Big Cam Cummins: Optimal Fuel Pressure For Performance

The Cummins 855 engine, also known as the Big Cam, is a stalwart powerhouse in the heavy-duty engine space. With a maximum horsepower rating of 605 at 1800 RPM, this engine is capable of delivering impressive power for demanding tasks. The high horsepower allows the 855 to handle heavy loads and provide the necessary torque for tough jobs. The engine features 6 inline cylinders, making it a reliable and efficient choice for a variety of applications.

The fuel pressure of a Big Cam Cummins engine is a crucial aspect of its performance and efficiency. While the exact fuel pressure can vary depending on various factors and modifications, it is important to note that making adjustments to the fuel pressure can impact the engine's power and fuel consumption.

One way to adjust the fuel pressure is by changing the \fuel button\ in the pump, which can increase the fuel flow and, consequently, the engine's horsepower. Additionally, modifications such as drilling out the throttle shaft and turning up the pressure can also be made, but these changes should be approached with caution to avoid potential engine damage.

It is always recommended to consult with experienced mechanics or professionals before making any significant adjustments to the fuel pressure or other engine components.

Drilling out the throttle shaft

To begin, the top piece of the throttle linkage is removed by taking off the nut. The bottom piece is then rotated one notch clockwise, and the tabs that make contact with the fuel line are ground off. This step ensures that the throttle has more travel before hitting the fuel line. After reassembling, the idle will be significantly higher, and the next step is to start grinding down the throttle shaft to gradually lower the idle to the desired range. This process requires patience and caution as removing too much material can result in an excessively low idle.

Once the desired idle is achieved, the next step is to address the throttle linkage and spring. Swapping their positions allows for a full sweep of the throttle arm. Additionally, the throttle linkage shaft may need to be lengthened by loosening the jam nuts and rotating the shaft. After these adjustments, the throttle response will be more immediate, and the engine will return to idle faster.

It is important to note that this modification can make the throttle more sensitive and touchier. Therefore, it is crucial to exercise caution and gradually adjust the throttle shaft to avoid removing too much material. Additionally, this procedure may void the engine's warranty, and it is recommended for experienced individuals only.

Changing the fuel button

The fuel button is located inside the bowl at the rear of the pump. To access it, you will need to remove the bowl and snap ring, and then pull out the valve. The fuel button is inside the valve and will have a number stamped on it. This number indicates the fuel pressure, with lower numbers corresponding to higher fuel pressure. For example, a "#12" button will result in higher fuel pressure than a "#22" button.

Before changing the fuel button, it is important to check the current fuel pressure using a "snap test". To perform a snap test, you will need a 0-500 lb pressure gauge. Start the engine and let it warm up, then quickly snap the throttle to the full position and release it while observing the pressure gauge. The pressure reading will give you a reference point to compare with future tests after changing the fuel button.

When changing the fuel button, it is recommended to start with a #12 button and then experiment with different buttons, gradually decreasing the number until you reach the desired fuel pressure. It is important to be cautious when removing the snap ring, as it can be difficult to catch, along with the shims and spring. Additionally, be careful when removing the valve, as the fuel button inside may fall out.

After changing the fuel button, perform another snap test to measure the new fuel pressure. The ideal fuel pressure range for a Big Cam Cummins is between 195 and 200 PSI. Adjust the throttle shaft screw as needed to fine-tune the fuel pressure.

It is worth noting that increasing fuel pressure alone may not result in more horsepower. Other factors, such as the fuel screw and governor shims, also play a role in engine performance. Additionally, be mindful of the engine's limitations, especially when it comes to RPM and EGT (Exhaust Gas Temperature). Always monitor your engine's performance and make adjustments as needed to ensure safe and efficient operation.

Turbocharging

One key aspect of turbocharging any engine is ensuring that the turbocharger is appropriately matched to the engine's specifications. For instance, when selecting a turbocharger for a Big Cam Cummins engine, it is essential to consider the specific model, such as the NH or NT series. Diesel Parts Direct, a supplier of turbochargers for these engines, offers a range of options, including the HT3B, BHT3B, and VT50 turbochargers.

When considering turbocharging, it is crucial to assess the current condition of your engine and determine if any adjustments or upgrades are necessary. For example, you may need to adjust the fuel pressure or consider adding an after-cooler to manage the increased temperatures that come with turbocharging.

In one discussion, a user seeks advice on increasing the power of their 250 hp 855 cummins engine. They are advised to perform a "snap test" to check the fuel pressure, which typically reads around 175 PSI. The user is then guided through the process of adjusting the fuel button, located inside the bowl at the rear of the pump, to increase fuel delivery to the engine. This process involves removing the bowl, snap ring, and valve to access the fuel button, which typically has a number like "18" or "22" stamped on it. By replacing this with a lower number, such as "12," you can increase the fuel pressure to the desired range of 195 to 200 PSI.

Another user seeks advice on putting a bigger turbo on a Big Cam 400 engine. They are offered a rebuilt HT4B Holset turbocharger with a modified end for a charge air cooler. This highlights the importance of ensuring that any aftermarket or modified turbocharger is compatible with the specific requirements of the Big Cam Cummins engine.

In conclusion, turbocharging a Big Cam Cummins engine can be a complex process that requires careful consideration and planning. It is essential to match the turbocharger to the engine specifications and make any necessary adjustments to fuel delivery and engine settings. While some resources and discussions are available, seeking expert advice and guidance is always recommended to ensure optimal performance and avoid potential engine damage.

Injector reflow

The first steps in injector reflow involve disconnecting the battery and removing various engine components to access the injectors. This includes the intake, heater grid, dipstick, wiring, intercooler boot clamp, valve cover, breather box hoses, and more. It is important to keep the work area clean and free of debris, as dirt can damage the fuel injection system.

Once the injectors are accessible, the high-pressure lines and connector tubes must be carefully removed, ensuring that the connector tubes are not damaged in the process. The injectors are then pried out, cleaned, and inspected for any damage. New copper seal rings are installed, and the injectors are reinserted into the engine, along with the connector tubes.

The next steps involve torquing the injectors, connector tubes, injector wires, and fuel lines to the specified tightness. The rocker arms are also reinstalled and the valve lash is set. Finally, the fuel lines are bled, and the previously removed components are reinstalled.

Pump recalibration

The first step in pump recalibration is to perform a throttle snap test. This test will give you a reference point for the current fuel pressure. To do this, install a 0-300 PSI gauge in place of the 1/8" pipe plug on the side of the shutoff solenoid at the top of the pump. Start the engine and let it warm up, then quickly snap the throttle to the full position and release while observing the pressure gauge. This will show you the current fuel pressure, which you can use as a baseline for further adjustments.

The next step is to adjust the fuel button, which is located inside the bowl at the rear of the pump. Remove the bowl and snap ring, and pull out the valve. The fuel button will be inside the valve and will have a number stamped on it. The lower the number, the more fuel will be delivered to the engine. For example, if the current button is a "#24" and you want to increase the fuel pressure, you can try a lower number such as a "#12". After changing the fuel button, perform the snap test again to see the new fuel pressure. Experiment with different fuel buttons until you achieve the desired fuel pressure, ideally between 195 and 200 PSI.

It is important to note that increasing the fuel pressure alone will not increase the RPM. To increase the RPM, you must also shim the governor (gov) spring under the bell cover. Additionally, be cautious when making these adjustments as too much fuel pressure can lead to smoking and high exhaust gas temperatures (EGTs). It is recommended to install a pyrometer to monitor the exhaust temperatures and keep them under 900 degrees Fahrenheit.

Another factor to consider is the turbocharger. Increasing the fuel pressure without upgrading the turbocharger can result in insufficient boost pressure. This can lead to inefficient engine performance and potential damage. Therefore, it is important to ensure that the turbocharger is capable of providing sufficient airflow to match the increased fuel pressure.

Furthermore, it is crucial to seek advice from experienced Cummins technicians or professionals who have spent time in the injection lab. They can provide valuable insights and guidance on pump recalibration, injector reflowing, and other necessary adjustments to achieve the desired performance safely.

Frequently asked questions