Harley-Davidson Stage Kits Explained
- Motorcycle Gear Reviews
- Feb 19, 2024
- 6 min read
Have you ever heard someone say, "I've got a stage 4 kit on my Harley" and just played along like you know what they are talking about? We are going to walk you through the basics of the stage kits from Harley and dispel some myths along the way.
The different stage kits from Harley are simply levels of performance upgrades to the engine. If you go to Harley-Davidson, the Stage Kits can either be ordered as a package, or pieced together. Usually, most riders are piecing together Stage 1, but the other packages are usually bought as a kit. This is usually because of the varied style options in a Stage 1 and lets face it, very few people run a Harley-Davidson exhaust.
There are 4 stages...only 4! I can't tell you how many times I hear people say they have a stage 5 or 6. That is not a thing. Of course, you can upgrade beyond the 4 stages, but there isn't a level assigned to whatever stupid thing you've done to your motorcycle. Okay, sorry. Rant over.
Harley-Davidson Stage 1 Kit
A stage 1 kit is very common and easy to do. A stage 1 kit is all about air flow. You are simply allowing more air to pass through the engine and programing the computer to match fuel to the increased air flow. So, lets start at the beginning. Air enters the engine through the air cleaner. Some stock air cleaners do a better job than others at maximizing air flow, as Harley has started to offer their upgraded air cleaners as standard. If you are looking to save some money, you might could just upgrade the air filter. I would recommend an oiled high flow filter, like K&N. Some of Harley stock air cleaners can use a kit to remove the backing plate of the stock air cleaner to increase air flow. In most cases, you will want to upgrade your air cleaner. There are a ton of styles and every one promises to maximize air flow.
From a performance aspect, you want to look for exposed surface area of the filter. The more surface area of the air filter that is exposed, the more air it can pull into the engine. A very common style of performance air cleaner is a Heavy Breather. They stand out by their design. They are forward facing and have an elbow pipe with either a 90 or 45 degree bend. They claim to allow the most amount of air into the engine. I can't swear to that. The real benefit may come to the placement. By being forward of a traditional air cleaner, the air they are allowing in the engine is cooler. Cooler air is more dense, which equal better performance.
Obviously, air that comes into the engine, has to come out. So, the next part of the stage 1 kit is exhaust. There are a lot of debates about exhausts. Generally, the freer flowing the exhaust, the better. The newer Milwaukee 8 motors are designed to like more back pressure than a twin cam or evolution engine. There are two main parts of an exhaust system. First, is the head pipe. This is the part of the exhaust that attaches to the engine and usually contains the Catalytic Converter, if equipped. The second part is the slip on, or muffler. The slip on attaches to the head pipe and usually contains the baffle. The most restrictive part of the exhaust is the converter. To maximize performance, you will want to replace the head pipe with something that has no converter (good luck finding that in today's market) or has a high flow converter that is placed as close as possible to the end of the exhaust. The vast majority of riders simply replace the slip on and leave the stock head pipe. It is a much less expensive option. You won't get as good of performance, but you will likely get the sound you are looking for.
The last part of a stage 1 is tuning the bike. When you tune the motorcycle, you are allowing the computer to compensate for the increased air flow with more fuel. The computer can compensate for about a 3% difference in air flow without having to be programmed. If you are only doing a stage 1, I suggest you read my tip about "Do I Need a Tuner". There are several very good tuners on the market, that will allow you to tune your motorcycle for the increased air flow.
What you will need for Stage I
Harley Stage 2 Kit
For the vast majority of riders, a Stage 2 kit is as far as they will go. Most riders are very happy with the performance of their bike with a Stage 2 kit. The Stage II kit builds on stage 1, but it replaces the cam. So, here is some mechanics 101. All Harley engines (except the Revolution) are push rod engines. That means the cam is at the bottom of the engine. The cam has oblong lobes that rotate. The push rods ride on the lobe of the cam. As the cam rotates, it will push the push rod up and down. The push rod will open and close the valves, which are at the top of the engine. So, by changing the cam, you are increasing horsepower and torque by adjusting the timing and lift of the valves. There are a ton of different cam choices on the market. They all achieve a specific goal. Some riders like a higher revving horsepower oriented engine, while others are all about torque. Most cams are designed to increase the bottom end torque of the engine. Increasing the lift in the valve not only increases performance, it also gives that lope sound that makes a Harley sound like a Harley.
Make sure your cam kit comes with adjustable push rods. If not, you will need those as well. Installing an upgraded cam can be a do it yourself project, if you are pretty mechanically inclined and have the right tools. If you do not feel confident, get a professional to do it for you. After the cam is replaced, you will need to retune the motorcycle again.
What you will need for Stage II
Harley Stage 3 Kit
A Stage III kit is increasing displacement. This is a pretty big job. You will be replacing the cylinders and pistons, which means the entire top of the motor has to be removed. I would suggest leaving this one to the professionals, if you have not done motor work before. Harley engines are measured in cubic inches with the exceptions of the XL Sportsters and the Revolution engine. When it comes to increasing displacement, bigger isn't always better. A consideration has to be the thickness of the cylinder wall. The outside of the cylinder will stay the same. That means the more displacement, the thinner the cylinder wall will be. As the wall gets thinner, heat can become a real issue. You can build a powerful machine that sounds incredible, but on a hot August day, it could overheat. Currently the largest kit offered by Harley-Davidson is a 131. I wouldn't suggest anything much larger than that on a Milwaukee 8 engine.
In my opinion, a Sage 3 kit is a waste of money. It is a pretty expensive upgrade and will only yield a little more power than the Stage 2. If you are going to spend the money for a Stage 3, just do a Stage 4. That is where you will see the full potential of your engine.
What you need for Stage III
Harley Stage 4 Kit
The stage 4 kit includes everything from the previous stage kits. So, we opened the bike up to allow for more air flow. We changed the cam to optimize the timing of the valves and we replaced the cylinders and pistons to maximize displacement. In Stage 4, you are upgrading the throttle body to increase the volume and velocity of air entering the cylinder head. You are also replacing the cylinder head as well. Upgrading the throttle body is a good idea, even if you are not doing all of the other upgrades. For most models, the stock throttle body is plastic and can be problematic as it call melt and cause vacuum issues. The replacements are metal. The throttle body may also be referred to as an intake manifold.
The Stage 4 kit can be expensive, but a lot of the cost is in the labor. It is a very time consuming job. Basically you are taking the motor down the the case and building it back with bigger and better parts. If you want the most performance out of your motorcycle, a Stage 4 Kit might be right for you.
What you need for Stage IV
I hope you found this helpful in determining the differences in the different stage kits! If you are in the planning stages of upgrading your motorcycle, be sure to check out our other articles and reviews under Performance!