How to Read and Analyze JB4 Logs for the Kia 3.3T
Datalogs are how your tuner sees inside your engine from a thousand miles away. Whether you're running a JB4, logging through an EK1, or sending files for a Map6 custom map, this guide covers how to capture a useful log on the Kia Stinger / Genesis 3.3T — and what we actually look at when we read one.
Why Logs Matter So Much
A tune is built on assumptions: your fuel, your mods, your climate. Logs replace assumptions with evidence. Every revision we make — timing, fueling, boost targets — comes from what your logs show, not guesswork. A five-minute log session is the difference between a tune that's probably fine and one that's verified healthy.
How to Capture a Log
With a JB4 (most common):
- Connect to the JB4 with the JB4 Mobile app (phone) or a laptop
- Start logging before your pull, not during
- Do a third-gear pull from ~2,500 RPM to redline on a safe road or track — third gear gives a long, steady load window
- Stop the log after the pull and export/share the file
A good log is: full throttle, one clean pull, no traffic lifts mid-pull, on the fuel your tune expects. Logs from half-throttle highway merges tell us almost nothing.
Without a JB4: the EK1 supports logging directly, and laptop-based OBD loggers work as well. If we've asked you for logs (Map6 customers — this is you), we'll tell you exactly which parameters we need for your setup.
The Fields We Actually Read
Make sure these are in your log — this is all we need. (Boost and timing targets aren't something you need to know or capture: your tuner already knows what the tune asks for.)
| What it is | On the JB4 | On the EK1 |
|---|---|---|
| Engine speed | RPM | Engine Speed |
| Boost / manifold pressure | BOOST2 | MAP |
| Ignition timing & knock activity | IGN1–IGN6 (timing and knock retard per cylinder) | Ignition Timing + Knock Learning 1–6 |
| Fuel pressure | FP_H | Fuel pressure |
| Air-fuel ratio | AFR | Actual Air Fuel Ratio |
| Fuel trims | Fuel Trims | (the EK1 doesn't log trims — one reason JB4 logs are often the more complete picture) |
Reading Your Own Logs: Spotting Trouble Without Knowing the Targets
You don't know the tune's boost or timing targets — and you don't need to. What a non-tuner can read is shape and direction:
- Boost should be a smooth curve. It climbs, holds, and gently falls off at high RPM. If the boost trace looks like a sawtooth — rapidly bouncing up and down mid-pull — that's a control problem worth a log review, whatever the numbers are.
- Timing should climb, not dip. Watch the IGN columns through the pull: a sudden drop of several degrees on one or more cylinders that then creeps back up is knock retard — the engine protecting itself. Once is a data point; the same dip in the same RPM range on every pull is a message. (Plugs are the most common culprit — check the gap guide first.)
- AFR should get richer under boost, never leaner. If the AFR number is rising (leaning out) as boost climbs, lift — that's the one pattern that shouldn't wait for a second pull.
- Fuel pressure should hold, not sag. FP_H bleeding steadily downward through a full-throttle pull means the fuel system is running out of headroom.
- Trims should hover near zero. The further they drift and stay, the harder the ECU is working to correct something — our fuel trims guide explains what the corrections mean.
That's the honest ceiling of self-serve log reading: shapes are yours, numbers are ours. If any of those patterns show up — or you're just not sure — send the log. That's what it's for. Don't keep pulling on a car that's asking for help.
Sending Logs to SleepyTuned
- Map6 customers: logs are a required part of your service — your custom JB4 map is literally built from them
- Tune customers: attach the log file to your order thread or e-mail reply
- Always tell us the fuel in the tank and any mod changes since your last log — context changes everything
After we review, you may receive a tune revision, flashed the same way as any file via your EK1, or adjusted piggyback settings.
Log first, pull later — your engine will thank you. Questions about your setup? Reach out or explore our tuning options.