More performance from Rotax Max evo by changing airbox?


I am checking my recently bought first Rotax Max Evo kart, and find it strange that the air intake is pointed to the rear of the kart.

My background is in motor racing, and there we wanted the air pressed in the airbox for more performance ( Ram-air )

Does anyone know if there is more performance to gain when I modify the inlet towards the front of the kart?

I do not need to follow rules because it is just for fun riding. And there is no dust on our track, only grass :slight_smile:

Also, I have a question about the air filter. It is soaked wet with some blue oil. I can imagine that it is harder to breathe when it’s this wet.

Is it better to get a new dry filter?

Here some pictures:

That airbox In mandatory if you are going to any Rotax races, anyway i highly doubt you will fell or see any gains In laptime if you change it.
The blue oil In the filter is either from when the filter was new, or added last time it was cleaned, there are dirrerent opinions on the filteroil, we use a very light coat on outside.

Thanks Bjorn,

But i do not go to Rotax Races , so no restrictions for me :slight_smile:

Do you know if there is performance to find when i modify the intake towards the front of the kart?

And can i wash out the filter with something like brake cleaner?

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I found out that KZ shifter karts use ram air

you can wash the air filter and yes its filter oil that im seen on the picture but its to mutch !!

the filter can be wash with water and cleaner like soap and then dry in the sun . Sometimes as a quicker method i wash them with gasoline but thats bad for the filter in the long run .

after its cleaned up you have to apply just a little filter oil to the outside skin of it before installation

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What do you think of the airbox modification idea?

Dude, if you’re not attached to any regulations you can try anything you want. Get a different airbvox entirely if you want. Put a big bore kit on… give the engine to a tuner and tell them do what they want :slight_smile:

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Yeah i know, but i just like an answer if it giver more performance, otherwise it is not worth trying it :wink:

I really cant see it being worth it, unless you really going for the last 0.01`s of a sec

Yeah that is your opinion. But is there somebody who tried it :wink:

It won’t make much difference, if any.

Given that you’re buying used tires, running a hard compound tire to save money, and haven’t been on the track yet, I would say that figuring out a ram air intake should be pretty low on your priority list. :wink:


I have driven KZ in the past, got a kid, so been 4 years out of it :wink:

But some free performance is always welcome, and its a 2 minute swap.

So asking is for free right :wink:

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The short answer is no, I haven’t tried it in a max, and I’d say very few have… so we might as well speak about ram air in general since it’s very interesting.

I can’t speak to Rotax Max specifically, it I can tell you that the position on the KZ airbox is mostly ergonomics for choking convenience. Most of the airboxes are reversible.

Ram air is not trivial to implement effectively on a two stroke carbureted engine. You will see examples of it on superkarts. (You might already be aware, it for others reading that are not….) In order for it to work you need to have an air bleed between the airbox and float chamber.

When I did back to back testing (KZ2) on a road course (average speed 95MPH, top 115/118) flipping the airbox around made no noticeable difference. Other than hitting my elbow on the airbox trumpets :laughing:

For what’s its worth, the most common hop
up for an FR125 is head volume.

So anyway, not really an answer to your question perse, but some nuggets to ponder.


Thanks for taking the time for this answer. Then we keep the airbox as it is :slight_smile:

Do you perhaps have an manual to change the head volume? Is it only a thicker gasket or more to do?

Honestly, the best thing about the Rotax is that it is “fast enough”, while being cheap (pump gas and 50:1 oil ratio - that alone cuts the operating costs to a fraction of other 2-cycle engines) and reliable to operate (i.e. just turn laps), just as it comes out of the box. Assuming that just turning laps is your goal, I’d just run it as it is, and focus on making yourself faster. It really is the ultimate 2-cycle karting package for “just turning laps”.

But - if you want to play with airboxes, go find somebody throwing out one of the original FR125 Max airboxes, and hack it up however you want… I don’t think there are any real performance differences between those old ugly things and the current less ugly ones anyways… at least the current ones have some value. As James said, ram air really isn’t a thing with these.

But why change head volume if your goal is just to turn laps on the cheap? With decreased head volume, something will have to be given up - whether it’s running more expensive and better fuel, or reliability.

If you want a faster motor, why mess with the Rotax? I’d just buy an OK… that’d be real fast!

yeah yeah, it where just come mind spins, and im leaving it as it is. When it gets boring i will try to find a Iame Super 175 some time. Thanks for al the answers

If you want to experiment you could put a diaphragm carb on, but it requires modification to the reed housing.

A guy at my local track did this, and boy it did fly😂


So the reason it won’t work is the rotax reeds won’t handle the air pushed in they flutter … the kz you can force air into because they have fine wire fingers that press on to the reeds so you can force air into it … even if you modify your airbox with bigger holes it’ll bog under load it will work on the stand but not under load the are very temperamental with all airbox mods