The first Griffon-powered Spitfires suffered from poor high altitude performance due to having only a single stage supercharged engine. By 1943, Rolls-Royce engineers had developed a new Griffon engine, the 61 series, with a two-stage supercharger. In the end it was a slightly modified engine, the 65 series, which was used in the Mk XIV. The resulting aircraft provided a substantial performance increase over the Mk IX. Although initially based on the Mk VIII airframe, common improvements made in aircraft produced later included the cut-back fuselage and tear-drop canopies, and the E-Type wing with improved armament.
The Mk XIV differed from the Mk XII in that the longer, two-stage supercharged Griffon 65, producing 2,050 hp (1,528 kW), was mounted 10 inches (25.4 cm) further forward. The top section of the engine bulkhead was angled forward, creating a distinctive change of angle to the upper cowling's rear edge. A new five bladed Rotol propeller of 10 ft 5 in (3.18 m) in diameter was used, although one prototype JF321 was fitted with a six bladed contra rotating unit. The "fishtail" design of ejector exhaust stub gave way to ones of circular section. The increased cooling requirements of the Griffon engine meant that all radiators were much bigger and the underwing housings were deeper than previous versions.
The cowling fasteners were new, flush fitting "Amal" type and there were more of them. The oil tank (which had been moved from the lower cowling location of the Merlin engine variants to forward of the fuselage fuel tanks) was increased in capacity from 6 to 10 gal.
To help balance the new engine, the radio equipment was moved further back in the rear fuselage and the access hatch was moved from the left fuselage side to the right. Better VHF radio equipment allowed for the aerial mast to be removed and replaced by a "whip" aerial further aft on the fuselage spine. Because the longer nose and the increased slipstream of the big five-bladed propeller a new tail unit with a taller, broader fin and a rudder of increased area was adopted.
The first batch of aircraft to fly with the Griffon 60 series engines were six converted Mk VIIIs JF316 to JF321 which were called Mk VIIIG. The first one of these was flown by Jeffrey Quill on 20 January 1943,
Changes to the aircraft were restricted to those essential to enable it to accept the new engine ... I found that it had a spectacular performance doing 445 mph at 25,000 ft, with a sea-level rate of climb of over 5,000 ft per minute. I remember being greatly delighted with it; it seemed to me that from this relatively simple conversion, carried out with a minimum of fuss and bother, had come up with something quite outstanding ... The MK VIIIG, with virtually the same tail surfaces both vertical and horizontal as the Merlin MK VIII, was very much over-powered and the handling in the air was unacceptable for an operational type ... I soon realised that a new throttle box would be needed giving a much greater angular travel for the hand lever ... The next essential ... was an improvement in the directional stability and control and a new fin was drawn out with a substantial increase in area (7.42 sq. ft) and a much larger rudder and fitted to the second aircraft JF317. This, though not ideal, produced a very marked improvement in directional characteristics and we were able to introduce minor changes thereafter and by various degrees of trimmer tab and balance tab to reach an acceptable degree of directional stability and control. The enlarged fin of JF317 had a straight leading edge but for production a more elegant curved line was introduced.One prototype, JF321, was fitted and tested with a Rotol six-bladed contra-rotating propeller unit; although this promised to eliminate the characteristic swing on take-off (caused by the propeller slipstream) the propeller unit was prone to failure. The pitch control mechanism controlled the pitch on the front propeller,
... and this was transmitted to the rear propeller (which was rotating in the opposite direction) through the transitional bearing mechanism. If this failed the pitch of the rear propeller was no longer under control and might do anything which was potentially dangerous.A similar contra-rotating propeller unit was later used on production Seafire 46 and 47s.
When the new fighter entered service with 610 Squadron in December 1943 it was a leap forward in the evolution of the Spitfire. Jeffrey Quill flew the first production aircraft, RB140 in October 1943:
So the Mk XIV was in business, and a very fine fighter it was. It fully justified the faith of those who, from the early days in 1939, had been convinced that the Griffon engine would eventually see the Spitfire into a new lease of life ... It was a splendid aeroplane in every respect. We still had some work to do to improve its longitudinal and directional characteristics, but it was powerful and performed magnificently. The only respect in which the XIV fell short was in its range.The Mk XIV could climb to 20,000 ft (6,100 m) in just over five minutes and its top speed, which was achieved at 25,400 ft (7,700 m), was 446 mph (718 km/h).
...a hairy beast to fly and took some getting used to. I personally preferred the old Mk Vs from a flying standpoint ... Even with full aileron, elevator and rudder, this brute of a fighter took off slightly sideways.In spite of the difficulties pilots appreciated the performance increases. Wing Commander Peter Brothers, O/C Culmhead Wing in 1944–1945 and a Battle of Britain veteran;
It was truly an impressive machine, being able to climb almost vertically – it gave many Luftwaffe pilots the shock of their lives when, having thought they had bounced you from a superior height, they were astonished to find the Mk XIV climbing up to tackle them head-on, throttle wide open!F Mk XIVs had a total of 109.5 gal of fuel consisting of 84 gal in two main tanks and a 12.5 imp gal fuel tank in each leading edge wing tank; other 30, 45, 50 or 90 gal drop tanks could be carried. The fighter's maximum range was just a little over 460 miles (740 km) on internal fuel, since the new Griffon engine consumed much more fuel per hour than the original Merlin engine of earlier variants. By late 1944, Spitfire XIVs were fitted with an extra 33 gal in a rear fuselage fuel tank, extending the fighter's range to about 850 miles (1,370 km) on internal fuel and a 90 gal drop tank. Mk XIVs with "tear-drop" canopies had 64 gal. As a result, F and FR Mk XIVEs had a range that was increased to over 610 miles (980 km), or 960 miles (1,540 km) with a 90 gal drop tank.
The first test of the aircraft was in intercepting V1 flying bombs and the Mk XIV was the most successful of all Spitfire marks in this role. When 150 octane fuel was introduced in mid-1944 the "boost" of the Griffon engine was able to be increased to +25 lbs (80.7"), allowing the top speed to be increased by about 30 mph (26 kn; 48 km/h) to 400 mph (350 kn; 640 km/h) at 2,000 ft (610 m).
The Mk XIV was used by the 2nd Tactical Air Force as their main high-altitude air superiority fighter in northern Europe with six squadrons operational by December 1944.
One problem which did arise in service was localised skin wrinkling on the wings and fuselage at load attachment points; although Supermarine advised that the Mk XIVs had not been seriously weakened, nor were they on the point of failure, the RAF issued instructions in early 1945 that all F and FR Mk XIVs were to be refitted with clipped wings.
Spitfire XIVs began to arrive in the South-East Asian Theatre in June 1945, too late to operate against the Japanese. It was this type which was rumoured to have been buried at an airfield in Burma after the war.