Engine Disparity in X30?

Quick question:

Does anyone know if the disparity between X30 engines is very much? I currently race in KA3 (KA100 engines) and there is a fairly big speed difference between some engines. I’m thinking of moving to X30 in a few years, so I’m wondering if it will be any different from the class I currently race in terms of equality between motors.

Pretty much non existent, the differences is in the drivers, setup and carburetion for sure

Despite what’s been written on some recent threads on this forum, stock X30’s have very good parity. Anyone claiming to see 2 or more hp differences between new, stock engines either does not have a proper dyno facility, or they don’t know how to use it.

What you are likely seeing in KA is a difference between how engines are built. The difference between a built KA and one out of the box is huge (much bigger than gains from a built X30). An out of the box KA is simply not competitive vs the way they are being built these days. For starters, in order for a KA to be competitive, the cylinder needs to be bored to at least within 4 piston sizes of the max allowed bore size. KA’s are also much, much more sensitive to carburetion than an X30, so there is a good chance you are seeing a disparity in tuning itself. An average engine properly tuned will destroy the best engine that is out of tune.


This is interesting because it goes against my observations. From what I understand an out of the box KA is pretty decent.

The 4 piston sizes within max bore I’m skeptical about too. Any ideas in that that’s supposedly worth? Not that I doubt it’s done (racers and money are easily parted) more that I sincerely doubt the benefits of it. The fact that it just so happens to add to the total cost of ownership perks my spidey senses.

Even at the highest level of 100cc world championships of the late 90’s/early naughties going at or near max bore was not worth it.

Admittedly, the KA is quite a bit different to those 100cc engines in FA/FSA. Bore, stroke, rod length, port timing etc. are totally different.

It’s always fun to try and separate the perception from empirical evidence. Especially given the dearth of the latter.

Curious to know how much extra volume/cc is 4 piston size adds…

This is definitely something builders are doing with the KA, but not with the X30. No clue why it benefits one but not the other, but I’ve heard of that practice from 3 or 4 different builders.

A Danish builder does this on Mini60 engines, bores it to the last dimensions.

Out of the box KA’s are not remotely competitive at a national level. I have a ton of back-to-back dyno and track proven empirical evidence to back this up.

It has nothing to do with an increase in displacement, but rather the change in size of the transfer and exhaust ports as the cylinder is cut progressively wider. The port timing changes, and in combination with optimal ignition timing and carburetion, the power gain is substantial. The reason it made no difference in the 100cc days is that they were open engines; you could change the porting as you like with a grinder and a degree wheel. As you saw, a small change in cylinder volume made no difference. The KA trick is a work-around on a spec engine.


Ah, that makes sense on the bore vs port area. Clever.

What can you share? I’m especially curious on carburation… I’m assuming this is beyond simply setting the jet screws and popoff for the sweet spot.

I just wanted to note that I agree with Christian here. I have a few points on here from 5-6 years ago when the KA was new here and none of the builders had figured them out yet, where I was noting a stock engine would be just fine. But as the years have gone by, engine builders have sorted them out more and now there is for sure things to be gained on a blueprinted KA.

Again, probably not critical for the club or regional racer, but nationally yes.


Interesting… 20 characterzzzz

Seeing that building engines is how I make my living, I probably share too much on here as it is…but as far as carburetion goes, the KA (like all pumper carbs) has a big lean spot at low rpms, so finding a combination of fulcrum height, spring weight, and needle settings that delivers a bit more fuel at low rpms while not running overly rich at high rpms, is the key. I do this on the dyno with lambda/afr readings. I find once a good combo is found on the dyno, it translates very well to the track, with only minor tweaks needed to the high needle to compensate for weather. That said, carb settings found on the dyno are only useful if you have enough volume of air flowing in your dyno room (and into the engine) to achieve real-world egt numbers. If a dyno operator finds his egt lower than he expects on the dyno, the air flow available to the engine is not sufficient.


Unless you run at a club like Badger, where in KAjr you’re up against 10-15 Regional racers and a handful of those are also national racer, at every club event!

1 Like

Just curious on this Christian, do you dyno all motors before you do your magic? If so, how consistent are bone stock motors and can even a soft motor be brought up to performance expectations?

Not every new engine…it’s just not practical time-wise, but I do frequent spot checks on new engines mainly to check the calibration of my dyno. New, stock engines are very well built and consistent these days. Yes, there are occasional outliers, but they are rare, and the differences are not very significant. If an engine is truly down on power, it’s almost always a problem that can be cleaned up with a proper rebuild. I’ve only encountered a couple of true duds in many years…pretty much any modern engine can be built to be competitive.

If a customer sends in a engine they feel is down on power, I will always do a pre-build test so I can see what I’m working with.


Defiantly isn’t as bad as the KA100’s where people buy 10 engines to build 1, but you do have X30s’ that are better and ones that are worse. The closer you get to the front of the field the more you’ll see the small disparities, half a kart length at the end of a straight, half a tenth exit of this corner etc. But, in my opinion, here in Australia anyways, if you have a trustworthy engine builder that you can have a genuine relationship with, and are willing to learn how to use a laptop with the appropriate software to read AFR with a track overlay, you can beat the guys who buy 5 engines to build 1.

I know this is a common perception in all spec classes… But is this actually happening with KA,or do builders just blueprint what they have and discard any that just don’t perform?