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.
For those of you searching for big power, the ceiling is still around 775 horsepower without supplemental fueling (650 hp with an LT1). The factory injectors can only spray so much fuel in the 75-percent reduction of the injection window compared to the Gen IV’s. Comp Cams fuel pump lobes are critical to enhancing the flow of the mechanical fuel pump with forced induction, and altering the intake lobes can also play a pivotal role in fueling. Opening the intake lobes sooner means we can spray the fuel sooner, before the exhaust valve opens. While piston shape was originally a larger concern with direct injection, we’ve had great results with a traditional-style piston in our twin-turbo 2016 Camaro SS. On a stock LT1 piston, the “bowl” is there to keep fuel more concentrated under cold start. Most of the heat during cold start is closer to the plug than against the cylinder walls. When we built our DI 416ci in late 2013 to early ’14 there was a lot of mystery to this design, so the piston manufacturers just mimicked it. We had worked closely with Wiseco on this and came up with a general consensus. Since then we have taken to test this out. Our 427ci engine in the 2016 Camaro has a fully dished traditional-style piston top. We have zero issues with power and most notably cold start. As a matter of fact, the car starts up better now than it did stock!
Once you’ve maximized the camshaft and injection timing, the only place left to go is supplemental fueling. Many tuners initially resorted to heavy doses of methanol injection, which has additional cooling benefits (high vaporization of heat) and octane, but ultimately many are moving in the direction of having more precise stand-alone port-injection systems like on our 2016 Camaro. The same strategy is used in IndyCar, which uses E85 and can’t supply enough fuel through direct injection alone. E85 requires roughly 30 percent more fuel, so its use is limited in direct-injection applications without supplemental fueling. The opportunity is there for aftermarket manufacturers to figure out how to integrate port injection with GDI in the most seamless way possible, but presently, there is no control available for GDI, and many ECUs don’t support more than 12 injectors.
Steve Miller, President of Ilmor Engineering, who developed Chevrolet’s IndyCar engine said, “The best solution would be to do the lower throttle stuff with DI only, and then bring on the port injection at higher loads. If the port injectors are too far up the runner, this can introduce transient problems due to the necessity of wetting the walls when they dry out during the DI-only operation, but of course this is less of a problem at high-mass flows.” He went on to say that the location of the port injectors is critical and should be “low and facing the inlet valves.”
There are two major concerns when using supplemental fueling with direct injection. Because of the difficulty in integrating two separate EFI systems, you do run the danger of dumping too much fuel into the chamber (or at the wrong time). Ultimately, that can wash the oil off the cylinder walls and even cause pre-ignition (aka detonation). However, this is not a concern with a properly designed and calibrated system. On the other hand, some loss in efficiency is unavoidable. Steve Miller warned that “a well-set-up DI engine can normally tolerate more compression ratio without crossing the knock boundary, so some caution needs to be exercised to make sure that an uprate combined with some port fuel injection does not get you into trouble.” By adding a less-efficient fuel delivery method you must account for this in the ignition timing and compression. Of course, the use of race gas, E85, or methanol would help. Regardless of what fuel you use, some cooling effect is observed when spraying on the back of the intake valve that can add torque. Often this is why carbs are credited as making more power than EFI, albeit much less efficiently. In addition to removing heat from the air in the process of vaporization, fuel also acts as a detergent. So adding port injection can actually help clean the intake valves as well. Despite the concerns, ultimately the good outweighs the bad.
As we look to the future, we can only hope that more aftermarket solutions will become available. When we asked Steve Miller about the potential for larger aftermarket DI injectors he said, “The flow limit of DI injectors are pretty well set by the seat diameter and the maximum size of holes that can be incorporated within this. Making new configurations of injectors is a big investment, and so each manufacturer tends to stick with sizes which are popular, bearing in mind that most auto manufacturers have modular engines based on a .5L pot size.” The alternative would seem to be using multiple injectors. “The spray configuration is also a critical thing to get right, so that the fuel is distributed evenly across the cylinder in the short time between the end of injection and firing the spark. To put more than one injector in the cylinder would demand a lot of development and there is not that much space, although the two-valve layout of the GM engines would help.”
As we look ahead to the 2016 SEMA and PRI shows, we can expect even more aftermarket solutions coming our way. With new challenges come innovation … and the opportunity for new heroes to emerge. Who will be the next Grumpy Jenkins, Smokey Yunick, or Don Garlits?
01. The supercharged LT4 received the most stout fuel system of the Gen V V-8s; with a higher flow 2,900-psi fuel pump and 25cc/second injectors with a unique spray cone for boost. Even the rails flow more fuel. The LT1 is rated at 2,175 psi and 20cc/second, respectively.
02. The Corvette C7.R race car, which competes in the IMSA WeatherTech SportsCar Championship GTLM class, uses an updated version of the 5.5L C6.R engine with direct injection to meet class rules. Fuel economy was improved 3 percent for fewer pit stops.
03. The twin-turbo V-6 developed by Ilmor for IndyCar also uses direct injection, but requires port fuel injection to supplement in order to meet the fuel mass required with E85.
04. On the right you can see various fuel pump lobes offered by Comp Cams, which have different lifts (5.7-7.5 mm) and number of throws (3-5) to increase the flow.
05. While aftermarket cams can be installed either with or without a phaser limiter, depending upon the specs, it is wise to lock out the phaser altogether on high-powered builds. Redline has a proprietary setup, however, you can also purchase this from Livernois Motorsports.
06. Both the MSD Atomic and Holley intakes were released at the 2015 SEMA Show, which made port injection available on the LT1. A separate controller is needed, though, to actually fire and tune it.