Fueling The LTG
By: Mark Gibson, Gibspeed Automotive Solutions, https://www.facebook.com/GibspeedAuto
Important Baseline Notes
The LTG, a 2.0L GM ECOTEC derivative, is fueled from the factory using a Direct Injection (DI) fuel system. This system consists of a Low Pressure or In-Tank Fuel pump (LPFP or ITFP), which lifts fuel from the tank to the second component in the system, a High Pressure or Cam Driven Fuel Pump (HPFP or CDFP). The HPFP, driven by the camshaft, compresses the fuel fed from the LPFP and sends it to the fuel rail and the final component in the system, your Direct Injectors. The injectors, mounted into the cylinder head, then fire fuel sequentially, directly into each cylinder.
• LPFP – Controlled by its own Control Module and feeds between 60-80psi at all times
• LPFP- ~180LPH and limited by Total fueling requirement only
• HPFP- Controlled by the PCM and feeds between 2500-3500psi at peak load
• HPFP and DI Injectors – Limited by total fueling requirement and RPM
These are very basic, but extremely important concepts to remember when thinking about fueling the LTG, understanding how GM uses these components for a STOCK vehicle, and understanding how best to use these components to make the increased power you desire.
DI and PI Basics
Direct Injection (DI)
Pros
o Extremely Efficient Burn (Can run leaner than PI)
• Better fuel economy
• More power with “less” fuel (leaner burn)
• Provides cooling directly in the cylinder
• More “safe” power available with the same total fuel volume
• Ethanol’s cooling ability increased exponentially over a typical PI system ▪ Allows a higher compression ratio • More responsive • Faster transient turbo response and overall spool
Cons
• Limited fueling window
• Can only spray for 300-350 Degrees of each engine cycle
• Fuel capability is more limited as RPM increases
• Limited Upgrade Ability
• Can only increase fueling capability by: • Increasing High Pressure Fuel Pressure • Increasing High Pressure Injector Volume
• Still need all Low-Pressure pump upgrades a standard PI system would need.
• Even with ALL AVAILABLE upgrades, still limited to 450whp on E85 or 640whp on 93 and RPM is still limited
• Cost/Availability
• No washing of the intake valves to keep carbon at bay
• Tuning, Difficult to tune and very few professional tuners have significant experience in this. (Shoutout to Cooper’s Camaro)
Port Injection (PI)
Pros
• No Carbon Buildup
• Fuel sprayed onto the intake valves to keep them clean o Unlimited Fueling Window
• Can spray during the entire 720 Degree engine cycle
• Fueling capability NOT limited by RPM
• Unlimited Upgrade Ability
• Replace little injectors with bigger ones
• Replace the Low-Pressure fuel pump with a bigger one
• Go big enough and there’s literally no limit to the amount of power you can make, even on E85
• Cost/Availability
• Thousands of options for PI injector upgrades
• Tuning
• Been around forever and easy to tune for any professional tuner
Cons
• Inefficient Burn (relative to DI)
• Significantly less cylinder cooling
• Higher octane and “more” fuel (richer mixture) needed for the same power output
Dual Injection (DI + PI)
Pros
• All Pros of DI + All Pros of PI
• If PI is a Pre-Throttle body, you get the added bonus of significantly cooled charge-air and still maintain the cylinder cooling of the DI injectors on top of it.
Cons
• Requires a secondary controller to tune (Any professional tuner can tune them together though)
Auxiliary/Dual Fueling Theory
The overall theory is simple. Add a secondary source of fueling, to supplement the DI Injectors’ limited ability to keep up with the demands of increased airflow and increased RPM necessary for increased power output from the LTG. Below, I’ll detail how that can be accomplished and why it works so well. Please DO NOT use any of these images as guidelines for the actual changes you should make in your tune. These were purposely skewed for illustration purposes and if you attempt to use these values, you will… fuck your shit up. Please instead consult your tuner and feel free to share this information with them. They will have the expertise and ability to translate these images into exactly what you need.
The Stock Airflow Model (MAF Based)
In the stock airflow model below (MAF Based), you can see the gradually increasing curve of a typical PCM. This curve is used to predict the amount of air that will be entering the engine at a given RPM and MAF Frequency. For any vehicle running a MAF sensor, with no supplemental fueling, this is the type of curve you’ll come across, with only smoothing and multiplications of the curve necessary to account for different bolt-on modifications.
The Aux Fuel Airflow Model (MAF Based – Meth Injection)
In this model, you’ll notice both the Blue line, representing the stock airflow model, a red line, representing the airflow model typically used with Methanol as additional fuel and a highlighted space between the two. This would be used on a high HP stock turbo or big turbo (BT) setup. In this case, you’ve hit, or come close to hitting, the limit of the stock DI Injectors and HPFP. To increase your headroom, you’re telling the PCM to expect LESS airflow into the engine, than will actually be coming in. This causes the PCM to limit the amount of fuel being injected in that range of the MAF curve. This reduction of the MAF curve is usually done by a global percentage change but follows the same curve as stock. The highlighted area on this image depicts the amount of fueling the Methanol is taking care of to keep your AFR on target, even while DI injection has been decreased. You must note that in the case of a progressive meth controller, your start of injection would be at the point the curves begin to diverge and the “full” injection point would be at the point your new curve hits its bottom. Once that methanol is spraying at its full amount, you once again start relying on the PCM to begin adding more DI fueling to stay on target. This means you’ll still eventually (and usually rather quickly) find yourself running out of fuel again, just at a higher RPM and overall airflow. For the reasons highlighted above, as well as some other meth-specific downsides, I only recommend using meth as fuel for small BTs, where you’re only asking the meth to provide cooling, octane, and ~15-20% additional fueling headroom. Once you begin leaning on meth for more than 20% of your overall fueling demands, bad things tend to start happening.
The Aux Fuel Airflow Model (MAF Based – Additional PI Injectors)
In this model, you’ll notice both the Blue line, representing the stock airflow model, a Red line, representing the airflow model able to be used with Secondary PI Injectors and again the highlighted area in between. This type of setup can be used in any of the cases a Methanol setup is used, while also being recommended for any truly high hp setup (>400whp). Much like the Meth-based airflow model, you’re telling the PCM to expect less air into the engine that will actually be coming in. This, once again, makes the PCM request less fuel to be injected by the DI Injectors. The overall concept is the same, tell the PCM less air is coming in, therefore less load on the DI system where its ability is limited, and the rest of the necessary fuel is injected by a different fuel source.
Where this system differentiates itself from the typical Methanol setup, and why it’s far better for bigger power setups, is in 2 ways.
1. Since you’re using a Pulse Width Modulated (PWM) Injector and controller, you have much more control over how precisely the additional fuel is being injected into the airstream.
2. You’re not limited to a static amount of additional fueling, like the “full” position on a meth nozzle. Instead, you’re able to Flatline your DI airflow model at the exact amount you want. This means you can look at a Datalog, decide where your DI Fueling is at the limit you feel it’s best suited for, and then tell the PCM to cap the amount of DI Fueling at that exact point. From there, you can use your Secondary PI Injectors to make up the entire remainder of the airflow curve. This design allows for all the benefits you’d see from a PI engine while keeping all the benefits from your DI system as well.
3. Since these injectors are much more precisely controlled and can increase flow almost indefinitely, you can supplement as much as 80% of your overall fueling demands with this PI system, provided you’re making enough power to need it.
Closing Comments
Direct injection is great, but it is flawed. If used correctly, in conjunction with supplementary PI fueling, you can have the best of both worlds, meaning endless power potential and endless fun. I hope this document can serve as a starting point which leads countless LTGs to great power and great success. And now I’ll crawl back into my hole and finish designing more parts.
