Been looking all over the Internet but can't seem to find a solid answer. I've noticed a lot of back and forth going on about whether or not disconnecting the red line to the wastegate actually makes more power or just increases heat and manifold pressure with no real benefit. I understand that this largely depends upon whether or not the stock ECU's fuel map can compensate for the extra air or not, because more boost without more fuel does not = more HP. Which leads me to my question; is a completely stock 7.3 ECU (L99 - 2003) able to compensate for extra air once boost goes past the stock 15 - 17 psi? I understand the ECU is set to defuel at ~24 psi, but is more fuel added after 15 psi up until defueling starts to occur? Once intake manifold pressure reaches whatever the max stock boost pressure is (~15 psi), does the fuel table flatten out from 15 to 24 psi and all the extra boost is just unnecessary hot air? Or does the injector pulse-width actually continue to increase as boost increases, which would ultimately result in more power?

 

I don't know but, you should not assume that an increase in boost w/o additional fuel does not increase power. Power is also proportional to cylinder pressure.

 

So you're saying cramming more hot air in the cylinders without adding any extra fuel actually increases power? That's not how I understand diesels to work, but I could be wrong.

 

Anything past 12 or so PSI.

The stock wastegate control is commanded to start sending boost pressure to the actuator at around 5.7 PSI. Disconnecting the red line most noticeably delays the opening of the wastegate even though it also slightly increases boost until drive pressure overcomes the actuator spring pressure.

 

You're right. More boost doesn't necessarily mean more power. Diminishing returns, especially with a stock turbine housing.

 

I understand that, but what I'm wondering is when the red line is disconnected from the wastegate causing boost to increase past stock values (12, 15, whatever it is), does the stock fuel map add more fuel past that point? Meaning, does it add more at 16 psi than it does at 15, and more at 17 than it does at 16, more at 18 than it does at 17, etc. Basically, as boost is increased past the stock value(whatever it may be, general consensus seems to be 15 - 17 psi), is additional fuel added to compensate for the added boost? Or with the stock fuel table, is the same amount of fuel being injected at 23 psi as it is at 16 psi? For example, I created some fuel tables with the tuning software I use (notice the highlighted areas). What I'm wondering is, does the fuel table look like the first picture (no additional fuel is added after 15 psi) or the second (additional fuel is added all the way to point of defuel)
Flat...fuel does not increase after stock boost. Fuel added as boost increases past 15 psi

 

Short simple answer..... yes and no.

Ok I lied, that wasn't a simple answer.

It really depends on what is happening. There are many variables built into the stock calibration that go into fuel delivery. Boost is just a teeny tiny part, and is only sometimes a factor in fueling.

However, it's not exactly cut and dry as more boost is better or more boost is worse. As Cody said, there are diminishing returns when adding boost without adding fuel, but there are quite a few factors that have to be considered.

If you're strictly talking "hot air", yes trying to cram in increasingly hotter air won't net you much, if any, gains. But you have to ask what is happening as boost increases? What are the RPM's, the load, altitude, ambient temps, etc?

In certain situations, additional boost without additional fuel is beneficial. In other certain situations, additional boost without additional fuel can have no benefit or even be slightly detrimental to performance.

Even more, it's not just the fuel map. There are other quite a few additional maps and parameters that go into the quantity of fuel delivered at any given point. For example, you can actually decrease FIPW in the PW map, but increase ICP and end up with MORE fuel injected.

Going even further, at what pressure is the fuel being injected? What is happening with timing? All of these factors will determine just how much air is really needed to give you the most power.

So I see what you're trying to ask, but the problem is you can't get an exact and correct answer. There's just too many variables happening at one time, and different scenarios will yield different results. The reason why? Diesels don't run at a set stoichiometric ratio like gas engines do. If you are looking at gasoline engine tuning, you'll see a very direct correlation between boost and fueling. Diesels.... all bets are off.

 

Right, and I get that there are a ton of different variables. I've tuned quite a few turbo, supercharged, and N/A engines so I understand engine dynamics and all the different variables and tables involved, but what I am wondering is, after all said and done, all trims and other variables factored in (let's assume the fuel pulse at 2,000 RPM @15 psi is 4ms), with ALL other variables the same, at 2,000 RPM but this time @23psi, is the fuel pulse still 4ms, or is it 6, 8, etc? I understand that other trims or tables may be adding to that 4 ms pulse on the main fuel table, but let's just assume the main fuel table is responsible for compensating for any added boost beyond 15 psi; will that fuel pulse at 2,000 rpm @23 psi still be 4ms, or does the actual fuel table have a spot on the map for 23 psi? Of course this depends upon the stock map sensor and if it can actually read pressures that high.

 

It would take me some time and studying to understand all that science data being provided (if I could get it at all) but...I'm thinking that anytime you provide more air to a fire it will burn cleaner and unless there is some sort drawback like less lubrication for the cylinder, a cleaner burn should provide better performance, even if it is minimal.

 

In that kind of scenario, most likely no difference.

Even if it does make a difference, it's extremely small. There's a little sub-parameter in the 7.3L diesel calibration that looks at boost, and only in certain scenarios does it make a small move in the mass fuel desired. But it's small changes. Did I say small changes? To give you an idea, there's an entire table and several more parameters dedicated to oil temps that influence fueling far more than boost does. Changes in PW related to boost pressures are most often so small that you probably can't even read a difference in PW, even if you had it set up on a dyno and did a controlled test.

Ok I see the approach that you're taking. Here's something that might help your thinking: The fuel tables your thinking of don't exist the same way in diesel tuning as they do in gasoline tuning. The approach you're using is coming from a gasoline world. It's kinda like left-brain vs right-brain.... two completely different things. Let me explain. The PCM on these trucks can only see boost and intake air temps. There is no mass airflow sensor, and there are no O2 sensors, so the PCM can't make fine tuning adjustments based on airflow. There simply isn't enough information that it can collect and read. It's doing a much more crude caveman style of reading airflow, which in the tuning world you can understand and appreciate that you don't want the PCM making big decisions with very little info.

Diesel tuning, and subsequently fuel injection pulse width (FIPW), are directly tied to pedal position, RPM, injection pressure, and timing. Oil temperatures play a significant factor since the injectors are "powered" by high pressure oil, and oil temperatures directly affect how the injectors fire. Way way way way way way down the list is airflow and boost, which has very little input.

Hope this helps.

 

Just something to add to this...

From my own experience with tuning my truck and a few others, the only time I paid attention to boost was to remove the triggering of the overboost code in the calibration, and to see if there is a problem (IE boost leak). But to be honest, there's no changes I ever made to fueling/ICP/timing based on boost. Even if someone asked to re-tune their truck because they swapped to a bigger turbo, there was nothing to change.

The only exception is if the truck was running far larger injectors than the stock turbo could possibly handle, and the fueling was limited to keep the turbo intact. In that scenario, yes boost was a consideration. But at any other time, it never was.

That's my perspective from doing my own tuning. Others in the tuning world may be different in their approach.

 

Yes, that helps a lot, thank you! Finally someone who understood my question to help answer it. That all makes sense, except what is the MAP sensor used for then? Speed density systems use MAP/TPS, whereas MAF systems use MAF. I get that these do not use MAF sensors, but they do have MAP sensors, so what is the MAP sensor used for? Is it strictly to measure boost so it can open the wastegate? The way it sounds from your post, it is not used by the ECU to increase fuel based on manifold pressure..?


That's very interesting, thanks for sharing! Although I don't really understand why more boost does not mean more fuel is required in order to make more power...? If that's the case, that would mean that disconnecting the red line from the wastegate causing boost to rise from the stock 16 psi to 23 psi does in fact increase hp...? I say 23 psi and not 30 because the stock turbo starts to move out of its efficiency range around 25 psi....

 

It's easy to talk to you because you understand some tuning basics. And I wanted to answer in a way that didn't just address your question, but also helped you understand a bit more into the diesel tuning aspect and how it's different from gasoline applications. So here's a bit more info to help with your line of questions.

Because diesels don't run at a set stoichiometric ratio like gas engines, there is not the same need to match fueling with boost. That's why you don't see a throttle body on 7.3L, the airflow isn't metered. Let's say if you're accelerating a diesel and running at the gasoline stoichiometric ratio of 14.7:1, you'll make power but you'll generate a lot of black smoke. Under peak torque conditions, the black smoke doesn't start to clear up until you're running lean, around 25:1 and higher (diesels can run far leaner than that, and often do). So having a turbo shove more air in helps the combustion process, but only up to a certain point. Again it's diminishing returns. Since we know that turbos eventually move out of their efficiency range, eventually the air becomes so hot that you actually start losing power. Or the drive pressures far exceed the compressor side and the turbo grenades. Which brings us to the next part.

The MAP is monitored mainly to protect the turbo. In the stock calibration if you go past a certain point you'll trigger an overboost code. But mostly it's used so the PCM knows when to direct some of that boost to the red line and start opening the wastegate. Ford never intended for the 7.3L to be a hotrod, so they stuck very reliable and quick spooling turbo on top, then set up an electronically controlled wastegate to open up early and keep the turbo operational for hundreds of thousands of miles. When you remove the red line, you're shoving in more air and not losing as much drive pressure to the turbo. However you do get closer to being out of the turbo's efficiency range, and of course there's the possibility of sacrificing a bit of reliability. The stock GTP38 will see a drastic rise in drive pressures when you start going much over 20 psi, thank's to its small exhaust housing (hence the quick spool). When drive pressures tick over 2:1 compared to the compressor side, you really start stressing the internals of the turbo.

So in the end, does removing the red line make more power? Yes and no. It really depends on the conditions, and how much boost that stock turbo is pushing out. If you're in the efficiency range of the turbo, yes you might see a teeny bit more power. Most likely won't feel much in the seat of the pants, but what you can gain under certain conditions might be a bit leaner of a mixture and perhaps clean up the emissions a bit more, giving you a slightly more efficient burn and hence that tiny bit of extra power.

If you need more explanation let me know.

 

Wow, that completely answered my question, thank you! I had read that AFRs with diesels weren't quite like gas, but I still thought for cooling purposes and to maintain a relatively similar AFR, more fuel would be needed.

Yeah I've tuned a few different engines using AEM's EMS. I had an old 1997 F-350 with a 460 that used bank-to-bank fuel injection. Since bank-to-bank is so inefficient, I ripped a wiring harness out of an old 1995 F-150, used an AEM EMS designed for an old Mustang, then added the appropriate sensors and re-pinned the ECU plug so the AEM ECU could read the new inputs for SEFI(had to add a map sensor and 6 new injector inputs). I then made a custom tune for that engine from scratch; idle, ignition timing, fuel tables, trims, etc. It was a challenge but I got it to run really well. My MPG went from about 10 to 12 on the highway. It was a fun project. I documented everything I did here

 

Adding more boost w/o fuel should yield more power because internal engine combustion efficiency is a function of cylinder pressure. If you can burn your fuel at higher pressure you should get more energy out of it. Diesel cycle engines do not have the same limitations as spark ignition engines. As all of you reformed hot-rodders know, if you can bump up your compression from 8.5 to 11.5 you can really make more power. As for those who have tried this, its a bit more complicated. Higher compression pistons typically have an effect of cylinder gas exchange efficiency and there is the bit about the fuel octane requirements. Diesels are not subject to pre-ignition. On the other hand if your turbo is only shoving lower density gas into the cylinder at higher pressure, you might not be doing anything useful. What you need is more air mass in the cylinders. Maybe we should check the efficiency of the IC. Have any of you guys pulled it and clean it (inside and outside). The inside could be totally covered in oil over a few 100K miles of blow-by. If the turbo hosing is clean, the IC should be as well I would guess.