How Fast Is a 125cc Go-Kart?

A few days ago, I decided to take my weekly trip to my nearest go-kart course. So, after I did a few laps on the 4-stroke kart that I was given, I had a cooldown lap to catch my breath. It was then that I noticed a couple of go-karts flying by me. I figured that it was because I was driving slowly, but even when I got back to pace, they were still going a solid 10-20mph faster than me.

Once I was back home, I did a bit of digging, and I found out why those karts were so fast. That’s when I found out they were 125cc karts, so I learned the following after some more research.

125cc karts can go up to 87mph (that’s 140 km/h!). Most 125cc go-karts can reach a top speed of 70-80mph. The kart’s top speed can vary depending on different weather conditions, tire degradation, and the track surface but the engine horsepower is the main factor when it comes to a go-kart’s top speed. That being said, I’ve included some ways to increase your go-kart’s acceleration and speed below.

While doing my research, I read about 4-stroke and 2-stroke engines. Even though I knew that they were different, I didn’t know that their output differs that much. 

Before talking about the top speed of each, let’s compare the two engine types.

2-Stroke Engines vs. 4-Stroke Engines?

I’ve driven 2-stroke and 4-stroke karts. While I didn’t notice a huge difference, the 2-stroke go-karts felt like they had much better acceleration than the 4-stroke ones.

From my research, 2-stroke engines are lighter and offer lightning-fast acceleration, while 4-stroke engines are more linear, efficient, and durable. This, in turn, makes 2-stroke engines better for sprint races, while 4-stroke engines are better for long races.

2-Stroke Engines

A 2-stroke engine has an upstroke and a downstroke on every crankshaft rotation. Simply put, when the piston goes up, air and fuel enter the engine (crankshaft) and are compressed and ignited. The piston gets pushed down (downstroke) when the fuel gets ignited, and the fumes leave through the exhaust.

4-Stroke Engines

Unlike the 2-stroke engine, the 4-stroke one requires two crankshaft rotations. Instead of two steps (two strokes), a 4-stroke engine operates in four steps. The initial step includes the fuel being drawn, followed by its compression on the second step. Once the fuel is compressed, it’s ignited on the third step, and the exhaust valve allows the fumes to exit on the fourth step.

Here’s a side-by-side of a 2-stroke and a 4-stroke engine kart on the same track.

Their differences become much clearer by looking at how the two engines operate. The 4-stroke engines are heavier than the 2-stroke ones since they need twice the piston strokes to function. 

That being said, 4-stroke engines are more complicated, which means they’re more expensive to buy and repair. But, they’re also more durable, quieter and the power is more linear. 

A 2-stroke engine also needs you to mix oil and fuel, which isn’t easy for everyone. This means it’s terrible for the environment since burnt oil can come out of the exhaust and into the air.

As a general rule, though, 2-stroke engines are harder to maintain and worse for the environment, but they’re more powerful and easy to repair.

All of this is great, but even if a 2-stroke engine is 10HP, it will get demolished by a 4-stroke one that’s 30HP.

How Much Horsepower Is a 125cc Go-Kart?

Engine horsepower is the main factor that affects how fast a go-kart can go. Well, you know, your driving skill is also a main factor. 

Anyways, unsurprisingly, not all 125cc go-kart engines have the same horsepower. From what I understood, most 125cc go-karts have an engine horsepower of 27-36HP. The majority of 125cc go-kart engines are closer to 30HP.

I decided to find some of the most famous chassis along with their engines so that you can get an idea of the numbers that revolve around 125cc go-karts.

As you can see, the horsepower of a go-kart isn’t the only factor. For example, a 2-stroke 30HP engine will most likely be faster than a 4-stroke 30HP engine because of the way it operates. The chassis might also be better aerodynamically, but I’m no Adrian Newey (Red Bull Racing Honda’s Chief Technical Officer in F1), so you’ll have to consult him for more information like that.

This is why there’s a difference in the top speed of the last three go-karts I mentioned above, despite having the same horsepower.

It’s worth noting that the top speed of any kart goes down for several reasons. As I mentioned above, the track might be slippery, or it might be scorching. The tires won’t be working optimally in both cases, so these top speeds can’t be achieved.

At the same time, your body weight plays a huge role in how fast your go-kart can go. For instance, you might be driving a go-kart with a Vortex RoK GP engine, but its top speed is less than 75mph. Unfortunately, none of us can turn to feathers once we’re strapped in the kart.

So, how can you ensure that your go-kart can go up to its top speed?

How Fast Does a 125cc Go-Kart Accelerate from 0 to 60?

Controversial opinion incoming: I hate it when people talk about a go-kart’s top speed. While I’m still a beginner, I’ve seen that I rarely reach my kart’s top speed on the track. Now, a go-kart’s acceleration is a much better metric to talk about.

I was amazed to find out that, under optimal conditions, a 125cc go-kart can accelerate from 0-60mph in a matter of 2.5 seconds! While most 125cc karts will do the same in around 3-4 seconds, that’s still really fast.

Generally, this is enough time to reach 60mph on a straight. While my maths aren’t the best, I don’t expect a go-kart in an ordinary go-kart course to get more than 65mph. So, what’s the point of a kart having a top speed of 75mph if it can’t do 0-60mph in under 7 seconds?

That being said, there are tons of factors that affect an engine’s acceleration. I remember playing some video games which had a slider for acceleration and top speed. The higher the top speed was, the less the acceleration. I never realized that the same is true for go-karts.

This is called tuning. Besides the weather conditions and the track layout, tuning is essential to your go-kart’s acceleration. The two things that you can adjust on a go-kart are the gearing and the jetting (also known as the fuel mix).

Fuel Mix

If you’ve played the F1 games from Codemasters (which I’d highly recommend), or you’re an avid F1 fan, you already know about fuel mixes (fun fact: F1 cars use up to 110 KGs of fuel per race). For those of you unaware, whether the engine is 2-stroke or 4-stroke, the fuel gets ignited by adding air to it.

Fuel mixes are affected by the track and the weather conditions. So, your fuel mix in Italy for example wouldn’t be the same as the one you’d use on a course in the U.S since the humidity level isn’t the same.

There are two main ways to name a fuel mix: lean and rich.

A lean fuel mix means that the engine eats up less fuel. You can do that by lessening the jetting in the kart’s carburetor. Lean fuel mixes offer higher top speed and subsequently lower acceleration.

A rich fuel mix is the opposite of a lean fuel mix. So, your engine will eat up more fuel by increasing the jetting in the kart’s carburetor. But, you’ll be getting much better acceleration and less top speed.

Gearing

Gearing is a bit more straightforward. Most engines, especially Rotax ones, only have one gear or a sprocket. The larger this is, the greater your go-kart’s acceleration will be. Subsequently, the top speed will be less. This also works vice versa (the smaller it is, the less your go-kart’s acceleration will be, and the higher the top speed will be).

I’m not the best mechanic in the world, so here’s a fantastic video explaining how tuning the gear ratio affects acceleration.

That’s pretty much all I had to say for 125cc go-kart engines. Hopefully, I’ll be able to get my hands on my very own 125cc go-kart in 2022. If you want to be the first to hear about my experience with that, be sure to sign up on our mailing list.

Stay tuned for more articles just like this one!