Double-Piston Design w/ Enclosed Spring: Whereas the standard S14 timing chain tensioner is a single piston design, the newer S50/S52 tensioner is a double-piston unit. This can be seen in the photo above. What constitutes the first piston is obvious. But note that the portion of the first piston that sticks out to the right of the orange line is actually a second piston, which resides inside the first. Furthermore, this second (smaller) piston also has a strong spring behind it. However, since the second piston is held inside the larger first piston via a circlip, the spring says compressed even when the unit is removed from the car. For a time I believed that it was the double piston design that allowed this tensioner to produce a greater oil pressure force on the tensioning rail. This greater force I presumed was responsible for the reduced timing chain rattle. However, it was pointed out to me by Jon Thelen (an early proponent of this mod) that my analysis was incorrect. In actuality, the spring in the S50/S52 tensioner is probably largely responsible for it's improved performance over the stock S14 tensioner. This is explained below:

Development of Forces: For the record, the diameter of the stock S14 timing chain tensioner piston is 19.5 mm. This yields a cross-sectional area of 299 mm˛ (0.46 in˛). With the S50/S52 tensioner, the diameter of pistons #1 and #2 are 18.0 mm and 15.0 mm respectively. This gives cross-sectional areas of 255 mm˛ and 177 mm˛ (0.39 in˛ and 0.27 in˛).

Now one way that the timing chain tensioner develops force against the tensioning rail is by converting oil pressure to a force. It cannot change the oil pressure that is delivered to it. That pressure is dictated by parameters within the engine (and is rpm dependent).

Force = Pressure x Area

Note that the schematic above depicts the tensioner in what I will call its "extended" position. However, imagine what happens when the oil pressure is zero. At this time all the forces in red (oil pressure forces) will be zero. The only force against the tensioning rail will be created by the spring (the blue arrows). Piston #1 will be completely pushed into cylinder #1 and the tensioner will be in its "retracted" position. It is not until the oil pressure is high enough that F1 = F2 + Fs that piston #1 will begin to move outwards. This happens at what I term the "break point" and as oil pressure rises beyond this point the spring forces completely cancel and the force on the tensioning rail is entirely created by oil pressure.

The spring force is shown as a horizontal line which implies that the spring force on the rail is constant and independent of oil pressure. To first order this is true, as the relative compression of the spring past the break point is presumable negligible relative to how far it has already been compressed inside the tensioner (and held in place via the circlip). Past the break point the spring does have to compress in order to maintain a force balance, but things are complicated enough already ;-)

So in conclusion, my current theory is that the reduction in timing chain rattle caused by the S50/S52 timing chain tensioner is due to the addition of a much stronger spring. As we have seen above, when oil pressure is low, most of the force on the tensioning rail is produced solely by the spring. So the only real way to reduce rattle on startup, when oil pressure is low, is by having a stronger spring. Quite possibly BMW chose to enclose the spring within the tensioner, as otherwise it would be very difficult to install. Furthermore, the double-piston design of the S50/S52 tensioner produces a "rising rate", so that additional force can be applied to the timing chain tensioner under high RPM conditions, when it is needed most.