How an easy transmission flash made our BMW 435i even faster

Once someone cracks the code, more performance is often just the press of a button away.

The MHD Tuning suite that we’ve been using with our BMW 435i project car has been an invaluable tool not only for logging engine performance data but for increasing power (once we got the intake temps sorted out) and subsequently reducing lap times.

So, could we up the ante even more by reprogramming the control unit for our BMW’s ZF 8HP eight-speed auto?

The 8HP is already one of our favorite automatic transmissions, offering quick shifts, crisp response time and excellent drivability. It brings near-DCT levels of control to a more traditional fluid-coupled automatic while delivering a paddle-shifter experience to some more affordable cars like our 435i.

We grabbed a Black Friday deal on xAutomotive’s xHP TCU Flash Combo. It usually retails for $354.

The software operates on either iOS or Android devices, and you’ll also need a way to connect to your OBD port. XAutomotive offers several options, but our MHP Wi-Fi dongle also works fine and is usually plugged in anyway.

Tuning your transmission is as easy as selecting one of the off-the-shelf tunes–which can also be heavily customized through various options–or individually setting parameters on those options through the interface. Our initial tune took a few minutes to complete, but each subsequent reflash has taken well under 60 seconds.

The xHP package gives fairly deep control over transmission parameters. We feel like we’ve barely begun to scratch the surface, but our initial impressions are overwhelmingly positive, even while just making some wild guesses at proper settings.

For our purposes, the most significant features seem to be the ability to adjust, or completely eliminate, gear-based torque limiters, torque reduction on upshifts, shift speed and torque punch.

According to xAutomotive CEO Clemens Mühlbacher, the removal of the torque limiter functions is one of the most important aspects in delivering more performance from a transmission tune, particularly on vehicles with tuned engines.

“The stock torque limiters are set only a small tad over the stock torque values of the engine and will prevent power gains [from properly reaching the drive wheels] when left untouched,” he told us.

Basically, your tuned engine may be producing more power, but the transmission is allowing slip through the clutches and torque converter, so the rear wheels only see stock torque. This torque reduction is particularly dramatic in the “money” gears like third, which is, coincidentally, where we spend a lot of time exiting corners at our official test track, the Florida International Rally & Motorsport Park.

Another side effect of that slip is heat. By eliminating the slip, xAutomotive notes, you also reduce the heat and wear inside the transmission, particularly with tuned engines that would otherwise be putting a lot of their additional power and torque into transmission heat instead of forward thrust.

So the claims seem sound, but how does it work on track? We loaded up a tune and headed to the FIRM to find out.

Our track tune–which may change with more testing, but this was our first shot–consisted of loading up the off-the-shelf Stage 2. It quickens shifts, removes torque limiters, and changes mapping for full drivetrain lockup to be far more aggressive, resulting in the engine more often being directly connected to the entire drivetrain with no fluid of clutch couplings operating to create slip.

We also selected a few custom options, like True Manual mode. It prevents automatic upshifting at redline and disables the kickdown function, which when stock can still occur given the right circumstances, even in manual mode.

We slightly decreased the shift torque reduction, too, which theoretically should reduce boost lag after shifts. We also increased torque punch, which likely doesn’t create much performance benefit but does give you a bit more seat-of-the-pants feel for shifts–and that definitely adds to the driver’s level of sensory input.

Switching on the xHP tune shed nearly seventh-tenths of a second. We were shocked, too.

We searched the VBox data for anomalies, and yeah, maybe there were a couple tenths hiding in noise or driver variation, but the most significant time gains we saw on the Circuit Tools graph were directly attributable to the xHP transmission tune.

One of the most dramatic time gains came at the exit for the FIRM’s Turn 5, a mid-50 mph, third-gear corner at around the 2500-foot mark on the data graph.

On corner exit, the xHP-tuned car (green trace) immediately jumped ahead of the non-tuned car (red trace), leveraging that un-torque-limited third gear to a more than 2 mph increase in speed at track out and a 4 mph speed advantage at the end of the subsequent acceleration zone.

We saw another big gain when accelerating out of the FIRM’s hairpin at the 4500-foot mark. The tuned car used that additional third-gear punch for stronger acceleration down the straight to a 2.5 mph advantage before the braking zone.

Somewhat hidden in the data traces was the subjective feel of the car, though. This was somewhat evident through the FIRM’s Turn 8, starting at around 5500 feet, where the tuned car entered this tricky corner faster and had a steadier speed through the corner than the down-up-down trace of the non-tuned car.

The car is now much, much easier to balance on the throttle, better engaging all four wheels in the cornering equation. The link between throttle and power delivery feels far more direct and responsive, even giving the ability to create on- or off-throttle oversteer, which wasn’t really an option before. Before, you only got to choose between mild or terminal understeer.

Shifting is improved, too, with the transmission reacting even more quickly and directly to commands from the paddles. The additional torque punch of each shift makes it easier for the driver to stay “on rhythm,” so to speak.

Having that additional bit of sensory input is more helpful than barely being able to perceive the shifts in OEM mode. The transmission simply feels more like a DCT than a paddle-shifted automatic.

So at this point, we’re sold and looking forward to diving deeper into the tuning options to produce an even more ideal setup for the track.

We were honestly a little surprised by the amount of those gains, but a call with xAutomotive confirmed what our data told us: The company’s programmers worked to improve mid-corner and exit speeds.

The software may be worth it on feel alone, but a seventh-tenths drop for $354 is a pretty serious bargain.