Hayate and Fuel Grade

After testing by US air force, the Hayate was owned by a collector. After that, the owner presented it to FUJI HEAVY INDUSTRY which used to be 'NAKAJIMA AIR PLANE". At that time, the Hayate was full flying condition. However, one day FUJI H.I. gave up on maintaining it and sold the Hayate to a museum. It was the start of this sad story. The new owner set it outside and the rain came. The Hayate could not be flown under this condition. After that, the museum sold the Hayate to another muesum. Then, the main undercarriage was cut for moving it to a new museum. The sad Hayate has been exhibiting in Chiran museum in Kagoshima Japan now. http://www.d1.dion.ne.jp/~r_dom/haya_index.htm

Allow me to quote from a book published by Dai Nippon Press, detailing the development of the Nakajima Ki-84 Hayate, code named the 'Frank' by the Allies:

"The following spring, another captured Hayate (believed ultimately scrapped) went through more rigorous testing at the Middletown Air Depot in Pennsylvania. Taking place in April and May of 1946, to their great surprise, the Americans found that in many performance categories, the Ki-84 was the equal of, or superior to, the P-47N and P-51K to which it was compared. Roughly the same in maximum speed at altitude, the Hayate proved superior in early climbing performance, maneuverability, and overall handling. On the down side, the testers noted the aircraft's relatively short range, and poor protection, as well as shoddy materials and workmanship in some components, especially the exhaust stacks. It must be noted that during these tests, the plane had an advantage it never had during the actual war: high grade gasoline. 87-octane was the best the Japanese forces could provide their planes during the conflict, and although the 'official' data on the Ki-84 indicated a top speed of 624 kph, in practice the aircraft could only reach 580-590 kph. However, with 140-octane gas in its tank, the normally troublesome Ha-45 [the engine] was able to pull the Hayate through the skies at a top speed of 689 kph in level flight at 20,000 feet, a stunning 60 kph more than the Japanese were ever able to produce, and just slightly faster than the P-51D-25 and P-47D-35 at the same altitude."

I'd guess the Japanese were even worse off with regard to high octane fuels than the Germans were. Although they had access to a considerable supply of high grade crude (at least until US submarines cut the shipping lines) I don't believe they had sufficient processing capability to take full advantage of it.

In a confidential report listing titled COMPARATIVE PERFORMANCE AND CHARACTERISTICS REPRESENTITIVE ENEMY AND ALLIED AIRCRAFT the FRANK 1, Nakajima is listed as follows:

Engine: Nakajima Ha.45: 1970hp/S.L. 1695hp/21,000ft. Armament: 4x20mm. Range: 1795mls/156mph/359gallons of fuel.
Performance at the test weight of 7940lbs.: Climb: 3780fpm/S.L. 3290fpm/21,000ft. 10,000ft/2.7min. 20,000ft/5.8min. Service Ceiling: 39,000ft. Maximum Speed: 348mph/S.L. 422mph/21,000ft.
Due to all the problems it encountered in production, but taking into account the ingenuity of some chief mechanics in the field, IMHO it was likely the Ki.84 the Allied pilot would meet in combat would be capable of 325-350mph/S.L. and 384-425mph/20,000ft. Just an opinion though. I have read that the performance of the Ki.84 varied greatly in the field during 1945.
The Ki.84 is the most controversial Japanese fighter to evolve from WW2. It definitely had the potential to be a world beating contender if it hadn't been for its constant failure of exhaust stacks due to poor materials, inefficient welding and problems with the hydraulics.
According to Wright Field Report No. F-1IM-1119B-ND released in January 1947, a Ki.84-1 with the Ha.45 Model 21 engine, Serial No. 302, performance was found to be as follows: Speed: 350mph/S.L. 389/10,000ft. 412/20,000ft. 426/23,000ft. 400/30,000ft. 370/35,000ft.
This was listed under Normal Fighter, Military Power. Climb Normal Fighter: 3790fpm/S.L. and 3195fpm/20,900ft under Military Power. 3615fpm/17,900ft under W.E.P. Service Ceiling: 38,800ft. No test weight or engine power settings given.

On another official PERFORMANCE AND CHARACTORISTIC sheet 156A-2 the following performance is listed: Engine at T.O. and WEP: 1970hp/S.L. 2040hp/3000ft. 1850hp/17,900ft. The following performance is at 7,940lbs.: Speed: 369mph/S.L. 427mph/20,000ft. Climb: 4275fpm/S.L. 3615fpm/17,900ft. Maximum Range: 1,815mls/173mph/1500ft./359gallons of fuel.

These test figures are under ideal situations and using USA high grade fuel. However, quoting Richard Dunn: “I have a copy of a translation of a captured document of unknown reliability which indicates two maximum speeds for the Ki.84-1 (light) and Ki.84-1 (improved versions). The two speeds apparently relate to the Ha 45 rating of 2000hp at 4,020ft and 1800hp. at 19,680ft: Ki.84-1 (light): 412mph and 430mph respectively. Ki.84-1 (improved): 409mph and 427mph respectively. The light version is listed at 7885lbs. and the improved is listed at 8507lbs.

Ever since the Frank and Mustang first met in China the dogfights that ensued were reputed to be among the most ferocious of WW2!

The Ki 84 potentially had everything a balanced air superiority fighter needed except reliability. It was fast, far ranging, light on its feet, fast climbing, armored and hard hitting, and in strong numbers. Only lacking high altitude performance to the degree of the USAAF or RAF. However it had the agility to turn inside all of them. It's like a Bf 109K or Yak-3 but with long range. Terminal dive was good at 495 mph. Initial climb was in the 4,000ft/min class and level speed was better than any mass produced fighter from Japan at 427 mph (with high octane fuel). But as the war was ending, factory quality was fading - even muzzle velocity of its fast (850 rpm unsynchronized!) world beating 20mm cannons decreased.