If you are in any involved in the world of karting, you’ll likely have heard of the Rotax Max engine. Rotax make the most well-known kart engines and run their own competitions exclusively for their customers. But you may be left wondering just how fast the Rotax Max 125 engine is.
The top speed of a Rotax Max 125 engine is about 90 mph (145 kph) under perfect, unrestricted conditions, and this speed will vary depending on several factors. It can also accelerate from 0-60 mph (0-97 kph) in just 2.5 seconds, which is the same as a Porsche 911 Turbo S.
There are many factors to take into account when looking at the top speed and acceleration of any kart engine. In this article I will discuss what affects the speed of your Rotax kart. We will also be looking at what makes Rotax engines unique and why they are so popular.
A Brief History Of Rotax Engines
Rotax kart engines are a sub-division of BRP, which is a Canadian company better known as Bombardier. BRP has been in the engine making industry for 90 years. You may also recognize their engines on various other vehicles, such as Sea-Doo jet skis, Lynx Snowmobiles, ATVs, motorcycles, and even some light aircraft.
The modern version of the Rotax kart engine, specifically the Max 125 engine, was introduced in 1997. The Junior Max class was introduced shortly afterward in the year 2000. The popularity of these engines grew quickly over a short period of time due to their reliability and simple maintenance.
The Rotax engine is constantly being improved year after year in order to keep up with their engine manufacturing rivals. Due to the relative simplicity of a kart engine, technology is rather limited, but there are always ways to make small improvements in your engines.
Famous Rotax Users
Some Rotax engine alumni include Kimi Raikkonen, Max Verstappen and Jenson Button. Having such big-name drivers representing their engines has massively helped Rotax in recent years, especially with the development of their customer racing series.
Why Is Rotax So Popular?
Rotax is a popular engine brand partly because they run an exclusive racing series for drivers who run their engine: the Rotax Max Challenge. The RMC is a racing series with a generalized set of regulations that most karting tracks can run. They are also approved by the CIK-FIA. Running this series allows the drivers from that specific championship to qualify for the Rotax World Finals.
The Rotax World Finals are hosted each year in exotic locations around the world. Recently the finals have been held in countries like Italy, Malaysia, and South Africa. The Grand Finals see 360 drivers from 60 countries around the globe attend the event each year.
A Hugely Popular Event
This series has become hugely popular and it’s somewhat of a Karting World Championship, which most drivers will aspire to partake in. All Rotax engines used in this championship are sealed, preventing cheating and engine tampering.
This creates an even playing field and eliminates some of the disadvantages caused by a lack of finances. In addition, the smaller RMC championships act as a ladder system, which promotes talented drivers who race clean and adhere to regulations.
In the Grand Finals, each participant uses the same engine, tires, and chassis, giving each driver an equal chance of winning the competition.
What Are The Engines For Each Rotax Class?
Rotax allows for different classes depending on age and skill level, to keep the playing fields as level as possible. These engine classes also have their own categories in races.
The horsepower figures may seem small, but do keep in mind that the total weight of the kart plus the driver is between 170 and 200 kilograms (375-440 lbs). This means the power-to-weight ratio is extremely high and can make these karts incredibly fast.
125 Micro Max
These restricted engines are perfect for kids looking to start karting. With 9.5 horsepower and up to 7000 RPM, this engine is small and restricted to keep the speeds low enough that any mistake made by the driver is forgivable and easy to correct. Being a kart for kids with no experience, the top speed is restricted to around 20-30 mph (32-48 kph).
125 Mini Max
The mini max is an engine built for kids with previous experience in karting. This engine has an increased power output of 13 HP and it can run up to 7500 RPM. The top speed of this kart will only be slightly higher than the micro max at around 40 mph (64 kph), as it is still designed for kids.
125 Junior Max
The next step up is a junior class kart. These karts are the smallest class used in international competition and this class runs for drivers between the ages of 11 to 15 years old. These karts have 23 horsepower and can run up to 8500 RPM. They can hit a top speed of around 60 mph (97 kph).
125 Senior Max
These are the most common adult race engines. This class can be entered by anyone over the age of 15. These engines have 30 horsepower and can run at a very high 12000 RPM.
These engines can be upgraded from the 125 junior max class by installing an electronically controlled power valve, which is where this unit gets its performance boost from. This is the top level of non-gearbox engines. The top speed of this engine is around 75 mph (120 kph).
125 Max DD2
This is the largest engine built by Rotax, putting out 34 horsepower. DD2 is an abbreviation for Direct Drive 2. This means that the engine has a 2-speed gearbox, which is operated by shift paddles behind the steering wheel. The other factor that makes this engine unique is that it has a chainless drive system.
This engine is for drivers who are experienced in karting and looking to go even faster than senior class drivers, reaching speeds of up to 90 mph (145 kph). It also requires some extra mechanical knowledge and maintenance.
KEY POINTS
• Rotax Max kart engines come in various different classes
• The most powerful ones will reach up to 90 mph
• It’s key to choose the right engine for your age and experience level
How Do I Maximize My Kart’s Engine Speed?
The top speed of a kart engine is not the most important factor to consider when racing. Most kart circuits will not have a straight long enough to reach your particular rig’s top speed anyway. Even if the straight is long enough to reach that top speed, if you set your kart up to max out on the straight you will be losing too much time on the rest of the circuit.
As an example, if you look at Formula 1 car speed traps, the cars with the highest overall top speed are not always the fastest in terms of lap times. You can see this clearly in our article on why some F1 cars are faster than others. With that said, having a higher top speed than your opponents can help you in overtaking.
Rotax engines can be tuned to achieve an optimum speed based on the circuit and the weather conditions using several variables, so that you can find that balance between raw top speed and full-lap performance.
Gearing
Firstly, you will need to get your gearing right. Rotax engines have one gear or sprocket (unless it’s the DD2 we mentioned earlier), and therefore can either be tuned towards a higher top speed or faster acceleration. The larger the sprocket (the higher number of teeth) the faster your kart will accelerate. However, this will be at the cost of losing top-end speed.
This works in the other direction too. Having high top-end speed by using a smaller sprocket will cost you acceleration out of the corners. Gearing is mostly determined by the circuit, and you will find that most if not all the drivers will be running the same sprocket at your local circuit.
Jetting
The next setup variable is your engine’s fuel mix, often referred to as jetting. The jetting determines how much air is allowed into the combustion chamber to mix with the fuel. You can adjust your jetting inside the carburetor.
Jetting is a very sensitive element of your engine setup, and it can be affected by track and weather conditions. For example, you will find that your ideal jetting is different in summer and winter.
Lean vs Rich
A ‘lean’ mixture is when there is a higher air-to-fuel ratio. This can be achieved by using a smaller jet and this will give you more power in the higher rpm range. In simpler terms: higher top speed, slower acceleration.
A ‘rich’ mixture will do the opposite, as more fuel will be used in the mixture. This is achieved by using a larger jet and this will give you more power in the low rpm range. You’ll get quicker acceleration out of corners, but a lower top speed.
Getting The Right Mixture
Naturally, you want to find a balance between the two. There are a number of signs to look out for that suggest your jetting is off. Your kart might bog down going out of a slow corner, which is caused by a lean mix, where fuel isn’t flowing in fast enough to combust. On the other hand, your kart might struggle and misfire on the straights if the mix is too rich.
Another easy check is to inspect the spark plug after running the engine for about 20 to 30 minutes. Normally if the engine is running too rich you will find some residue of fuel and oil on the plug. Ideally, the plug should be dry.
These are always dependent on the track and the weather conditions. Some circuits will have tight, technical sections that require a lot of acceleration, whereas others have fast flowing corners and long straights that will benefit from a high-top end speed. It’s all about what suits the specific track you’re racing on.
Besides these tweaks, there are some smaller changes that can give you slight advantages, but they are much more technical and do not offer as much of a performance boost as these two setup elements. If you get these two sections of your setup spot on, I can assure you that your kart will be running as close to optimum performance as is legally allowed!
Maintenance
In addition to these setup tweaks, proper and regular maintenance on your entire engine unit can help you to keep it operating at peak performance. Here are some regular maintenance checks you should be doing to keep your engine running smoothly:
- Clean the carburetor
- Check your exhaust
- Clean your clutch and replace it if necessary
- Replace your oil regularly
- Check and clean the air and fuel filters
Final Thoughts
The Rotax Max 125 engine can hit almost 90 mph (145 kph) under ideal conditions and on a long enough straight. It can also accelerate just about as quickly as most modern supercars when tuned correctly. However, under normal race conditions, you likely won’t be reaching these speeds.