Why do the 737-100's JT8Ds, and no others, have suck-in doors? Of the various use cases of the Pratt &amp; Whitney JT8D (the most-produced low-bypass turbofan in history, and the second-most-produced turbofan of any description1), those installed on the 737-100 appear to use a set of auxiliary suck-in doors, similar to those so familiar from the earlier JT3D:

(Image by Steve Fitzgerald at Airliners.net, via Fæ at Wikimedia Commons, modified by Fæ and Marc Lacoste at Wikimedia Commons.)
The always-useful 737 Technical Site confirms that these are, indeed, suck-in doors:

History
The original choice of powerplant was the Pratt &amp; Whitney JT8D-1, but before the first order had been finalised the JT8D-7 was used for commonality with the current 727. The -7 was flat rated to develop the same thrust (14,000lb.st) at higher ambient temperatures than the -1 and became the standard powerplant for the -100. By the end of the -200 production the JT8D-17R was up to 17,400lb.st. thrust.
Auxiliary inlet doors were fitted to early JT8D's [sic] around the nose cowl. These were spring loaded and opened automatically whenever the pressure differential between inlet and external static pressures was high, ie slow speed, high thrust conditions (takeoff) to give additional engine air and closed again as airspeed increased causing inlet static pressure to rise.

In contrast, the JT8Ds installed on other types of aircraft, so far as I can tell, do not have suck-in doors:

DC-9

(Image by Cory W. Watts at flickr, via Josve05a at Wikimedia Commons.)

727

(Image by Pablo Andrés Ortega Chávez at flickr, via Wikimedia Commons.)

737-200

(Image by Andre Gustavo Stumpf Filho at flickr, via Wikimedia Commons.)

Caravelle 10

(Image by duch.seb at Wikimedia Commons.)

Mercure

(Image by Michel Gilliand at Airliners.net, via Fæ [again] at Wikimedia Commons, modified by Fæ at Wikimedia Commons.)

Why did the JT8D need suck-in doors on the 737-100, and not on any of the many other aircraft it was attached to?

1: Behind only the General Electric/SNECMA CFM56 high-bypass turbofan, used on (among many, many other aircraft) the 737-300 through -900.
Since the production of the 737-100 and -200 was concurrent, there were in fact -200s with blow-in doors, and more often than not, the inlet was replaced during the airframe's lifetime. An example is United's N9003U, line number 12, a 737-200:


Sources: jetphotos.com and airliners.net

The doors are spring-loaded and open by differential pressure at medium-to-high thrust settings and slow speeds, i.e. during takeoff and landing, i.e. near communities. The openings created in the inlet allows the inlet noise to escape, which is a major source of noise.
Therefore, and evidently, both manufacturers and customers have favored the sound-absorbing inlets.

(...) the noisiest test configuration had an inlet duct with simulated blow-in doors. When the blow-in-door inlet was installed, the A-weighted sound power levels were approximately two decibels greater than when the inlet with no blow-in doors was installed.[1]


An additional design feature for minimum noise is the use of a single opening inlet at all operating conditions. The use of devices such as blow-in doors to bring additional air into the engine at some engine powers has some aerodynamic advantages but increases engine noise. No blow-in door systems are in use on the DC-10.[2]

From a regulatory aspect, at least in the 747's case – which sported those inlets in its early days – the noise-aimed regulations of the 70s were the reason for their replacement:

After promulgation of FAR Part 36 and application to the 747, substantial changes were required for the engine installation. The blow-in-door inlet was replaced with a fixed lip inlet providing a large reduction of the fan source noise while the surface area of the acoustic lining contained in the engine and nacelle was more than doubled.[3]


1: Marsh, Alan H., Robert L. Chapkis, and Gary L. Blankenship. Study of Noise-Certification Standards for Aircraft Engines. Volume 3. Selection and Evaluation of Engine-Noise-Certification Concept. DYTEC ENGINEERING INC LONG BEACH CA, 1983.
2: McPike, A. L. &quot;Community Noise Levels of the DC-10 Aircraft.&quot; Journal of Aircraft 9.8 (1972): 542-547.
3: Little, J. W., and R. E. Russell. &quot;Status Report: Subsonic Aircraft Noise Reduction.&quot; Turbo Expo: Power for Land, Sea, and Air. Vol. 79757. American Society of Mechanical Engineers, 1976.