Exposing the truth behind, overpriced (£700), Overpromised, Non-Dyno Tested Map on a BMW M5 (E39) from a big name tuner!

In our opinion the map is little more than Junk, and proves that you don’t always get what you pay for.

Before we begin, for those who don't know, the AFR ratio (Air Fuel Ratio) is the amount of fuel to the amount of air inside an engine. In petrol engines, if it runs too rich, the AFR will be low, causing the engine to bog down and potentially wash the cylinder bores with fuel. Conversely, if the AFR increases (more air than fuel), the engine runs lean, increasing temperature, risking melted parts, and causing improper combustion, often leading to a dead or damaged engine.

We often encounter subpar “calibrations” on cars, but the one on this M5 stands out because it endangers the engine and is far from cheap. Typically, poor-quality maps fail to deliver the promised power or torque, though most are at least “safe.” They might make the car smoke a bit or merely alter the driver’s wish (i.e., how the accelerator pedal responds) without much else.

This E39 M5, equipped with an S62 engine, came to us for a power run with AFR (Air-Fuel Ratio) monitoring after being remapped by a well-known BMW tuning and styling specialist. Given the tuner’s reputation, I had high hopes for the calibration but was disappointed.

The tuner had deleted the MAF (Mass Air Flow) sensors, charging the customer around £700. Despite the hefty price, the map wasn’t dyno-tested, and our findings indicate it wasn’t a properly developed map either. It might have been tried on many cars, but tested? Hardly.

The customer was promised a 20HP gain, but the car ran stock on the dyno. So, what did they get for their money? A car that, if driven hard, is at risk of catastrophic engine failure whether it burning a valve or knock caused by the lean running. Chances are they advanced the timing too, so its a recipe for a big issue.

The Main Issues Identified:
Issue 1:
• The car runs lean between 2300 RPM and 3500 RPM with the MAF sensors plugged in or not plugged in.
Issue 2:
• With the MAF sensors unplugged, there is no MIL light, but the car then runs leaner across the entire RPM range, making it even worse.
Issue 3:
• The car made stock power. The only gain compared to a stock E39 M5 is a bit more midrange torque, likely due to the uprated exhaust manifold the customer had fitted.

Before anyone suggests the Bosch LSU Lambda Sensor was out of spec, we are a professional outfit. We validate our Lambda probes using a freshly calibrated Bosch emission tester and, if necessary, propane for verification (though in this case, the AFR readings from the LSU sensor and the BEA emission tester were almost identical).

The fact is, the tuner should have caught the lean RPM range. Any map—especially on a petrol engine—requires thorough testing, whether on a dyno or on the open road. Applying a map without proper testing or the tuner’s willingness to test it is a gamble. This case exemplifies a major issue waiting to happen. The car might drive fine for a while, but on a hot day with hard driving, the engine will enter the lean zone, leading to knocking and potentially severe damage, likely after any warranty has expired.

You can take your diesel to a tuner, and unless they are amateurs, the tune will work without issue, probably even without testing. The same cannot be said for a petrol engine, which is at greater risk of running lean and causing serious damage.

The Two Approaches to Tuning:

1. Off-the-Shelf Files: These are not tested specifically for your car but are claimed to have been tested before on similar cars. This is the kind of thing sold by agents, men in vans, and many “tuning centers.”
2. Custom Files: This is our method, where we create a custom file for each car and customer based on our knowledge and experience, followed by rigorous testing and, of course, after-sale support.

Every tuner encounters occasional issues, such as a MIL light indicating “overboost.” These can be resolved with minor tweaks. However, allowing a car to run lean is unacceptable. Anyone pretending they never have an issue has never done any tuning. But its what they do after they’ve become aware of an issue that sets a good tuner apart from a bad one.
When dealing with agents or resellers, you often get what you pay for. They are simply making a quick buck, and when something goes wrong, they become impossible to reach.

A £150 remap is just that—a £25 file from someone unknown. Yes, it might work, but it might be doing damage that you’ll find out about months or years later. Given that the customer paid £700 for a dangerously flawed tune, I recommended they contact their credit card provider to seek a refund and have the map removed, returning the car to its stock settings.

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