Some of my answers will differ a little, so I'm just starting from scratch.
are the mk17 and mk25 discontinued ? i only see mk17 evo and mk25 evo on the site now.
It's just an evolution in the MK 17 and Mk 25 designs.
The Mk 17 EVO has some differences inside that improve the flow rate on two of the ports, so now all 4 low pressure ports are "high flow" ports (and that's why none of them are marked. There are also some differences in the trim boots.
The Mk 25 EVO has a different swivel cap with different threads that is not backward compatible with the older Mk 25s. The springs in the Mk 25 and Mk 17 have always had a Teflon coated spring, the difference on the EVO iterations is that the coating is a little thicker and it's blue.
what is the difference between the mk11 and mk17 (the original, not the evo) ? is it only the extra heat sink fins on the mk17 ?
The Mk 11 and Mk 17 use the same basic regulator body. The Mk 17 adds a sealed ambient chamber but in addition to the outer part with the fins it also incorporates a diaphragm to seal the chamber, a black rubber trim boot to protect the diaphragm, and a smaller diameter spring pad to fit inside the sealed ambient chamber.
In addition, the Mk 17 uses a thinner diaphragm as well as an extra washer which effectively increases the working range of the valve and thus the maximum flow rate.
All of the above changes can be incorporated into a Mk 11 to make it into a Mk 17. To do that you need the smaller spring pad, the finned ambient chamber, and the trim boot, along with a Mk 17 annual service kit.
what is the difference between the mk21 and the mk25 (the original, not the evo) ? is it only the swivel turret on the mk25 ?
The Mk 21 is an entirely different regulator that actually has more in common in terms of over all layout with the old Scubapro Mk 9. The Scubapro Mk 9 was a Mk 10, but without the swivel turret. Like the Mk 8 before it (a Mk 5 without the swivel turret) it was made in part to appease die hards who felt the swivel cap imposed a weakness over the original Mk 1 design.
The Mk 21, like the Mk 9 and Mk 10, uses a smaller diameter piston head, which keeps the regulator smaller and lighter, and like the Mk 10, the 0-ring sealing surfaces are located in the main regulator body, rather than in a separate swivel cap that screws onto the body. However, like the Mk 10 Plus, the piston uses a the same seat and the same piston sealing surface design as the Mk 25.
The Mk 21 is also uses a cold forged regulator body, which minimizes the milling steps required to finish the regulator, which allows Scubapro to offer it for about $100 less than a Mk 25, and it's smaller and lighter.
In effect, with the Mk 21 Scubapro finally responded to requests for something similar to the discontinued Mk 10 Plus, although without a swivel turret. Still, the hose routing options are good fro recreational divers, and with the optional end port divers used to Mk 25 style technical diving hose routing can still do that with the Mk 21.
why is the mk2 evo only avail with the 195 second stage ? can you not get it with any other second stage as a package ?
Scubapro likes to package it's regulators in "entry level", "intermediate" and "advanced" packages, presumably with the intent to sell divers three regs over their career. As such they also make you move up a level to get certain features.
Want an adjustable or balanced second stage? You have to move up to an intermediate or advanced reg to get it as you won't find them on the Mk 2 or Mk 11 first stages.
It really is as simple as that. The flow rate on the Mk 2 is more than adequate to support the higher performing second stages such the G260, S560, S600 or A700, and I'll argue that a balanced second stage makes a great deal of sense on an unbalanced first stage as the balanced poppet design is not affected by decreases in IP as tank pressure falls with an unbalanced piston first stage.
Way back in the day, you could get the balanced and adjustable G250, a non adjustable but balanced G200B or an unbalanced and non adjustable G200. If you put all three of them on a balanced first stage (The Mk 10 or Mk 10 Plus back in that era) and tuned them to peak performance, you wouldn't be able to tell the breathing performance apart. However, mount them on an unbalanced first stage (a Mk 3 back in the day) and the G250 and G200B would breathe increasingly better as the tank pressure dropped.
The reserve is also true. An well designed unbalanced first stage can deliver excellent performance when placed on a balanced first stage. The unbalanced and non adjustable C200 and the non balanced and adjustable C300 were designed as high performing non balanced seconds stages to be paired with high performance balanced first stages, primarily for the European market, and they gave great performance on the Mk 11, Mk 17, Mk 21 and Mk 25
Today in the US you can still get the C350 on the balanced piston Mk 21 first stage where it still offers excellent performance for around $500. But you won't see that second stage on the Mk 17 and Mk 25 because of Scubapro's marketing/packaging practices.
You can however still order any Scubapro first stage and any Scubapro second stage separately, and then assemble them in any combination you want. They all work well together.
