I was trying to keep my explanation short and sweet. :4-dontknow:
The Hemi being a 90 degree V8 engine with power strokes at every 90 degrees of crank shaft rotation is operationally inherently balanced by design. When the MDS system deactivates cyl 1,4,6 & 7, (every second cyl in the firing order), the engine still maintains operationally inherent balance as not only are the power strokes at every 180 deg of engine rotation, but every power stroke is 90 off axis from the previous. This is a huge difference from an inline 4 cyl engine where although the power strokes are at 180 deg of crank rotation, they are in the same vertical plane.
Although the V8 benefits hugely from this 90 deg power stroke offset, there still remains parasitic deflection in the vertical direction, however it is not a direct vertical deflection, but rather a sinusoidal or oscillating deflection. This also plays a huge roll in why the 90 deg V8 design is capable of such smooth operation....especially when running on all 8.
Could this deflection (and associated 'vibration') be reduced with the deployment of balance shafts...Absolutely! Similar results could achieved by increasing the mass of the engine, but for purpose of staying on topic, I'll limit my discussion to the balance shaft(s).
In order to achieve a vibration reduction using balance shafts, you would likely require 6 balance shafts. 2 would be required to be aligned with each cylinder bank rotating in opposite directions so as not to add vibration in the + and - 90deg plan from the bank. Then an additional 2 would be required aligned to the vertical plane, and carefully adjusted to be slightly out of phase to contract the sinusoidal deflection mentioned earlier. (I can only assume this can be done with only 2....but have never tested this in the real world).
If you install the 2 shafts per bank, why would you need the extra 2+? After all, if you have reduced/eliminated the effects of the power strokes with the bank shafts, there should be nothing left to cause the sinusoidal deflection and associated vibration!
Well......If your going through all the trouble to design a balance shaft system to reduce the effect of MDS, why would you stop there? Might as well complete the job and remove the minimal vibration caused by the rotating mass and get rid of the harmonic balancer which is nothing more than a inexpensive and somewhat effective replacement for a balance shaft system.
To answer your question -
So when do you think we`ll see a 5.7 hemi come with balance shafts in the engine to get rid of all the torsional vibrations that this MDS technology is causing.
Never!
I am not disputing the fact that balance shaft systems have been deployed with excellent results on some engines. But in these cases, there is an inherent imbalance that virtually dictated the necessity of balance shafts. In the case of the Harley engines, they actually did it too well to start with, and had to reduce the effect of the balance shafts to give the operator some 'seat of the pants' feedback as to what RPM the engine was turning at, so they obviously work.
In the case of the Hemi -
a) the additional cost to deploy such a system would increase the cost of the engine (likely not going to be very popular)
b) would increase the weight of the engine (which would further reduce the payload of the truck)
c) would add additional complexity to an already complex engine (DIY is tough enough as it is)
d) would reduce the available HP by whatever amount the balance shafts used (IDK about you, but I didn't buy the Hemi because of it's Rolls Royce super smooth engine operation)
And to add to all of this, the balance shafts would have to change in mass between MDS and non-MDS mode, or the vibration would become an 8cyl characteristic at more then double the effect! :str:
Simply, it does not make sense to deploy this type of expensive complexity to an engine that really doesn't have a problem to begin with. Is there a small amount of vibration from the engine when MDS is activated - YES! But even with the factory Alpine sound system, I can drown out this vibration at even a modest listening level, and the Alpine Sub is not that punchy.
