"I think the main difference between the [more advaced Kilo diesel] 7-2D42M and [older] 4-2DL42 is the increased performance [of the 7-2D42M] using 2 stages turbochargers and intercoolers.
[The 7-2D42M's increased performance (see the April 3, 2018 Table) includes 1500 kW mchanical output at 750 rpm. This compares with the 4-2DL42's 1000 kW at 700 rpm.]
[The 7-2D42M and 4-2DL42] trace their origins back to the 30/38 in the 1930s. 
In my view the 30/38 is similar to the old British Gardner engines , they are both very economical low speed engines. None of the 30/38 variants, including the 7-2D42M, can run at speeds higher than 750rpm.
I am speculating given these engines [have] long crankshafts, long stroke high torque, [the] Kilo SSK is optimized for low speed patrols [around 4 knots?], meaning running their engines at 400-500rpm.
Specifically for the six Kilo 636s exported to Vietnam or all 636s? One of two possible pump jet outlets can be seen as the hole immediately to the right of the propeller shroud and below the tail plane (or is that an exhaust outlet?) (photo courtesy DANVIET.vn via FDRA)
One difference I notice between the Russian [Navy only] 636.3 and the [for export] 636.1 (looking at photos of the Vietnamese Kilo [above)]) is the existence of pump jet outlets on both sides, at the rear just below the horizontal tailplane on the 636.3. They are for very low speed [2 to 3 knots?] shallow depth operation I think.
[I'm] not sure where the auxiliary propellers on the [plain 636, 636M or] 636.1 are located. In my view the main weakness of the Kilo 636.3 or 636.1 is the sonar suite. Few Russian submarine designs have the prominent spherical bow [eg. on the Collins with torpedo tubes underneath] sonar due to the [obstructive] location of the Kilos torpedo tubes. With sonar, passive performance is directly related to the surface area. There are also no flank sonar arrays that I am aware of.
Russia needs to sell SSKs to get cash, especially in light of the vaporware that is the Kalina SSK. [not to mention endless delays in exporting a mature Amur class SSK]. So I think there are 4 potential areas for performance improvements on the Kilo SSK:
1. improved sonar performance. May be add the flank arrays. Improved electronics (I have read [the
Russians] use dated CPUs)
2. replace Lead-acid batteries (LABs) with Lithium-ion batteries (LIBs) [on newbuild Kilos,
Lada-Amurs and Kalinas]. This will require new [diesel-generator] gensets.
3. optimize the sail/fin, better meshing with the hull
4. better skewed propeller design. If you compare the German type 212 propeller to the 636
propeller, you can clearly see the German propeller is much better."
 This may refer to the six cylinder Kolomna [D42?] 30/38 with 30/38 meaning 300mm bore and 380mm stroke. (see IUSS Alumni Association website).]
 During World War II (1939–1945) British multipurpose diesel builder, L. Gardner and Sons Ltd's war work consisted mainly of building diesel engines of their own design. Their 4LK bus engines were also used as the main powerplant in the Royal Navy’s X class and XE class midget submarines.
KQN and Pete