The center post of the SR-71 is a press fit design and makes electrical contact on the side of the rod, with the contact strips on the heating rod. The press fit design causes a few components of the bottom assembly to come apart when you disassemble it, as the heating rod and center post tend to stay together when you pull it apart. The SR-74 is easier to service as it uses a spring-loaded center pin, that makes contact with the bottom of the heating rod, as opposed to the side. The rod stays with the upper chamber and the center post always stays with the bottom assembly when you separate the chambers, making the -74 easier to scoop reclaim out of if you haven't dialed in your cooldown technique.
SR-71 tends to function better when bathed in oil; neglected; etc. There is no spring loaded pin to get gummed up with oil, although when adjusted correctly the SR-74 is also extremely reliable even when dirty. The bottom insulator of the SR-71 was a mechanical flathead screw adjustable design; the SR-74 was a double spring loaded pin design. Newer SR-74x we've gone back to the solid, SR-71 style bottom adapter due to the increased reliability of that design. (In case anyone out there has the -74 style spring loaded bottom insulator, and wants to upgrade to the mechanical, adjustable bottom insulator; open a ticket and I'll help you out;
http://www.w9tech.com/vaporizer-contact-us/ )
The SR-74 heating rod has a higher resistance (in the neighborhood of 2.3Ω IIRC) as compared to the SR-71 (at 1.5-1.7Ω). Sorry if these may be slightly off, I'm not looking them up anywhere just recalling what I see on my own personal meter (although I have multiple heating rods and some of them may be slightly higher or lower resistance than the next, they are all within a certain tolerance range). This means that either the usage technique or applied voltage needs to be different between the two.
Either SR series unit can use either style heating coil, stainless steel or titanium, although I think the consensus at this point is that the Ti coil is superior to the stainless steel coils that required trimming, both in ease of use and in flavor. The Ti coils simply require a slight squish to create an interference fit, and they'll last a long time- one of the two coils in my -71 right now originally came in my -74 I purchased a year ago, and I've put well over an ounce of oil through that coil... Cleaned via iso soak, boiling, torching...
In addition to new post styles,
@THC SCIENTIFIC has changed up the chamber lengths with each iteration. The SR-74x's rod protrusion depth seems to be perfectly optimized for one Ti coil, and really shines in this configuration; while past models like the SR-71 worked well with more coil length (the rod protruded further into the chamber). Experienced users, and those who tend to use their Hercules personally (instead of sharing w/others who might dump oil) usually don't need to separate the chambers nearly as often as 'newbies' or people using the Herc in a group setting; and often prefer the reliability of the -71. People new to the herc and who share it might prefer the ease of maintenance, disassembly, and assembly that comes with the spring loaded parts of the newer -74 and -74x models. Either group of users will appreciate the performance with one Ti coil on the newest SR-74x chambers.
I came into the whole thing with the SR-74, bought the x upgrade; and only much later came across a -71 in a smokeshop and have been able to try all three SR series personally; though I had my hands on the original Hercules 3.7 and 7.4V versions with tanks; never got to use those guys, sadly...
