Thank you to BMW Ag: for B series engines (this time more about B58) in the newest versions of ISTA, the Expert mode is available – and the first menu is Rough run data.
In the menu description regarding the depiction of the cylinder performance are several quite nightmarish sentences, which do not give any clarity. This has not changed since the E series. True though, in none of the internet forums, I haven’t read the entries of a knowledgeable person who would confirm that yes, someone has managed to understand how to use this menu! But maybe I am the only one who doesn’t understand anything from this description.
Measuring units of the cylinder performance are also “interesting”. In several ISTA releases and menus (including Call-up), I have seen:
Doesn’t look convincingly. But all options indicate acceleration. Is this true? It turns out that… To save the intrigue, I will reveal it later.
What do these performance data show?
DME measures the time of each combustion cycle of each cylinder. The more efficient the cylinder, the shorter this combustion cycle – the connecting rod turns the crankshaft faster. These performance data indicate the relative differences in the mechanical efficiency of each cylinder (time of the combustion cycle of the cylinder). Well, such a short description, in my opinion, is much clearer.
The next step is to understand the measuring units of these data. For example, what does +5.0, or -7.5 mean?
To find out this, I acted simply. I disconnected the ignition coil of one of the cylinders for the time of the test and started the engine. Here is the result of the Call-up functions:
The good news (unlike, for example, MSV/MSD) – the result in the Expert mode coincides with the Call-up menu. So:
a. these data do not show the data of the acceleration (as indicated by BMW) but retardation. So – a positive value means REDUCED mechanical efficiency, reduced – INCREASED;
b. in the idle, for 0 performance of cylinder (disconnected cylinder) correspond around 70 units, which means: 10% difference of ideal is about +/- 7 units.
As for these engines, DME can measure mechanical efficiency of the cylinders on the go; I measured the “sensitivity” by torque 35Nm and 1500 RPM. The disconnected cylinder indicated a delta of around 100 units in this mode. Conclusion: in this driving conditions +/-10% corresponds +/-10 units.
What menu units are acceptable?
Correctly performing DME evens out the performance of the cylinders, and it should converge to 0. In my evaluation, a +/-5% difference from the ideal is acceptable for the engine not to vibrate, and the fuel mixture of cylinders would be correct enough.
Here is an example:
What do we see in the menu:
a. for the short term, the performance of each cylinder is even +/-7 .. 10% from ideal;
b. the average mechanical efficiency of all cylinders is close to 0 – the situation looks fine.
This time conclusion – the idle is “trembling”, but all cylinders on average (in the long term) are working correctly.
In this menu:
a. the idle looks much more even;
b. it is clearly visible that the 1st cylinder is “lazy”; its average (marked in blue) efficiency reaches level + 7 .. 10 units, which means: mechanical efficiency is reduced by -10 .. 13%.