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13 Popular Modifications to Increase Horsepower in a Diesel Engine

Diesel Engine Horsepower Modifications UpgradeThe number one question with any engine, whether it be gas or diesel, is how do I get more horsepower? Raw power is the name of the game. We want to go fast and we want it now! The good news is that there are plenty of ways to add more power to a diesel engine that don’t require expensive modifications. This article will address what is capable by changing the hard internal components of the engine as well as 13 common aftermarket modifications to add more power to a diesel engine.


Before we dive into the various fun ways on the aftermarket side on how to increase horsepower let’s first look at the physics of what is structurally possible and what is not possible on an existing diesel engine. There are only five mechanical ways to increase horsepower out of a diesel engine: Increase Pressure in the Combustion Chamber, Alter the Stroke Length, Alter the Cylinder Liners, Increase the Power Strokes (RPM) and Decrease Friction.


Most likely you are not going to be able to alter the stroke length (up down cycle of the piston). Clearances are usually very tight as far as piston length goes. You can alter the weight of the pistons by using less dense material such as a lighter aluminum composite but you will always run the risk of pistons breaking. Your diesel engine is engineered for a certain piston configuration based on the engine serial number and deviation from OEM spec could damage your engine if not modified properly. That being said the changes to the cylinder liners go hand in hand with oversized or undersized pistons. Overall, reducing the weight of the engine will increase the horsepower. Modifying an engine to make it lighter without sacrificing compression will produce a more efficient distribution of energy to the crankshaft and thus higher horsepower. Using aluminum versus cast iron heads and blocks will reduce the weight of the engine but also decrease the durability of the core. Aluminum blocks, commonly used in automotive engines, are much more prone to cracking. Common lighter but less durable part swaps include the flywheel, flywheel housing, camshaft and ancillary fuel, water and oil pumps.


Cylinder Head Porting of a Diesel Engine

Cylinder Head Porting

Boring out the cylinder liners is a common way to increase horsepower. The greater the area for compression the more fuel burned and thus more horsepower. The cylinder liners or sleeves make up the walls of the combustion chamber. Usually there isn’t much you can do to increase the diameter of the cylinder liners without some help of a machine shop. A machine shop will measure, magnaflux for cracks, properly bore, surface the deck, install injector tubes and pressure test. With the newly larger cylinder liners you are then going to need to use oversized pistons and rings to create dynamite compression.

Diesel engines have both wet and dry cylinder liners depending on the make and model. Wet cylinder liners are a removable liner sleeve that is inserted into the cylinder block. It is called “wet” because it is surrounded by the engine coolant. Dry liners simply use the cylinder chamber bored into the existing cylinder block; there is no removable liner. The goal of the cylinder is to create a tight seal that generates good compression. Going out of OEM spec through boring could cause a loss of compression if not measured properly and/or replaced with oversized liners. By using oversized pistons you get higher displacement which equals more capacity for fuel and oxygen and subsequently higher horsepower. Our take on oversized pistons is that if the sleeves are bored out of OEM spec we generally do not advise it however it is a common way for those in the modification community to increase horsepower.


The last way you can increase the horsepower from a structural engineering standpoint is to increase the RPMs. More combustion cycles per minute will equate to more RPMs which subsequently increase horsepower. Revolutions per minute in an engine measure the number of times the crankshaft rotates on its axis. Consequently, the number of RPMs is exactly double the number of power strokes (up and down strokes of the piston) in a 4 stroke engine. In an old 2 stroke engine the RPMs and the Power Strokes are exactly the same. Unfortunately, there is no quick fix to structurally increase the RPMs in an engine however various modifications can be done. Before making any major internal modifications know that any deviations from OEM spec will void the warranty. The stock engine and ancillary hard internal parts of the valve train are rated for a certain OEM spec.

Both gasoline and diesel powered engines have something called a Rev Limiter. A rev limiter is a device that restricts the rotation speed or the crankshaft or RPMs. It is a fail-safe to protect the valve train. The rev limiter keeps the engine from crossing the “redline” which is the maximum speed the engine can safely operate without damage to hard internal components. A tachometer gauge measures the RPMs in the vehicle. The term “redlining” means crossing into higher RPM territory marked as a red or critical zone on the tachometer. In newer electronic engines the ECM (Electronic Control Module) will automatically prevent redlining. The ECM does this by cutting the fuel to the injectors to lower engine to a safer operating speed. Usually the ECM does this at the redline or a few hundred RPMs over the redline. In an automatic transmission the vehicle will upshift to higher gear automatically before redlining. If the driver in a manual transmission without an ECM is at redline and attempts to upshift to a higher gear but accidentally downshifts to a much lower gear (5th to 3rd for example) the RPM of the engine will try to match the speed of the transmission. The result will be a highly damaged engine.

Now that we have given our spiel about the issues of with trying to increase the RPMs here are a few ways you can increase the RPMs of an engine. The first thing that needs to you need to do is eliminate the rev limiter if present. Once the rev limiter is removed you can then proceed to upgrade the camshaft, valves, valve springs, crankshaft, pistons and connecting rods. As discussed further in this article the lift and profile of the camshaft can be increased to allow more air into the cylinders. More RPMs will put much more stress on your transmission. Look at the nominal torque rating of the transmission installed. Is it rated for the RPMs of your newly upgraded engine? The orbitals, clutches, input and output shafts will all have to be upgraded to handle the increased pressure, wear and temperatures.

Diesel Cylinder Head Porting DiagramIncreasing the RPMs will require a lot of valve train work which will change the performance of the engine. Overall, the additional RPMs will put a lot more strain on the entire system. It is imperative that if you upgrade one aspect of the engine that you also consequently modify all other components to compensate for the additional strain. Upgrading all of the springs, retainers, camshaft and lifters to the RPM you are trying to achieve is a wise thing to do before embarking on multiple upgrades at once. One system plays on another. For example, haphazardly doing modifications to the cylinder head can cause catastrophic failure to the head gasket or head itself. Each component and connecting components all have to be taken into account. The timing of the valves opening and closing enough has to be perfect so the pistons don’t smash into the valve. Other internal components such as the connecting rods and rocker assemblies also have to upgraded to compensate for the additional horsepower.

Other things to consider when increasing the RPMs include the wear on all rotating parts. The flywheel, camshaft, crankshaft and connecting rods are all rated for a certain spectrum of speed. The higher the speed the more kinetic energy is stored and used by the moving parts. Also with increase speed comes increased temperature. More moving parts will simply increase friction all throughout the engine. When doing the modifications you want to use parts that are rated for higher temperature and stress failure points. The most common parts that can overheat are the cylinder head, pistons, gaskets, liners, rings and exhaust inlets and outlets.

As the speed and temperature increase so does the pressure. To increase the speed of an engine typically more fuel will be pumped and burned. Air inlet and exhaust pressures will also increase as more oxygen is required for higher compression. All tubes, pipes and hoses must be more durable and larger to handle the increased pressures moving throughout the engine. It’s real easy to see a frayed hose or loose clamp. You are going to want to make sure the fuel pump, oil pump, water pump, aftercooler and cylinder head are all upgraded to handle the larger capacity of fluids being pumped throughout the engine.

Reducing friction will increase the horsepower and efficiency of a diesel engine. The amount and type of oil and assembly grease will reduce the friction coefficient. This means that every rotating wearable surface that is properly lubricated will use less energy to move thus freeing up more potential energy to convert into horsepower. Another overlooked area to increase horsepower and torque is the wheels. The larger the wheels and differentials the more surface area there is to transfer grip and traction to the road. The stickier the tires the more it changes the frictional coefficient. Remember, Power = Torque * Angular Velocity!

Now that we have discussed the more complex ways to internally increase the horsepower let’s look at some of the most popular aftermarket modifications and add-ons.


1. ECM Flash and Chip Upgrades -

ECM Diesel TruckWith electronic engines the easiest way to increase horsepower is to do an ECM Flash which is simply reprogramming the onboard diagnostic computer to handle more power. Depending on the engine you have a reputable reflash company or the dealer will be able to dial up or down the horsepower to a degree. Typically diesel engine models are grouped into families such as A, B, C, E etc… For example a Caterpillar 3406E is a different build than a Caterpillar 3406B. The pistons, block, head and more can be different. That being said there might be 3 or 4 horsepower and torque ratings within a family. Reprogramming the ECM only takes a few minutes but will allow the onboard diagnostic computer to remove certain power restrictions. The ECM will allow more fuel, air intake and turbo boost during compression. NOTE, as stated above, make sure to check the data plate on your transmission for the Nominal Torque Capacity Rating. The first two digits of that number multiplied by 1000 is the maximum torque allowed for the transmission. Upgrading the horsepower and not the transmission is a good way to ruin your transmission! For those that do not have an ECM in the engine you can reprogram the ECM in truck for various fuel improvements and optimal transmission shift timing. There plenty of various modules and aftermarket chips for those looking to upgrade the engine themselves. Power modules modify the ECM and allow the driver to manually select engine settings for desired performance.

2. Turbochargers and Superchargers -

Diesel Engine Performance TurbochargerWhen you think of upgrading the horsepower on any engine one of the first things that come to mind is a turbocharger. A turbo works by forcing more air into the engine than normal. The internal components of a turbo include a turbine which is force fed exhaust gases which are then pushed through a compressor and then expelled out through the intercooler. The entire process pressurizes the air intake. A stock turbo is designed to increase air flow three to four times over a naturally aspirated engine where as a performance turbo can increase air flow five to ten times more. An upgraded turbo will also keep exhaust gas temperature down which will increase the efficiency and performance of the engine. By not upgrading the turbo (neglecting air flow) and only increasing the fuel flow, you end up burning way more fuel which creates higher exhaust temperatures and dramatically lowers fuel economy. Turbo air pressure is referred to as “boost” and measured in pounds per sq inch. The most optimal setup is a twin turbo design. The larger the turbo the larger the boost however, the big ones really only work at higher speeds and have a considerable amount of lag time. The smaller ball bearing turbos work great a lower speeds and reach peak performance relatively quick. The answer is a dual turbo setup where you have a smaller turbo for most normal driving speeds and a larger one for higher speed and torque situations. The quick spooling turbos work in tandem with larger ones.

Turbo Boost GaugeSuperchargers are sometimes the solution to the dual turbo design. A Supercharger will spin at roughly 50,000 RPM vs. 15,000 RPM with a Turbocharger. Unlike a turbocharger which uses exhaust gases a Supercharger is directly connected to the engine by chain or belt and uses the crankshaft for its energy source. The advantage of a Supercharger is there is no lag, it supplies constant boost even at lower engine RPMs it is easier to maintain and it does not uses a wastegate for emission abatement. Superchargers aren’t used all that much with diesel engines as they already run at much lower RPMs than gas engines and you always have the possibility of a belt or chain breaking. Superchargers can add about 40-45% more horsepower to an engine and, like turbos, work best at higher altitudes.

3. Fuel Pump / Injectors –

Once you have increased the air flow with a larger intake and have upgraded the ECM or modules you then should look at a new set of aftermarket injectors. Aftermarket injectors are built to allow more fuel to flow into the combustion chamber over stock models. Finding the correct injectors for your engine and modifications can be tricky. Injectors are rated for the horsepower output of a particular engine. The goal is obviously more horsepower but you don’t want to overdo it with overpowered injectors. Burning too much free flowing fuel can put more stress on the engine. Ideally the fuel and air flow ratio should be increased in stages ie don’t upgrade the injectors or fuel pump and skip the air intake or turbo. Too much fuel and not enough oxygen or too much oxygen and too little fuel will not give your engine the optimal performance. Most high performance injectors use a special injector nozzle which increases the pressure of for better fuel atomization.

P7100 Cummins Diesel PumpThere are many advantages of older diesel engines from 80s or 90s with indirect injection (IDI). Indirect injection uses a pre-combustion chamber coupled with a glow plug where fuel is mixed with high pressure air and is ignited automatically. The explosion is shot through the injector nozzle when the valve opens and expands in a bowl shape cutout on top of the piston. With indirect injection engines the use of a high pressure injection pump is needed. Older style Cummins fuel pumps such as the P7100 can be modified to directly increase the fuel flow to the injectors however you will be sacrificing miles per gallon. The in-line fuel pumps use a pulse system to deliver a pattered flow of fuel where as a rotary style injection pump allows for a steady constant flow of fuel. The advantages of IDI is lower initial cost, no emissions technology costs and greater advantages for modifications.

Newer direct injection (DI) technology coupled with an ECM allows for a much more fuel efficient design. The injectors open and close automatically to allow a metered amount of fuel to be burned. With direct injection a very precise amount of fuel is injected directly into the combustion chamber right at the top of the piston. There is no need for a pre-combustion chamber as pressures are already extremely high. Upgrades to direct injection applications usually are more on the ECM side of things. Changing the amount of times the injector opens and closes as well as how much fuel is sprayed per cycle can be modified for more horsepower. Advantages of DI over IDI include reduced noise, vibration, higher thermal efficiency, greater fuel economy and overall greater engine performance.

4. Bigger Air Intake/Inflow Kit –

Diesel Engine Air IntakeCompression in a diesel engine can’t happen without the marriage of fuel and air. The stock intake usually impedes air flow by having a restricted opening. The restricted opening usually contains a sound baffle to reduce engine noise. Removing this baffle allows for more air to be sucked into the combustion chamber and mixed with fuel. In addition to the baffle most stock air intakes have a paper filter. This filter is of poor quality and can get clogged rather quickly. New intakes have synthetic fibers that allow for greater airflow even when grimy.

Secondly, OEM Intakes typically don’t suck cold air in from the best places; most likely the air is coming from the radiator wall or just underneath the front fender. A modified intake will draw in cold air that is outside of the engine housing. This colder outside air is much denser and holds more oxygen. The more oxygen available in the air intake the more that can be burned during compression. By upgrading the air intake system you can expect 20%-30% HP Increase as well as improved fuel economy.

5. Intercooler Upgrades –

Diesel Engine IntercoolerOnce you upgrade the turbo you are definitely not going to want to neglect the intercooler. The intercooler is designed to maintain the boost pressure after leaving the turbo while simultaneously cooling the air before it reaches the combustion chamber. The intercooler drops the ambient air temperature from the turbo exhaust which makes the air much denser. Colder, denser air contains more oxygen and thus will lead to more horsepower during combustion. Aftermarket upgrades feature larger fin inlets and outlets as well as an expanded grill and radiator. This is a must have for the diesel enthusiast.

6. Performance Exhaust System –

Diesel Engine Exhaust SystemWith additional modifications to the turbo, intake and cylinder heads it is imperative you also upgrade the exhaust system. The exhaust system in your engine is designed to reduce engine heat and expel post combustion gasses. Much like air intake systems stock exhaust ports are geared towards noise reduction versus performance. Stock mufflers and exhaust tubes are designed with kinks and odd bends to limit noise as well as emission reduction. The overly kinked design makes it hard to push exhaust gas out which only work at optimal levels with newer emission compliant engines the EGR Coolers. The aftermarket exhaust will feature a much straighter design with a wider diameter opening. An aftermarket exhaust and muffler kit will reduce exhaust gas temperature (EGT), improve horsepower roughly 10 – 20 HP and improve torque.

7. Performance Camshafts –

Diesel Engine Performance CamshaftInstalling a performance camshaft is another great way to increase horsepower. The camshaft’s function is to open and closes the valves. The engineering behind a performance camshaft versus an OEM spec camshaft is that the lift and profile are altered to allow more air into the inlet valves. The higher lift on the lobe of the camshaft increases the valve’s stroke length. A greater stroke length equates to more horsepower. You can also alter the valve timing (ie when the valves open and close). With a performance cam you can leave the valves open for a longer period of time allowing more air into the combustion chamber. Leaving the valves open longer changes the compression ratio, horsepower and emissions created. A machine shop can weld, grind to a specific size, polish and measure the lobes with a dial indicator to determine optimal specifications. In addition to camshaft grinding a machine shop can bring back the crankshaft to OEM spec to decrease friction and increase rotational velocity.

8. Transmission / Torque Converter Upgrade –

NV4500 Transmission PartsOnce you add more power to a diesel engine you are subsequently going to put a lot more stress on all of the internal and ancillary components. One point of failure that is often overlooked is the transmission. A high performance torque converter will be able to handle the increased torque created during the all of the modifications. A torque converter upgrade will prevent clutch surface deterioration and pad surface wear weakness. A high performance torque converter will typically be constructed of stronger rated steel to withstand higher internal temperatures and feature more clutch discs to prevent slippage.

If you are really increasing the horsepower by a considerable amount a simple torque converter simply won't do. A higher performance transmission will be necessary if you are seriously upgrading horsepower and torque. There is a lot of strain on the transmission with increased RPMs. Without upgrading the transmission you run the risk of slippage, increased temperatures and premature wear of the internal parts. Aftermarket Billet transmissions from ATS Diesel are considered some of the best in the performance modification world. Ideally if you are going to upgrade your engine you want to make sure you do not neglect your transmission too.

9. Transmission Filter Kit –

Diesel Engine Transmission Filter Kit 2A transmission filter kit will help add life to your transmission. These are very specific filters that reduce clutch wear, adds cooling capacity and removes containments to keep transmission cooler.

10. Nitrous Oxide (NOx) –

The last step and the “Holy Grail” of horsepower upgrades is the addition of Nitrous Oxide or “NOS”. Nitrous Oxide is popular with drag racers and heavy haul rigs needing a short burst of additional power. Nitrous Oxide is a mixture of Nitrogen and Oxygen that is injected into the intake plenum. The compound is broken down by the internal engine heat and injected into the combustion chamber. The quick influx of oxygen allow more fuel to be burned in a quick period of time. There are various levels of NOx from a 75

NOx Kit for Diesel

Popular Brand "NOS" Nitrous Oxide Horsepower Upgrade Kit

HP, 100 HP, 150 HP, 200 HP and a 300 HP boost. For optimal performance the timing of a NOS shot should only occur when the turbo boost pressure is high. There are other additives that can be injected into the air intake such as chemical mixes of water/methanol spray. The Water/Methanol comes in a 1 or 5 gallon tank and is sprayed in conjunction with the boost pressure. The mixture lowers the intake air considerably as well as creates micro explosions in the combustion chamber. The mixture, in the form of a vapor, further breaks down diesel fuel droplets to create a more robust explosion of air and diesel. Chemical intercooling can give an additional 75-100 HP which is perfect for pulling heavy loads.

11. Additives –

Diesel Fuel AdditivesThere are a million fuel additives that are on the market that all promise the world. The truth is most are pretty much the same. Fuel additives are chemical compounds that are designed to mix with your diesel fuel to provide a more clean burning combustion. They are meant to remove engine sludge, stabilize the fuel, clean out impurities and decrease friction. Various additives and blends of diesel fuel will improve fuel efficiency and improve output by roughly 10 HP.


12. Performance Gauges: (Exhaust Gas Temperature, Transmission Temp, Boost Pressure Gauge –

Exhaust Gas Temperature GaugeOnce you have done all of your modifications it is imperative that you monitor all of the new systems you just installed. With performance engines you are going way beyond OEM spec. There is no guarantee the engine will not suddenly blow up on you. Therefore it is wise to install some new gauges in your vehicle to keep an eye on things. As discussed added horsepower puts a considerable amount of strain on your transmission. Installing a transmission temperature gauge is a good idea. Another crucial gauge you should consider is pyrometer. Nothing will ruin an engine faster than getting it hot. A pyrometer measures exhaust gas temperature (EGT) and is installed behind the exhaust manifold or turbo outlet right before the intercooler. Consequently, a boost gauge will measure health of the turbo. It will allow you to see the turbocharger’s temperature as well as the boost performance at desired RPMs. Sometimes a secondary Tachometer (RPMs) maybe be installed next to the boost gauge. Many boost gauges allow the driver to adjust turbo settings within the cab to fine tune the unit.


13. Regular Tune-ups –

One of the most overlooked ways to increase horsepower is to do regular maintenance. Changing out oil and air filters will cleaner models will instantaneously improve performance. Changing out the oil will reduce overall friction, lower internal temperatures and extend the life of your engine.


There are many ways to upgrade the horsepower of a diesel engine. The fun of any gearhead is tinkering and tweaking all of the various systems that so majestically weave together right under our noses. It is important to not over do any modification project as it will consume both your time and your budget. A piece here and there will get you to where want to be. Remember that with any modification you run the risk of blowing up your engine. There is no full-proof fail safe modification. Getting the engine too hot or increasing the horsepower too much too quickly is a great way for your truck to end up in the junk yard. Whether you are working on a 5.9 L or an old Cummins 4BT diesel, learn as you go and remember it's the countless hours of in the garage that is the enjoyable part not the destination.

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