Here is a Closer Look at Direct Injection on the Gen V V-8
The Present & Future of GDI: An advanced look at direct injection on the Gen V LT1 and LT4 V-8s
By now, no doubt, you have come to realize that direct injection (DI) is not the performance-killer it was once thought to be. There is a literal race to see who can make the most power with the new Gen V DI engine, and so far we are all winning. In the race to the top, there is most certainly a cap on horsepower that can be hit with even the more stout LT4 fuel system. As a result, some have used methanol injection nozzles, throttle body gasoline injection, and even a separate port fuel-injection system. But before we add fuel sources or abandon direct injection all together, it is important to understand just what is going on under the hood of our C7 Corvette or sixth-gen Camaro SS.
Assuming you have gotten past the concept that fuel is now shot directly into the combustion chamber, let’s talk about what happens next. The chamber on a Gen V engine has been designed with over a million man-hours of effort. Yes, a million. Much of it is done through CFD (computational fluid dynamics) and lots of engine dyno time. As with any engine, the intake valve opens to allow air to enter this chamber. The air does not always come in smoothly. In fact, it comes in as a tumble, swirl, and axially. In port-fuel injection this was very helpful in creating a homogenous mix, something we will revisit shortly. In DI, it’s raw, fresh air that enters the chamber. This is also referred to as “charge motion.” The fuel injector has six small discharge orifices that are strategically aimed at various parts of the chamber. This is where the engineering comes in. It is critical that every droplet of fuel can bond with the incoming oxygen and that it mixes evenly, or homogenously. The timing of when and where the fuel is injected is easier said than done—especially when we start modifying it!
So we wake up one day and decide we want to throw a supercharger on the C7, and while we are there let’s throw a cam in it, too! The supercharger is an air pump in front of the air pump and its function is to shove air into the engine above atmospheric pressure. The camshaft is the gatekeeper, and it controls when this air is allowed to enter and leave. Both of these go-fast parts will most certainly work against the design (as noted) in the chamber. If you have ever seen an Olympic bobsled race, you will witness how the bobsled follows a path in the corners that is usually dictated by entry and speed. It is fairly predictable. But if we increase the speed, it will track differently into the corner. The same thing occurs when 10 psi of boost is stuffed down into the chamber. Since the injector spray pattern was designed around a stock engine’s charge motion, you can see where we can run into an issue now that the target has moved. This can cause huge problems, such as raw fuel (unburned) being left in the chamber. Eventually this raw fuel leaves the exhaust port wasted, leaving power on the table. Now we add a camshaft that determines when the introduction of this air is allowed to enter and leave, and we just twisted the game again. Since the point of injection is in the chamber, the amount of time available to introduce fuel has been significantly reduced. From a power perspective, that pushes us backward since we need a certain “mass” of fuel to make “X” amount of power. Ideally, we would like to start spraying the fuel when the intake valve opens and stop spraying when the exhaust valve starts to open. This is the injection window. There is a small window overall to get air in, spray enough fuel mass, mix it, light it completely, make it work, and then get rid of it. Fortunately, after all this doom and gloom there are some solutions and still challenges.
With great industry tools such as HP Tuners and EFILive software, we are able to manipulate some of the tables that control the timing of the injection. If we are able to back calculate the stock injection timing with a stock camshaft, we can figure out an offset based on the new camshaft timing events as related to the injection window. Although this is not exactly the correct way to do it, at least it gets you close to being in line. If you really wanted to do it right, start investing into a 5-gas analyzer and a cylinder pressure transducer. The “5 gas” reads the five major gases of combustion: oxygen, hydrocarbons, carbon dioxide, carbon monoxide, and oxides of nitrogen. The proper way to know if combustion is complete is through these readings. But you say: I have a wideband! Well, it’s game-changer time for all you tuners. Let’s go back to the simple days of making lemonade. If we follow the instructions, we note that 2 quarts of water and one scoop of sugar thoroughly mixed will create the perfect lemonade (or proportion). We will assume for this explanation the water is air and the sugar is fuel. Now we take the same ratios but we pour the sugar in without mixing it and serve—it’s not going to taste good until you have the correct proportions. What this leads to is a false reading at the wideband NOT because you have the wrong mass of air and fuel, but because you didn’t mix it. This misconception can lead to a poor-running engine and also engine failure. Needless to say, be sure you are working with a shop that understands this process as the game has definitely changed.
