If you’re building a 5.9L or 6.7L Cummins, your fuel system matters — a lot. At Full Send Diesel, we’ve built dozens of these trucks, and the same things hold true every time: injectors, CP3 sizing, and a proper rail relief valve are key to making clean, reliable horsepower.
Choosing the Right Injectors
Too many people buy injectors for what they’re doing right now, instead of where they’re headed. If you plan to upgrade your turbo in the future, size your injectors now to match your end goal.
For example: if you’re planning to throw on an S467 later, grab a set of 100% over S&S Diesel Motorsport injectors now. With custom tuning, the truck will run great on your current setup — and you won’t have to tear everything apart again later.
You can always tune big injectors down, but you can’t squeeze more out of ones that are already maxed out. Think ahead — it saves time and money.
Choosing the Right CP3 Pump
Same logic applies to your CP3. You need a pump that can keep up with your injectors — otherwise, you’ll run out of fuel and fall short of your horsepower goals.
For most street trucks, a standard 10mm, 12mm, or 14mm S&S CP3 is all you need. High-speed CP3s are great for race builds, but they’re harder to tune and usually not necessary for street or towing applications.
If you're running big injectors or planning to add dual CP3s later, size accordingly now. Buy it once, not twice.
Why You Should Run a Calibrated Rail Relief, Not a Rail Plug
Both the 5.9L and 6.7L Cummins come from the factory with a calibrated rail relief valve. The issue is — over time, the spring inside that valve weakens. Your tune might be calling for 27,000 PSI at wide open throttle, but your stock relief could be blowing open early at 20,000 or less. That pressure loss kills performance, and in some cases can cost you hundreds of horsepower.
Some guys replace it with a solid rail plug, thinking it’ll solve the issue. Sure, it holds pressure — but it gives you zero protection. If your tune isn’t perfect and rail pressure spikes, that plug has nowhere to bleed it off. Now you’re risking cracked injector bodies, split lines, or fuel leaking into the motor.
The right move? A calibrated rail relief from S&S Diesel Motorsport. It holds pressure when you need it, but vents safely if things go sideways. We stock them here at Full Send Diesel for 5.9L, 6.7L Cummins, and even Duramax platforms.
Skip the plug. Skip the worn-out factory valve. Run the calibrated relief and protect your investment.
Fuel System Horsepower Reference
All horsepower values referenced below are measured at the crank and are dependent on supporting modifications. The following injector and CP3 charts were provided directly by S&S Diesel Motorsport.
5.9L Cummins Fuel System Chart
6.7L Cummins Fuel System Chart
Need Help? We’ve Got You.
If you're serious about building a reliable, high-horsepower Cummins, your fuel system needs to be dialed in — injectors, CP3, and rail relief included. We trust S&S Diesel Motorsport in every build we do, and we stock their full lineup at Full Send Diesel.
If you're not sure what setup is right for your truck, contact us — we’ll get you dialed in.