying to be an egotistical, narcissistic jerk (not my intention at all), I just wanted to see what other people had going on in their heads about the subject at hand. I like to see other folks' points of view because if I didn't I wouldn't be any better than Chris Matthews when it comes to being a good, non-partisan journalistic anchor.

 

Because there is only 140cc per 1000 avalable with the stock injectors...

More fuel per injection cycle...

Fuel... The PCM cant even open the wastegate unless you have enough air pressure to bleed to the wastegate rod...

Because of the increased air flow going thru the motor.

Increased fuel rates for a given air flow...

Less back pressure for more air movment...

Lower boost pressures for a given RPM...


About the best I can come up with, considering the mood I am in...

 

But oh so good......


Ah, to heck with it. Here come the goods!

 

I'm not picking on Tugly. I know he put a lot of thought into his responses and I admire him for that. He just gave me some really good "talking points".

1. If there is more to making power than adding fuel, then why do tuners (the people AND the products) hit a power wall at 290-320 HP on stock injectors?


The stock injectors are very limited in their capacity. They hold about .14cc of fuel. It takes larger amounts of fuel to make power above the 300 HP range.

Also, diesel engines do not rely on a "stoichiometric" air/fuel ratio to run like a gasoline engine or when filling a room with hydrogen and lighting it off with a telephone ringer. The only thing that controls the engine RPM and how much twist is transferred to the crankshaft is how much fuel you can get in the cylinder just before top dead center and immediately after top dead center. All in all, it works out to a maximum of almost 72 degrees mathematically but shouldn't last that long. Obviously, too much fuel without enough air just results in a poor burn and a lot of black smoke. However, the only way to tell that a diesel engine is too lean is that it WILL NOT RUN.


2. Why does it take "larger" injectors to get above that number all else being equal?

The stock injectors in a 99.5-03 7.3L are capable of only holding and injecting enough fuel per injection event to make roughly 300 RWHP give or take 20. By using injectors that are capable of delivering more fuel in the very short window of injection (the aforementioned 72 degrees based on the IDM's capabilities), the more power we can make. ALL 7.3L engines have to live by the same exact set of rules for injection event duration. The IDM CANNOT command more than ONE injector "on" at any given time. With a V8 engine, there is one combustion stroke every 90 degrees. The IDM is programmed to allow up to about 80% duty cycle (80% of 90 degrees is 72) so the absolute maximum duration of an injection event is 72 degrees. However, the mechanical delay of the injector cuts pretty severely into that equation and the time that it takes the crankshaft to travel 72 degrees gets much shorter for every additional revolution per minute. The delay is the time that it takes for the injector to actually "open" and start spraying fuel once the IDM energizes the circuit. The injector does not start spraying the fuel right when the IDM supplies current to the coil. Therefore, the more fuel we can get out in less time, the more power we will be able to make.

Speaking of more fuel in less time.....
Did you ever wonder why nobody offers a 400cc/1000 shot injector with stock or even 100% nozzles? It's because although the capacity of the injector is enough to get you in the 600 RWHP range, there isn't enough TIME to get all of the fuel out using nozzles with such small nozzle orifices. The faster the engine is spinning, the less time we have to inject fuel. Time is constant; it never changes. RPM is constantly changing. Just to add some ACTUAL VALUES to this exchange, I'll throw in some tidbits using a simple excel-based calculator I came up with.....

At 1000 RPM, the maximum pulse width we have to play with before the IDM has to think about firing another injector is 12 mS (milliseconds). This is the "commanded" value and doesn't take into account the actual delay of the mechanical aspects of the injector. At 2200 RPM, that time becomes 5.445 mS. Now, at lower injection pressures, 12 mS will empty the injector pretty easily. However, to get that same amount of fuel out in less than half the time, obviously the pressure has to be increased. At redline of 3300 RPM, the IDM gives us a whopping 3.63 mS of commanded PW per cylinder in which to inject the fuel. I can tell you that stock nozzles on an AD injector are not capable of emptying their total capacity in the actual 2.9 mS worth of "open" injector time we have there. Heck, 100% nozzles on 160cc injectors (stage 2's) won't even do that.....it's close, but on injection pressures of 3000 PSI and lower, it's not happening.

The aforementioned scenario is precisely the reason that torque falls off sharply after 2600 RPM (and even earlier with tuning); there's not enough time to get enough fuel in the cylinder to continue making torque past that point. "But Cody, the engine is rated at 250 HP at 3000 RPM. I thought you said that torque falls off after 2600 RPM!!!" Ah, young grasshopper, power is a calculation of torque and RPM. If the RPM increase exceeds the amount of torque decrease, then power will increase or stay roughly the same. I am not an advocate of torque because in fact, while not entirely a useless value, it's power to the ground that gets work done (horsepower is, in reality, a measurement of work done over time) but that's a story for another time.

Power boils down to fuel over time.

Another tidbit of information:
Larger nozzles at lower-than-maximum power levels (say 300 HP on 100% nozzles vs. 300 HP on stock nozzles) will have less of an issue with EGT for the given power level. Why? Because the burn time is a lot shorter. Instead of the stock nozzle injecting over a long period of time (more crankshaft degrees) with fuel droplets searching for remaining oxygen molecules in the combustion chamber, the fuel is injected over a much shorter time period consuming the oxygen more efficiently. Also, with the shorter burn time, a larger percentage of the overall heat created by combustion is absorbed into the piston, cylinder walls, etc. and less heat is carried out of the exhaust port. It's also the reason that when tuned properly, 300 HP on larger nozzles will smoke far less than 300 HP on stock nozzles.


3. What controls air flow through a diesel engine? Specifically a 7.3L. I don't care about the "throttle plates" on newer diesels to make the EGR system work better. They have nothing to do with air control in any other capacity.

The only things controlling the airflow through the engine are the cylinder heads, the exhaust manifolds, and the turbine housing. Boost is simply a measurement of restriction when dealing with all things equal. Air MASS can be increased with higher boost, but the volume of the engine and how much airflow goes in and out are 100% restricted to how well the engine is able to get air into the cylinders and how well the exhaust can be removed allowing clean, non-diluted air to be drawn into each cylinder during the intake stroke (read: no passive EGR).

The main point of the question is that diesels do NOT restrict or meter airflow. The ONLY thing that alters engine speed and ultimately the power output is how much fuel you start injecting into the cylinder just before top dead center on the compression stroke.

4. Going back to the last question, why do you think EGT goes down when you're not lugging the engine up a grade in 4th gear?

This goes back to the air mass question above. It has to do with how much air mass is passing through the engine over time. When towing up a 7% grade at 2000 RPM in 4th gear, the engine speed is low, the mass air through the engine is low (low boost), and there is a lot of fuel being injected to make the power....however, without the air mass present, EGT goes up due to a very "hot" and long burn of excessive fuel. Downshifting allows the engine to move more air over time and therefore get a much better burn of the fuel because there's more oxygen present in the more dense package of air per combustion cycle.

I'm not sure what you're getting at with the changing "compression" theory though. If you're talking about cylinder pressure and stresses at lower RPM, then we're in agreement. You have to take into account though that (as an arbitrary number) 1800 RPM is 1800 RPM whether you're at a light cruise, deceleration, or under full load. The only thing changing is the amount of fuel being injected; and that fuel is what makes the heat which drives the turbine side of the turbocharger to spin the compressor wheel faster. I can't add 35 degrees of SOI advance in the base "timing" map and expect the engine to rev up; it's not going to happen. It will probably make funny noises and quit running after it chucks a connecting rod though......

5. Why do EGTs become such a huge issue when dealing with higher-than-stock power levels (even 40-60 HP additional) when all else on the truck is the same?

First, I'm not sure we understand the "stock tune delays fuel as much as the 135 degrees". If that were the case, we'd be injecting fuel on the following cylinder's compression stroke. NOT GOOD. The tuning values I responded with in an earlier post give the PCM the ability to command a calculated injection event up to 15 degrees AFTER top dead center. This is "retarded" injection timing. The tuning also allows for "advanced" calculated timing of up to 135 degrees; BEFORE top dead center. I can tell you that from personal experience (and I'm sure that you've seen forum posts about "whisper mode" or "drive through" tunes that make white smoke), that any "RETARD" more than about 8 degrees (-8 deg.) on the base SOI map causes white smoke (unburned fuel). To actually go to -15 on the base map (which still isn't really -15 degrees because of other advance adders and multipliers) will usually cause the engine to stall at an idle....or run so poorly that there's no doubt something is wrong. To "retard" the timing 135 degrees would equate to an advanced SOI value of 45 degrees on the next cylinder in the firing order; enough to blow a rod out the side if it were to run.

The "timing" has nothing to do with "compression", but everything to do with cylinder pressure. Cylinder pressure is your friend when it happens at the right time but it's your enemy when it happens too soon or too late. Too soon and the full brunt of the combustion of the fuel tries forcing the crankshaft back in the direction from whence it came. Too late and either the mixture won't even ignite due to low cylinder pressure or it will rob power and create white smoke.

The main reason EGT climbs higher and faster with aftermarket tuning is because of the burn rate of the fuel, the amount of fuel and the duration of the injection event. There are two ways to get more fuel out of a set of stock injectors: raise the injection pressure to force more fuel out of the nozzle orifices in less time OR keep the injection pressure the same as stock and keep the injector "on" longer. Most people will tune to take advantage of both of these functions. In either case, more fuel is being injected and I think we can both agree on that. Raising the injection pressure will only get you so far though because of mechanical limitations of the high pressure oil pump and the limits on stock tuning regarding injector pulse width. Add to the mix that diesel fuel, being a relatively slow-burning fuel, is being sprayed longer throughout the power stroke and more heat is absorbed by the cylinder walls, piston, etc. and being carried into the exhaust stream to be measured by a pyrometer.

6. Why do "larger", less restrictive turbine housings make the same or more power on less boost? Why do they become a problem at low engine RPM while towing?

It's all about airflow. The more you can get out, the more efficient the fuel will burn and the less heat you will make under load. This is precisely the reason vans and 95-97 Powerstrokes have "larger", less restrictive turbine housings. They do not have intercoolers so they can get away with having a slightly less responsive turbocharger (as far as boost is concerned) and they have slightly less problems keeping EGT in check (the lower capacity injectors in the 95-97 trucks help there too a little bit). The downside is that at lower engine RPM (sub 2000 range), there is not enough exhaust heat or flow energy to spin the compressor at a speed that will get the air density (boost) high enough to create a good burn. Hence the reason that less restrictive turbine housings often cause slightly higher than stock EGT levels at low load cruising and high load at low RPM. The stock turbine housing works well for boost response, but is HORRIBLE for creating a parasitic loss called backpressure at mid-high RPM under load. In addtion, with the higher boost levels of the later-model trucks, more heat is created by the compressor. The intercooler is there to regain some of the air density lost by heating the air so much. It's primary function is not strictly to cool the incoming air (although it obviously does), but to bring the density back up to pre-boost levels by way of cooling the air.

Don't confuse volume of a raft or tire with the volume of the engine though. The volume and pressure examples given VARY while an engine's volume is fixed (hence the capacity of 7.3L or 444 cubic inches). With our engines, we want density because the volume never changes. A gasoline engine's maximum cylinder capacity (volume) doesn't change but instead of having near atmoshperic density up to the density provided by whatever boost level and air temperature we have with our diesels, street-driven gasoline engines rarely ever have atmospheric density available to them because of the throttle plate. Volumetric efficiency comes into play with gasoline engines because the flow of air is actually dictated by the throttle plate. The VE of a diesel never really changes by much until you are able to get exhaust gasses out efficiently. The answer to the basic question (since I've totally gone off the deep end here) of why the same or more power can be made with less boost is that the engine can breathe better because there is less "backpressure" (technically called drive pressure in the turbocharger world) constipating the flow of air. The same amount of power can be made with less boost (restriction) because even though the boost is lower, more fresh air is able to flow through the engine! However, with the intercooler, piping, etc. that we have on our SuperDuty trucks, it is more desirable to mom-and-pop to have good boost response like the stock turbine housing has.

This might be a little over-the-top, but it's information nonetheless.

 

Thanks Cody. I always appreciate how well you can explain things.

 

While these electronic engines are beyond me, I've had enough mechanical engines to back you on fuel being the key to power.

I'll take a shot at the questions.

1. Stock injectors can only flow so much.

2. See 1.

3. Fuel. Add fuel to a mechanical engine and boost will rise, clog a fuel filter and it drops. The turbo is driven by exhaust gas, more fuel burnt=more exhaust gas=more boost.

4. Trying to burn large amounts of fuel with out the air flow to support it.

5. More fuel at a given rpm/ air flow rate.

6.a) Volume of air moved. Fill a 5 gallon bucket with a garden hose and a pressure washer and watch the results. The hose should have 35-85 psi and washer will be 3000(+-1000) or so psi. At best they fill at the same rate, but most time the hose will fill the bucket faster.
6.b) Lack of drive pressure.

These being said, how do some tunes produce the same, or greater, perceived power with with lower EGT's? Is it all powertrain management and would not hold true for a manual transmission truck?

I type way to slow and get distracted to easily. This was much more relevant at midnight, but the DVR'ed Walking Dead was more interesting..