Telemetry & Ports – Overview

RaceLab needs live game data (telemetry) to draw overlays and to log your laps. This section explains what telemetry actually is, how different sims send it, and which ports and files are important so that RaceLab can see your car and session information.

1. What “telemetry” means here

In sim racing, telemetry is simply the stream of numbers your game sends out while you drive: speed, throttle, brake, gear, track position and so on. RaceLab reads this stream and turns it into overlays, fuel estimates and comparison tools.^[1]

Different games send telemetry in different ways:

• Built-in SDK / shared memory – e.g. iRacing exposes live data through its SDK and a shared memory block.^[2]
• UDP broadcast – e.g. F1 and ACC can send telemetry over the network to tools that listen on a port.^[3]
• Plugins / DLLs – e.g. rFactor 2 and Le Mans Ultimate can load a plugin that exposes telemetry for tools like RaceLab.^[4][5]

RaceLab is a third-party overlay/tool that understands these different methods for the supported sims and connects to them so it can show you information on screen while you drive.^[6]

2. How RaceLab connects to each sim

Each sim has its own setup steps, which are documented in the official RaceLab “First Steps” guides.^[7] Here is a simple overview:

• iRacing – Uses the built-in iRacing SDK / telemetry system. There is no extra plugin to install, but RaceLab expects iRacing to run in windowed or borderless mode so overlays can sit on top.^[8]
• Assetto Corsa – Uses the Racelab library plugin placed in the AC install folder and enabled in game.^[9]
• Assetto Corsa Competizione (ACC) – Uses the broadcasting.json file in your Documents folder to send telemetry and race data over UDP to RaceLab.^[10]
• Le Mans Ultimate (LMU) – Uses a Racelab telemetry plugin DLL placed in the LMU Plugins folder plus a small JSON config file for the plugin.^[5]
• rFactor 2 – Uses the rFactor2SharedMemoryMapPlugin64.dll plugin in the Bin64\Plugins folder and needs borderless window mode enabled.^[4]
• F1 series – Uses the built-in UDP telemetry output, which you enable in the game’s Telemetry Settings menu.^[3]

The detailed step-by-step instructions live on the official RaceLab Garage pages for each sim. This overview page is here to explain the common ideas in one place.

3. UDP vs shared memory vs plugins

From your point of view as a driver, there are three main “shapes” of telemetry:

• UDP telemetry – the game sends packets over the network to a port (for example ACC’s broadcast port, or the F1 games’ UDP telemetry). Tools connect by listening on that port.^[3][11]
• Shared memory – the game exposes data in a region of memory on your PC; tools attach to that region and read it. This is common in PC sims and is fast and local.^[2][11]
• Plugins – the game loads a plugin DLL which then writes telemetry either to shared memory or to a UDP stream for tools to read (rFactor 2, LMU).^[4][5]

RaceLab hides most of this complexity from you; it just needs the sim to be configured correctly so that the data actually flows.

4. RaceLab’s local web port (overlays)

RaceLab serves overlays via a small local web server. By default this runs on http://localhost:8080 unless you change it in the RaceLab settings. Other tools or services can sometimes also try to use port 8080, which leads to “port already in use” or “no overlays visible” situations.^[12][13]

If overlays or preview windows fail to show anything, one of the first checks is whether another program has taken over port 8080 and whether your firewall is blocking RaceLab from listening on it.

5. When telemetry doesn’t connect – quick checklist

If RaceLab shows “no data” or overlays stay empty, a general checklist looks like this:

1. Is the sim actually on track?
   Most telemetry only flows once you are loaded into a session, not from the main menu.
2. Follow the correct “First Steps” guide for your sim.
   Make sure you have completed the full setup for that game: window mode, plugin installed or enabled, broadcasting.json set correctly, etc.^[7]
3. Check firewalls and antivirus.
   Confirm that neither RaceLab nor the sim is blocked, and that UDP ports used for telemetry are not being filtered.^[11]
4. Check for other tools using the same port.
   If another overlay app or telemetry tool uses the same port as RaceLab (especially 8080 for the local web view, or ACC’s broadcast port), only one can win at a time.^[12][13]
5. Restart both the sim and RaceLab.
   Many telemetry settings are only applied on a fresh start of the game. If you changed plugins, window mode or UDP settings, restart the sim before testing again.^[4][5]