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Is Bigger Truly Better When it Comes to Modding Your Diesel?

Diesel Performance Parts, Dodge Cummins, Ford Powerstroke, GM Duramax, Tips and Tricks

Written By: Lawrence “LT” Tolman

You might’ve heard that age-old adage about beginning with the end in mind, but what does that really mean? Let’s say you want to get to a certain destination, maybe the south coast of Florida. You could start your journey with the basic knowledge that your goal is south and east of your current position and just aimlessly drive in that general direction until you hit the water. You don’t have to be a rocket scientist to understand the likelihood of reaching your destination without some planning, a map, or GPS is pretty slim, and likely you’ll end up in New Hampshire or something. A better idea would be to enter an address into your favorite GPS app, and it will tell you the most direct way to reach your destination with very little wasted effort. Our passion is building trucks, not taking blind road trips, so what does this analogy have to do with building a diesel? It’s all about making a plan, having a realistic goal, and following very specific steps to reach your goal without wasting time, money, hurting your engine, or doing things twice. Here’s what I mean:

Before you start a new build, you should have a solid idea where you want to ultimately end up, whether your goal is to have a tow rig, a hot daily driver, or a race only truck. Depending on what parts you purchase, you can either make forward progress along the path you set out on, or if you get a little trigger happy and ignore the plan, you can buy the wrong parts and make several steps backward. Often, many of the parts you will add to your build will come in many different sizes, and it can be natural to think that the largest will be the best and will make the most power. While the biggest injector or turbocharger certainly has the capacity to make more horsepower than a smaller version, what impact will that have on the rest of the build? For a hypothetical build, lets consider a 5.9 Common Rail Cummins, with an end goal of 800 horsepower at the wheels, to be used as a hot street and weekend play toy.

Fuel the Fire

If you want more power from your diesel engine, the first thing you’ll need is more fuel. On a stock truck this is accomplished with a tune. By telling the ECM to leave the injectors open for a longer amount of time, more fuel is allowed to enter the combustion chamber. This is OK for a little more power over stock, but once you push an injector to the limit, its pulsewidth (or on-time, measured in microseconds or μs) becomes so long, the combustion event is no longer happening in the bowl of the piston when it’s near top-dead center. Without diving too deep into a technical discussion, a longer pulsewidth means the flame front is “chasing” the piston down the bore, which puts more heat onto the weaker outside edges of the piston and is a much less efficient way to make power. In simplest terms, keeping a small injector open too long is bad for your pistons, so to make more power safely, you need to install larger nozzles or injectors which can deliver more fuel in less time. So you should just go out and buy some 250% overs for your stock turbo 5.9 CR and hit the track, right? That’s a negative ghost rider.

When selecting turbo size, its imperative to match the size to the amount of horsepower you are currently making, or will be making in the very near future.

Just as there are issues running an injector that’s too small, running an injector that is too big can also lead to issues, and possibly catastrophic failure. First of all, to make a larger injector idle and run correctly at part throttle, the pulsewidth (μs) will have to be cut down a ton, which can give poor fuel atomization, a hazy idle, and poor partial throttle response. Furthermore, at wide-open throttle your EGTs will go through the roof if you don’t have enough air coming from the turbocharger especially in towing conditions, not to mention your fuel mileage will go out the window. A 250% over stock injector on a 5.9 Cummins has an ideal power range somewhere between 900 to 1,600 horsepower, so if your end goal is only 800, then realistically you only need an 80% larger than stock injector. To get the correct amount of fuel into the engine, you could run some BD-Power Stage 4 180hp injectors which flow 93% more than stock. These would allow you to meet your power goal without being too big to cause drivability issues.

While the injectors are primarily responsible for CREATING horsepower, both the lift and high-pressure injection pumps also need to be scaled up to SUPPORT the increased fuel demand of the larger injectors. For our 800hp 5.9 example, the stock CP3 won’t be adequate, so you have a couple choices. Either you can add an additional CP3 with a dual pump kit which will support about a thousand horsepower with two stock pumps, or you can upgrade to a single “stroker” pump which has more capacity than stock. A Fleece Powerflow 750 is a 10mm pump which is rated at 750-wheel horsepower, or the next step up would be 12mm pump which can support nearly 1,000 by itself. You can go a little larger on the injection pump side of things without as many negative side effects, but keep it reasonable. While a pair of 14mm pumps could support around 2,500hp, you’re only going to be using a fraction of the available capacity, you will be raising the temperature of the fuel excessively and wasting a tremendous amount of money. With the high-pressure fuel taken care of, the next issue is the lift pump, and to support 800hp you’ll need a pump that move 165 gallons per hour (up to 900hp capacity) of fuel up to the inlet of the CP3’s, and a FASS Titanium Signature Series is just the ticket. Plus, the added filtration and water separation is a great insurance policy which will keep your new (expensive) injectors and CP3 nice and clean.

The FASS Titanium Signature Series is thought of by many as the ultimate in fuel delivery and filtration.

REST STOP #1: It’s worth noting there is an ideal sequence and order install your modifications. Still working with the 800hp goal on a 5.9 CR Cummins, the total cost of that caliber of build will no doubt be close to the cost of purchasing another truck, so it can take some time to plan and save for the big purchases. While a 180hp injector might be what you need, it might take a couple years to save up enough money for the built engine, big turbos, built trans, injection pump, and other drivetrain upgrades, so you may be tempted to throw in the injectors while you wait for the rest of the parts to arrive. For the same reasons discussed earlier, it doesn’t make a lot of sense to install a set of big injectors with a stock turbo for an extended period of time, since it can’t provide enough air to burn all the fuel being fed into the engine. It makes more sense to purchase the parts one at a time and stick ‘em on the shelf and install everything all at once. I do understand it’s easy to get impatient running around with a mildly tuned truck, but in the long run it’s a smart move to make for your truck’s longevity. If you do want to make some upgrades, a smarter move would be to upgrade some smaller items first, like the lift pump, head studs, valve springs, and the air handling system first.

With a Bigger Hair Dryer

Just like with the fuel system, the air system is an important upgrade to a diesel truck when you’re chasing a big power number, but also just like with the fuel system, there is such a thing as too big of a turbo. If you ask 10 different people what the ideal size turbo is, you’ll get 10 different answers, since everyone’s opinion of what’s drivable and what spools quick is different, so we’ll speak in general terms. Keeping with the same 800hp example, it’s a fact the stock turbo won’t come close to that number by itself, regardless of how much fuel you push through it. The turbine and exhaust housing are simply too small, and the harder you push it with fuel, the more exhaust back pressure you’ll have, along with higher EGTs (DANGER WILL ROBINSON!). You could do a second-gen swap and install an S475 with the thought that its extra airflow will come in handy “just in case” you want to go further than 800, since a 475 can support almost 1,000hp by itself. In the real world, the 475 as a single will be very laggy on the street, especially with stock fuel. Again, everyone’s opinion of spool up and drivability is different, but why give up low RPM boost and torque if you don’t have to? At 800hp, a single S467 or S468 will be right in its sweet spot and will have much better street manners than a single 475. A compound turbo kit can also be sized too large, but the benefits of compound boost give you a much broader window to operate in. Adding an 80mm charger on top of the stock HE351 with an ATS Aurora Plus 7500 kit will make a 5.9 spool just like stock, but the added airflow up top will support tons of power, which is why I am a firm believer in compounds for any truck that spends time the street.

REST STOP #2: We’ve been discussing the air and fuel systems as stand-alone components, but the truth is both sides need to be addressed to balance everything together. You can think of it just like in high-school algebra class: if you add something to the fuel side of the equation, you’ll also need to make adjustments to the air side of the equation to keep things in balance. A larger turbocharger needs more fuel to spool, and larger injectors need more air to keep the combustion temps under control. It’s a symbiotic relationship, and that also extends to the rest of the powertrain as well.

Upgrading the turbo is a necessary part upgrade when looking to increase the performance of any diesel. That said, turbo selection is extremely important.

The Rest of the Build

Once you have the air and fuel system sorted out, a lot of the hard work and planning is done, but you’ve just barely crossed the Florida state line. There are several other parts of the drivetrain that still need to be strengthened up, and careful parts selection is critical here as well. The torque converter you select will need a stall speed that matches what RPM the turbos spool at; too tight and your turbo will take ages to light, or if you go too loose (high stall speed) your engine will be out of its power band before your truck starts accelerating. The final drive will need a gear ratio that meets your tire size, and mile-per-hour goals, your exhaust needs to be the proper diameter, and there are about a dozen other smaller items to choose, like head studs, vibration dampener, flex plate, transmission shafts clutch material and more. When you consider and budget for every item that needs to be changed to meet a horsepower goal, the final dollar amount might surprise you. But, it’s better to know ahead of time what the final cost will be and adjust your final goals accordingly. And if you get stuck and need help making choices on what parts to select, or even want to discuss what a reasonable horsepower goal is, feel free to call in the backup, and give the guys at Diesel Power Products a call. They have a tremendous amount of experience with diesel trucks and will be happy to help you figure out what direction to take your build and recommend a parts combination to best suit your goals.

Patience is a Virtue

I will admit, the hardest part of building a truck is having realistic goals, and the second hardest is waiting for the parts to arrive. Sure, everyone wants to hit that magic 1,000 horsepower number, but is it worth it? Can you afford it? And do you need it? Or are you just chasing a number that sounds cool so you can tell your friends you have a thousand horsepower rig. And while we used an 800 horsepower 5.9 Cummins as an example for this discussion, the principal of balancing components can be applied to any type of build, whether you’re after 500 or plenty more . Just remember to stick to your plan and select parts that aren’t too big or too small, and you’ll be in Key West in no time. The last thing you want is for your build to go sideways and wind up with a connecting rod hanging outside of your block somewhere on the side of the road in New Hampshire.

I’ll leave you with this parting thought: if this is your first time building a high-power truck that will spend a lot of time on the street, it might be worth considering the limits of your stock pistons and rods and use that as a target HP goal. While a six to seven hundred horsepower engine doesn’t sound as cool or impressive as a thousand, it is much more affordable, and cost is the final (and arguably the most important) variable in the equation that needs to be balanced. Besides, you can always go nuts the second time around…


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