Harley Stage 3 vs. Stage 4: Which One Will Improve Your Ride

Bikes became a popular vehicle for generations now and Harley Davidson has a great role in it. People find it liberating and easy to travel on bikes. It is not just a part of our lifestyle now but beyond it. Bikes have their own show business now; for instance, races, trips, etc. which is why people tend to upgrade stages according to their needs.

Harley-Davidson, also called just Harley, is an American motorcycle company based in Milwaukee, Wisconsin, that was founded in 1903. It was one of only two significant American motorcycle producers to withstand the Great Depression.

While the emission and intake on a Stage 1 design might be rather lenient, a Stage 3, 4, or 5 design involves a highly particular composition. The main difference in the stages like Harley Stage 3 Vs Stage 4 might surprise you. The components chosen will either enhance or damage the upgrade. Let’s find out which part upgrades you the most.

A Short Comparison Between Harley Stage 3 Vs. Stage 4

Stage 3Stage 4
Big diameter and valve modificationHuge diameter, higher compression combo
HP improvements30%-40%HP improvements 50%-60%
Increased compressionIncreased velocity compression and speed
Torque and power kit upgradeCylinder upgrade
No cam upgradeCam upgrade
Lower HPHigher HP
Approx.  $1800 + labor costApprox. $2700 + labor cost

Harley Stage 3 Vs. Stage 4: A Complete Overview

Stage 3: Most people refer to a Stage 3 construction as a modest big diameter and valve modification that includes all of the lower Stages’ parts. The original carburetor mechanism and the majority of the other factory components are used in most of these builds, such as HP improvements of 30-40% or more are possible when compared to stock, 117 big diameter kit 10.8 comps, Wood WM8-222 cam, FM AC/DC air filter, high duty clutch springs.

Stage 4: Stage 4 includes a huge diameter, higher compression combo. It has a greater valve design than Stage 3. Typically, this would entail a larger carburetor mechanism and pistons varying on the different models. Further maintenance alterations, such as gearbox, brake, clutches, and other enhancements, would be needed as well.

However, power requirements will be as high as 50-60% or even higher. For example,  2018 Road Glide, 11.4 comps, 120 big bore kit, Wood WM8-999 cam, FM Level B CNC parts, 64mm TB, 5.5 pumps, Jackpot 2/1 engine exhaust, FM AC/DC airflow filter, AIM SDR lock-up clutch & coils.


Stage 3

A Stage 3 Upgrade is for bikers who desire the most engine capacity and power across the whole RPM range, with up to a 40% boost in the mid-to-upper RPM speed. The improvement gives instantaneous driving horsepower and exceptional intersection to highway speed and on-ramp gear without downshifting.

Stage 3 involves the following changes:

  • Intake, exhaust, and turning upgrade
  • Torque kit and power kit upgrade
  • Big bore cylinders and pistons

Stage 4

It is the most advanced performance enhancement accessible, resulting in the greatest possible increase in velocity, compression, and speed. A larger diameter throttle body and Cylinder heads with CNC ports that flow more efficiently enhance velocity and deliver enormous amounts of  fuel and air into the engine.

This upgrade maximizes horsepower on a larger volume engine. Along with enlarged cylinder heads and a stronger throttle body, you will need valves, compatible pistons, and the right cam.

Stage 4 involves the following changes:

  • Intake, exhaust, and turning upgrade
  • Torque kit and power kit upgrade
  • Big bore cylinders and pistons
  • Heads and throttle body upgrade
  • Cam upgrade


By increasing the diameter of the pistons’ and cylinders compression, Stage 3 increases the engine’s size. This combination of parts will allow you to produce more energy by increasing the compression ratio and result in more powerful combustion. You might need time developing these upgrades to make sure that they do not only deliver on the performance but are also well integrated into the motorcycles.  

Stage 4 involves all the modifications of stage 3 along with a higher quality of cam upgrade. The cam upgrade increases the power of the engine and performs with the maximum efficiency that the upgrades can offer. You will see the change in compression combustion which increases the ability of the bike to give out more energy in the process. This will give you a feel of instant push in the speed, smoothly.  


The increased engine displacement enables the engine for ingest more air, and fuel, while a stronger compression piston compresses the air and fuel into a more dynamic combination that, when sparked, produces more horsepower.

You will gain this by setting larger valves, matched pistons, and the right cam modification. These changes in the bike will allow you to have a different experience than you have had in stage 1 or 2.

Stage 4 is mostly for those who want to experience the most powerful engines that can generate higher torque than stage 3. It includes all the upgrades of stage 3 and after that the addition of advanced cylinders heads and larger throttle bodies.

And just like the other stage kits, the other components like the high capacity lifters, cam bearing, clutch components, and all the necessary gaskets are included in the kit to make sure that the integration into the motorcycle is seamless.


It delivers the highest horsepower possible while still maintaining high quality to ensure the durability and reliability of your engine. This stage uses the same high compression pistons as stage 3 and larger diameter cylinders from the stage 3 kit. Now, in addition to that, stage 4 feeds air through the larger throttle body compared to the original one that already feeds a ton of air into the engine. So you can imagine how much more power you will get from this upgrade.   

High-Velocity Air

The air is fed through a cast aluminum manifold. That manifold feeds unique screaming CNC ported cylinder heads. These are known for their unique casting to deliver not only volume air but also high-velocity air. 


The stage 3 upgrade delivers up to 24% torque improvement, 39% HP improvement. After the stage 4 upgrade the throttle body altogether, the whole combination increases the torque and HP throughout the entire RPM range.

Cost comparison

The difference between the upgrade of stage 3 and stage 4 is half. For example, a stage 3 upgrade on a Milwaukee Eight Engine costs approximately $1800. Although this is an estimation of the kits only without including the labor cost.  On the other hand, a stage 4 upgrade on a Milwaukee Eight Engine costs approx. $2700 excluding labor costs which are almost double the cost.

This is obviously because of the advanced kits used in the stage for gaining better performance.

What is the best to choose, stage 3 or 4?

Honestly, this is a personal choice and it completely depends on the rider. For riders who want to have a good performing bike with a low-medium speed range, I suggest stage 3 is a good option for you considering the cost as well.

But if you are someone who wants the highest possible performance and is willing to spend as much as needed, stage 4 will surely meet your needs. The experience is total with it, with the amount you will be investing for the upgrade.

General Outline According to the Installation

Similar to stage 3, this stage 4,  is still a high-end upgrade. Therefore, the motorcycle engine stays inside keeping the bolt-on properties so that no specialized machine is required to integrate the components into the engine.

So like stage 3, the engine’s cylinders and heads need to be removed, replacing the cam with the eagle camshaft. Therefore, this allows keeping the installation compact and durable to carry out all the activities and stunts that the ridge will be expecting.


Is there any specialized machine required in the upgrade of stage 3?

There is no specialized machine required to integrate the components into the engine. The cylinder heads come off the engine for the installation of the parts.

Are adjustable pushrods installed in the stage 3 upgrade?

The cam is changed just like stage 2 but the adjustable pushrods are included since the top end of the engine is removed. This top-end upgrade puts this in 3 ranch categories for installation complexity. 

What is the key to delivering more horsepower in stage 4?

In the earlier applications, Harley has been able to take a  stock cylinder head and CNC port to deliver more flow. They learned that the cylinder head flows a ton of air, therefore to increase horsepower, they had to cast their old ports and make them a little smaller to increase the velocity of the air.  That is the key to delivering more horsepower.

Why is cam upgrade necessary in stage 4?

To take advantage of all the mentioned top-end improvements, it is necessary to include an upgraded cam in this stage.
The SE8 515 cam, delivers the highest lift among all the other cams and just like with the stage 3 kit that required high-performance valve springs. So those come pre-assembled into the heads.


Now, you know the difference between stage 2 and stage 3 and what are the changes that you will have after the modifications. I have mentioned the changes in the key features that will help you to choose which one you need. Also, answered the questions that consumers get confused the most about. 

You must understand the power, durability, performance, and velocity of the two stages. This understanding will not only help you before choosing the stage that you will upgrade to, but also you can know different mechanisms of the parts; how they work, their limitations. This knowledge will assist you after your installation as well.

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