In the first entries regarding the B58 engine, I mentioned that its performance subjectively differs from N53. N53 so masterfully evens out the performance of cylinders that the engine’s work (any vibrations) can not be felt at all. With B58, it is different – the vibration appears in driving conditions of average high and high required torque. Now, I have more information regarding B58 engines, and now I can also explain this vibration.
The vibration can be felt in comparably low RPM range ( up to 2500 .. 3000 RPM) if the required torque is above 250 .. 300 Nm. Vibration is more strongly felt by the cars with manual transmission because it radiates to the gear selector. And stronger, probably, it can be felt if the vehicle is equipped with RunFlat tires.
As usual, let’s start with technical information – live data.
Here how the cylinder performance in the mode of low torque looks like (required torque around 90 Nm):
As we see, all cylinders are performing very similarly. The difference is only several %. Accelerator pedal: 20%, curve in red color, bold (Axis 3).
And in this graphic – the accelerator pedal is pressed for 90% (Axis 3; corresponds to the torque 400 Nm):
How so? Has something “gone wrong”? The performance (mechanical efficiency) of cylinders is enormously different! This difference increases with the RPM increase. By 2500 .. 3000 RPM, this difference reaches at least 10 .. 15%!
Here, the situation becomes even worse in the range of higher RPM! As we know, till 3000 RPM, the flywheel data are accurate and reliable; in the case of low (to 3100 RPM), we don’t have any reason to doubt these data. Above 3100 PRM, the flywheel data are not precisely accurate, but what we see in the graphic still is much more “harsh” than it should be!
What do we see in this image? The engine has separated into two parts:
a. cylinders No.1, 2 and 3 are significantly more “active”;
b. cylinders No.4, 5 and 6 are “lazy”, and their performance is reduced.
I would like to add at once:
a. such a “picture” of the cylinder performance is stable. The flywheel is adapted; for other engine systems, the adaptation procedure has been performed several times;
b. the same situation has been observed for other B58 engines!
Suppose such a situation would be for only one engine. In that case, we could assume that the Bosch hasn’t introduced injector adaptations for the mode of large loads (or they are not working). So it happened that the injectors (they are as if parted on 2 Rail parts: cylinders 1 to 3, cylinders 4 to 6) are with significantly different flowrate parameters. Relatively unlikely, but – it could happen if one “mini-group” of the injectors is from one batch, another – from another. But the same problem for more than enough B58 engines? The injectors will not be the ones to blame. Bosch also is not a culprit!
Before I move to the culprit, I will justify why this time, neither the DME developer nor other mechanical problems of the engine are to blame. In the range of ample required torque:
a. Valvetronic is deactivated (HVAs will not cause the problems); all cylinders receive the same amount of air;
b. If there were problems with the injectors, in part or the cylinders (less active ones), the fuel mixture would be lean; in these cylinders, the detonation would be observed.
The DME itself would see the detonation, and the driver would feel it, as well as the development engineers. Finally – the engine would not be able to handle such torture for long. Obviously, the fuel mixture in all cylinders is correct – the wideband probe reports the average fuel mixture as Stoichiometric (Lambda around 1.00).
In the image: Lambda – blue color, bold (Axis 4)
So – what had happened to the three cylinders, which have lost the energy?
The answer is actually simple – the TwinScroll Turbo technology is to blame!
These engines have one common TwinScroll turbo aggregate for all cylinders. True though, this is not a “regular” turbo aggregate (as you can guess from its name). Every three cylinders turn the turbine’s rotor via its separate channel of gases. So the turbine works in Pulse mode; its rotor is turned not evenly but with quick “beats”. Every three cylinders are loaded as if with a different turbo aggregate. One “turbo aggregate” (with an air-channel of a smaller cross-section) is intended for a low RPM range. It ensures unique torque in the range of low RPM (for B58, full throttle of 450 Nm is available already by 1300 RPM!) and quick reaction time; a second turbo aggregate (with a larger cross-section of the air supply) is more active in the range of higher RPM and higher required torque. Each cylinder group is working on exhaust (inlet of the turbo aggregate) channels of different cross-sections; exhaust gasses of each group are “hitting” other blades (another angle) of the turbo aggregate. The thing that both turbo aggregates are mechanically connected (they have a common axis) does not reduce the problem.
In the image – picture of the TwinScroll turbo aggregate:
These different “turbo aggregates” are causing two problems:
a. different dynamics of the exhaust gases – due to this, the cylinder (their group) fulfillment with air (oxygen) changes. Accordingly – the amount of supplied fuel also is (has to be) different. The cylinders really work with various “power”; DME knows that and takes into account;
b. different average “load” means gas resistance, which occurs when the turbo aggregate starts to spin.
If I get ower my laziness, I will explore how significant the problem of air fulfilling is and how precisely the mechanical load actually impacts this situation.
As the ignition order of cylinders is 1/5/3/4/2/6, then by “groups”, the situation looks as follows: 1/2/1/2/1/2.
Accordingly, the engine runs in following mode – “active/less active/active/less active”. And, as we understand, the engine’s vibration appears, and its frequency is exactly 2 times lower than the performance of cylinders. So, if the engine, for example, runs with 2000 RPM, this vibration corresponds the 1000 RPM. The lower the frequency/RPM, the worse the vibration is suppressed by the flywheel and the engine itself. More about this read here.
Unfortunately, this vibration is a side-effect of the TwinSroll Turbo technology. Nothing can be done – just “enjoy”. So – BMW managed to widen the range of the RPM, in which the max torque is available; reduce the reaction time of the engine, but – the engine vibration “comes in the set”. Is this vibration critical? It is possible that many users of these engines have not even noticed it or considered it normal. Maybe someone has visited the dealer and asked to find the problem. Perhaps someone even enjoys it – it gives the feeling that the engine below the hood is “alive”. For me? Initially, when I noticed it, and the cause was not clear to me (there was a version regarding the incorrect performance of the DME), I felt discomfort. Now, when I know everything is fine with the engine performance, this vibration reminds me that there is a 450 Nm torque aggregate below the hood! Such remainder is nothing terrible.
True though, this “little” nuance significantly changes everything regarding “upgrading” these engines. “Chipping up” of any type means an increase in the power (changing the management maps in the DME itself or connection of other/external units, for example, JB4) disrupts these differences in the fuel mixture of cylinder groups. Accordingly – even if the DME keeps the average fuel mixture correct, the individual fuel mixture of each cylinder will be incorrect! Incorrect fuel mixture – detonation, too high temperature – damages the piston group of the engine. Incorrect fuel mixture – Large load to CO catalytic converter, overheating and destruction. Disbalance of cylinders – increased vibration, which means increased load to the flywheel, gearbox, and transmission box, as well as – output shafts, differentials, engine, and transmission cushions.
In addition, the following unpleasant nuances should be kept in mind:
a. this difference of a group of cylinders (and it is possible that the individual difference of each cylinder) is taken into account in different multidimensional management maps. These maps are not available to the “chiptuners”; they are not/can not be modified easily. Creation and modification of these maps require highly complex equipment, profound knowledge, and a significant amount of other resources, which the “chiptuners” simply don’t have;
b. it is impossible to create these correction maps (which describe the differences in the performance of cylinder groups) as adaptive. In theory, DME could perform the tests on the chemical performance of cylinders; as a result, the fuel mixture of each cylinder would be defined (not taking into account that the engine is equipped with a single Lamba probe, joint for all cylinders). Unfortunately, a static mode (driving conditions) of several tens of seconds is required to perform such tests. Such mode is not possible in actual driving conditions; accordingly, the only option – use exact, fixed management maps;
c. any type of interference in the management of these engines has critical consequences in stationary (slowly changing modes), but especially catastrophic – in transition (swiftly changing) modes. During transition processes, the situation after “chiptuning” is uncontrollable. Yes, possibly, the several seconds of incorrect fuel mixture will not melt the CO catalytic converter but will be able to damage it. One such overload cycle, probably, will have no consequences, but what about 100 cycles? And 1000? The same situation is with a lean fuel mixture in any of the cylinders. One spurt – without consequences. Two – without consequences. But 100?
Of course, it is very tempting to “cheat” MAF and MAP sensor reading for a little bit (indicating if slightly lower airflow and boost pressure indications the mode of large required torque) and gain some power increase in such a way. I believe it is possible to reach a slight (to +10%) increase of torque (DME will re-adapt the adaptation maps of the fuel mixture; the formulas of the energetical module will more or less match) without error messages in the DME error message memory. Unfortunately, this previously mentioned problem of the cylinder disbalance is not compensated or controlled in any way. Consequences – as I already mentioned before: overload of the CO catalytic converters and damages or even destruction of the piston group. In the best case – reduced the actual gain of the torque due to late ignition (because DME has noticed the detonation). Thank you, I don’t need such an upgrade, even for free!