In April 2025, I asked KTuner about supporting my Japanese Honda Fit. It is now October 2026, and I have found a new way to pass the time.

I am writing my own tuning software.

Apparently, waiting for somebody to support my car was the sensible option. Developing software to communicate with its ECU is what happens when the sensible option leaves me alone for eighteen months.

The project is called Renntrace Flash Tuner. I am building it with Codex, using the KTuner v1.2 interface, and I am already using it to tune my own Fit.

Yes, the same Fit whose engine parts have been sitting in boxes while I developed an increasingly intimate relationship with the phrase “ECU support.”

Harry’s orange Japanese Honda Fit RS with carbon-fiber hood and front fenders, its license plate blurred.
The same orange Fit. Renntrace is already tuning its current setup; the bigger engine parts are still boxed.

Eighteen months of waiting

I explained the original problem in All the Parts, None of the Software. My 2018 Japanese-market GK5 Fit RS uses a Japanese-market ECU that KTuner did not support. Owning a device with “Fit” on its application list did not make my particular computer more cooperative.

The first exchange was on April 14–15, 2025. KTuner said the 5R1 ECU was unsupported and that there were no plans to add it. Another exchange that April, about forum registration and getting help, left me bouncing between the forum and email.

I found the tone dismissive. The messages are below, so you can judge for yourself.

In January 2026, there was a possible route forward: reading the Japanese ECU through the interface. KTuner made clear on January 24 that reviewing the data did not guarantee support. I uploaded it on January 25. On July 24, the work was still in the queue.

That is roughly eighteen months since my first request, and nine months since the upload. KTuner had left the timing open. I simply stopped believing support was going to arrive soon enough to build a car around it.

The emails behind the wait

Excerpts from the April 2025 emails, with dates and context below each image.

KTuner email stating that 5R1 ECUs were not being considered for support at that time.
15 April 2025: KTuner said it was not looking into support for the 5R1 ECUs at that time.
My email explaining that a forum registration challenge required me to email KTuner.
17 April 2025: I explained that I was trying to register for the forum, whose challenge question told me to send an email.
KTuner replying that support questions should be handled by email.
17 April 2025: the reply directed support questions to email. I had already emailed to explain the registration problem.

I could build it with Codex

Then I had the thought that usually precedes a new project and the disappearance of several evenings: could I build the software myself with Codex?

I already had the car, the interface, and a very specific reason to care. The missing part was the application that would let me work with my ECU.

So I started Renntrace Flash Tuner.

I bought a tuning interface, waited for software support, and eventually became the software support. I wanted to install pistons. Somehow, the prerequisite became developing a desktop application.

This is what happens when a car project acquires side quests.

Renntrace is already tuning my Fit

I am already tuning my Fit with Renntrace.

The support I have built into Renntrace covers KTuner’s supported ECUs and adds my Japanese ECU.

Renntrace Flash Tuner interface concept showing a fuel table, timing and VTEC tabs, and a sample log.
Renntrace Flash Tuner interface concept: a calibration table, parameter controls and a sample datalog.

What I want to control

My priorities are the mixture, ignition timing, and VTEC. I need control over all three once the bigger changes finally move from cardboard to aluminum.

I also want the software to feel better to use. In my own use, Renntrace is faster, has fewer bugs, and gives me a better interface than the workflow I was dealing with before.

I wanted less fighting with the application and more time working on the car. The interface should help me do that without making every task feel like I am negotiating with a printer.

The software has to help me understand and control the car. A beautiful screen that quietly lets something important go wrong would be a rather expensive design award.

Connecting through the KTuner v1.2

I use the KTuner v1.2 interface to connect the computer to the car. Renntrace handles the tuning software on the computer.

Anyone using Renntrace will need compatible hardware, including if I eventually release the software for free.

I am also considering selling my own flashing interface someday.

I have already bought enough things for this project. I would like some of them to start earning their shelf space.

The KTuner End User Kit beside the UK-market Fit ECU from my earlier support experiment.
The KTuner kit and the UK ECU I tried during the earlier support search.

Engine work in spring 2027

The forged pistons, rods, cams, and the rest of the engine parts are still in their boxes. Developing tuning software has not secretly assembled an engine while I was looking at the screen.

The plan is to install the parts and tune the combination after winter, in spring 2027. I want to compete on track in 2027, and the front brakes and new wheels are also on the list.

I am still aiming for 180 whp, naturally aspirated. I will post the numbers once the engine is built and tuned.

The Fit has already become the carbon-fiber family car I apparently needed. Now I would like the engine to catch up with the amount of visual confidence attached to the trunk.

The parts waiting for spring

The same engine parts from my earlier Fit articles, ready for the next stage of the build.

The full collection of Honda Fit engine parts arranged on concrete.
The engine-build collection: forged internals, cams, springs, intake manifold, throttle body, ECU and KTuner.
Four forged pistons and four custom connecting rods for the Fit.
12.5:1 forged pistons and custom connecting rods. Installation is planned for after winter.
Two custom camshafts and a set of upgraded valve springs.
Custom camshafts and upgraded valve springs for the planned engine build.
An LRPTech carbon-fiber intake manifold attached to a large throttle body.
The LRPTech carbon-fiber intake manifold and 70 mm throttle body.
The rear of my orange Honda Fit, with carbon-fiber trunk and GT wing, its license plate blurred.
The Fit with its carbon trunk and GT wing. The engine work is planned for spring.

Sharing Renntrace

If this project works out, I might release Renntrace as free, open-source software.

I would love to see other owners tuning their cars with it, especially people who already own the hardware and are stuck waiting for ECU support.

First, I want to finish something I can stand behind. Then I can work out how to release it and which cars I can support.

Naturally, I bought an Evo

While all of this has been happening, I bought a white Mitsubishi Lancer Evolution IX MR.

It cost ¥3.3 million—about US$20,886 at ¥158 to the dollar. It has more than 300,000 kilometers, and to me, it runs like new. It is awaiting delivery while the air conditioning is fixed.

The white Evo waiting for delivery

The actual car in its dealer listing, photographed before delivery. The air conditioning is being repaired.

The white Mitsubishi Lancer Evolution IX MR viewed from the front, with Phoenix dealer branding on the plate.
The white Evo I bought. Listing photo: Phoenix Mie Matsusaka.
The white Evo from the rear, showing its large wing and MR badge.
The rear view and MR badge. Listing photo: Phoenix Mie Matsusaka.
Front Recaro seats and manual gear lever inside the white Evo.
The Recaro interior and manual transmission. Listing photo: Phoenix Mie Matsusaka.
The Evo’s engine bay in its dealer listing.
The engine bay before delivery. Listing photo: Phoenix Mie Matsusaka.
The Evo’s instrument cluster showing 322,618 kilometers.
322,618 km in the listing photo. A substantial amount of experience. Photo: Phoenix Mie Matsusaka.

Photos: Phoenix Mie Matsusaka.

The FK8 is being sold. The Evo may take over daily duty from the Fit, including family and winter use, although I have not even received it yet. It is a family-car plan with considerably more attitude than the original brief required.

So the current situation is that I am building tuning software because my sensible Honda would not cooperate, keeping forged engine parts in boxes until spring, and waiting for a high-mileage Evo to arrive.

I have solved the problem of waiting for one car project by purchasing another car project.

Fucking excellent management.