Japan had desperately sought major qualitative improvements in the A6M4. A6M3 had appeared before the end of the guadacanal campaign, and the Japanese deluded themselves into thinking it could solve the ever increasing superiority held by the Allies. The model 32 itself was delayed by Mitsubishis divided attentions between the J2M, the Zeke and the A7M. Design work on a replacement for the A6M2 began well before Midway, but the A6M3 was a watered down interpretartion of that new mark.
The a^M3 was meant to be a lightweight version of the J2M, and the J2M aimed to boost speed, climb firepower, dive and protection to the same level as allied fighters, whilst retaining the unmatched horizontal manouverability of most Japanese aircraft. The A6M3 incorporated some changes to boost zeke capabiliies, but really, fell short of the mark. the main differences was a switch to belt fed cannons, with ammunition increased from 60 to 105 rounds per gun. Ammunition counters were also fitted, and the radio installatioon improved. Engine power was significantly increased, to 1130hp, with a two stage blower also fitted. great things were expected in the performce increases arising from this new engine, but the increase in performance was really very modest. Top speed of the model 32 was 345mph @ 20K ft, an increase of 14mph over the A6M2, and slightly more over the model 11. The Model 32 was slightly less manouverable, with a smaller wing area, and slight increase in weight, and it was the model 32 that initially faced up to the hellcat. There were compensations for this however. Mitsubishis nearly doubled the number of skin fasterners in the wing, and the wing skins were increased in thickness. this increased sustained dive speed to over 410 mph, which was still a lot less than its American counterparts, but significantly better than the old model 21. Roll rate was superior to the hellcat, its chief adversary.
in one aspect, the model 32 was inferior to thge model 21. The larger engine fit and supercharger increased fuel consumption, and also necessitate3d shifting the firewall 8in back which further reduced fuel capacity. This of course affected its effective range, though in this area it remained superior to the hellcat. However the P-51 easily outclassed it. The model 32 had 24% less range compared to the Model 21. At a range of 650 miles, its typical operating range in the Solomons, it had just over an hours endurance over the target. Often model 32s operating over Guadacanal or goergia carried two 45 liter drop tanks under the wings.
The Jpanese did not produce the fighter they needed in late 1942 until late 1944, and by then it was far too late. After the a6M, as suggested above they had intended to improve Zero performance in a major way with the A6M4
Full details of the A6M4 are still not known, even today, but very recently, acclaimed aviation historian Jim Long has discovered some translated Japanese documentation that supports the turbosupercharger theory by referencing an intercooler, a device commonly used with a turbocharger that works like a radiator to cool hot compressed air. However, as he points out, it's not a lock, and questions remain as to the layout.
Tracking down the A6M4 is frustrating, elusive and often contradictory
Here are some comments thaqt i am aware of
The Mitsubishi A6m3 Zero-Sen ("Hamp"), by René J. Francillon, Aircraft In Profile No. 190 , ©1967
"THE TURBO-SUPERCHARGED SAKAE
"
The A6M4 version of the Reisen has long been conspicuously missing from the various historical studies yet published on this aircraft and even the designer of the Reisen, Mr. Jiro Horikoshi, could not remember what the A6M4 was! However, Mr. Horikoshi had the kindness to inquire among his friends of the former Imperial Japanese Navy and, recently, was able to confirm to the present writer that the A6M4 designation was applied to two A6M2s fitted with an experimental turbo-supercharged Sakae engine. The design, modification and testing of these two prototypes was the responsibility of the Dai-Ichi Kaigun Gijitshusho (First Naval Air Technical Arsenal) at Yokosuka and took place in 1943. Lack of suitable alloys for use in the manufacture of the turbo-supercharger and its related ducting resulted in poor operation marred by numerous ruptures of the ducting, and fires. Consequently further development of the A6M4 was cancelled, the aircraft still providing useful data for further aircraft, and the manufacture of the more conventional A6M5, already under development by Mitsubishi Jukogyo K.K. was accelerated."
Japanese Aircraft of the Pacific War, by René J. Francillon, Naval Institute Press, ISBN 0-87021-313-X, ©1970. Pages 369-371 have a couple of paragraphs that say:
"
In Japan at that time Mitsubishi and the Navy were attempting to improve the Reisen. At low altitude it could still hold its own against Allied aircraft, but at medium and high altitude it was hopelessly outclassed by the Lightnings and Corsairs. In an attempt to correct this situation two A6M2s were modified by Dai-Ichi Kaigun Koku Gijitsusho at Yokosuka and, designated A6M4s, were powered by an experimental turbosupercharged Sakae engine. Major teething troubles with the experimental engine precluded the placing of a production order and the Navy had to settle for an interim version of the aircraft, the A6M5, pending availability of the new Mitsubishi A7M Reppu... To improve diving speed Mitsubishi modified the 904th A6M3 in August 1943 by fitting a new set of wings with heavier gauge skin and with redesigned non-folding rounded wingtips..."
Zero Fighter, by Martin Caidin, Ballantine weapons book #9 ©1970. Page 158-159 shows a table that says:
"
Experimental version with turbo-supercharger. Only two built. Basically an A6M2.
A6M1-2-2N Zero-Sen by Richard Bueschel, Schiffer, ISBN 0-88740-754-4, ©1995 (This book is a reprint with some minor updating from the original ©1970 Osprey and ARCO-AIRCAM publication). Pages 62-63 have a chart that shows the A6M4 and a footnote that says:
"
Model 21 with turbosupercharger"
Zero: Japan's Legendary Fighter, by Robert C. Mikesh, Motorbooks, ISBN 0-87938-915-X, ©1994. Page 89 has a paragraph of text that says:
"
The assignment of this designation to a Zero model has been in question for a long time, since no authoritative records have ever been found to prove its use. The designation may have been set aside for a proposed model that never materialized. Some think that it was associated with an A6M3 that was to be equipped with a turbo-supercharged engine, as suggested in 1968 by the Zero's designer Jiro Horikoshi. But the reason is not really known."
Famous Airplanes of the World - A6M models 22-63, #56, 1996. Page 14 has a brief paragraph, basically translated, that the A6M4 design has not been adequately researched.
Interestingly, there are virtually NO references to the A6M4 in Japanese texts.
At
www.csd.uwo.ca/~pettypi/elevon/baugher_other/a6m.html
"
By late 1942 and early 1943, the Zero Fighter was beginning to be confronted with newer, more-capable Allied fighters. At high altitude, the A6M2 and A6M3 were hopelessly outclassed by newer Allied fighters such as the P-38 Lightning and the F4U Corsair. In an attempt to correct this deficiency, two A6M2s were modified by Dai-Ichi Kaigun Koku Gijitsusho at Yokosuka to take an experimental turbosupercharged Sakae engine. The short designation A6M4 was assigned to this project.
"However, major teething troubles were encountered with the A6M4, and no production order was placed. As a substitute, the A6M5 interim version was introduced pending availability of the A7M Reppu." At
A6M Zero
"A6M4 (2 modified A6M2's as prototypes) Turbosuper charged Sakae engine "
The most relaible source i know of on this subject is from a recognized expert who has corresponded as follows:
Jim Long of AIR'TELL Publications Research Service,
"
At the time that I first read this explanation, I was skeptical of its validity, and as the years passed and no proof was forthcoming, I became even more uncertain. But after all of these years, I finally found a scrap of evidence to support René Francillon's pronouncement. It is fragmentary, but I think it is enough to make us all believe that there was something to the report of the A6M4. But what I've found is small and only gives evidence of the A6M4 designation in connection with an aircraft that had an intercooler, and which probably means that it had a turbosupercharged engine. There are no other details of that sort, however. We'll all be left with questions, I'm afraid.
"What I've found comes from microfilm reel JP-26 which contains images of Bulletins 67-45 through 78-45. These documents are CINCPAC-CINCPOA or JICPOA intelligence bulletins issued by the Intelligence Center, Pacific Ocean Area, or the Joint Intelligence Center, Pacific Ocean Area, or the Commander in Chief Pacific and Pacific Ocean Area. They are available to the public on microfilm from the Department of the Navy, Naval Historical Center, Washington Navy Yard, Washington, D. C. 20374-0571.
"I have excerpted the material of interest to you, and it runs as follows:
"Excerpt from CINCPAC-CINCPOA BULLETIN NO. 67-45, 30 MARCH 1945 QUARTERLY REPORT ON RESEARCH EXPERIMENTS VOLUME 1 SPECIAL TRANSLATION NUMBER 52
"QUARTERLY REPORT ON RESEARCH EXPERIMENTS
(Naval Air Technical Depot); dated 1 October 1942, Captured on SAIPAN.
whatever the truth about the A6M4, what we do know is that it never materialised. The Japanese were forced by that failure to try and extract as much as they could from an already proven failure....the A6M3
more to follow