By William Pearce
In the mid-1930s, the United States Army Air Corps (AAC) was interested in a long-range bomber. Boeing won a contract to build the aircraft, which was originally designated XBLR-1 (eXperimental Bomber Long Range-1), but ultimately became the XB-15. By 1935, the AAC realized that current engines, and those under development, lacked the power needed for such a large aircraft. At the time, the AAC was pursuing its next experimental long-range bomber, the Douglas XBLR-2. The AAC requested the Allison Engineering Company build a 1,600 hp (1,193 kW) engine for the XBLR-2, which later became the XB-19.
In 1935, Allison was in the middle of developing its 1,000 hp (746 kW) V-1710 engine. The AAC requested that the new 1,600 hp (1,193 kW) engine have a single crankshaft and use as many V-1710 components as possible to keep development time to a minimum. After evaluating a few different configurations, Allison decided to double the V-1710 to create a 24-cylinder engine in an X configuration. This engine became the X-3420.
The X-3420 would have an entirely new crankcase, crankshaft, gear reduction, supercharger, and accessory section, but it would keep the basic V-1710 cylinder and head. The X-3420 had a flattened X arrangement with a left and right cylinder bank angle of 60 degrees, an upper cylinder bank angle of 90 degrees, and a lower cylinder bank angle of 150 degrees. The fuel-injected engine would produce 1,600 hp (1,193 kW) at 2,400 rpm for takeoff and 1,000 hp (746 kW) at 1,800 rpm for economical cruise. The engine would have an 8.5 to 1 compression ratio and weigh 2,160 lb (980 kg).
While using as many V-1710 components as possible made Allison’s job easier, the X-3420’s single crankshaft and its master and articulating rods required much design work, as did its fuel-injection system. Very quickly, Allison realized it did not have the resources to develop the X-3420 and needed to focus on the V-1710, which was encountering technical issues. Development of the X-3420 was effectively abandoned in 1936. As an alternative, Ron Hazen, Allison’s Chief Engineer, proposed a new 2,000 hp (1,491 kW) engine that had two crankshafts and was more closely based on the V-1710. The engine would produce more power than the X-3420 and be developed in less time. The AAC approved of Hazen’s proposed engine, which became the V-3420. The engine was often referred to as a W-24 or double Vee (DV) and was occasionally called the DV-3420.
The Allison V-3420 design was more complex than just coupling two V-1710 engines together. As with the proposed X-3420, a new crankcase, gear reduction, supercharger, and accessory section were at the center of the engine, but the V-3420 would utilize many V-1710 components. The use of two V-1710 crankshafts along with their connecting rods made the V-3420’s design and development much more manageable for Allison. The engine consisted of two 60 degree V-12 engine sections mounted on a common crankcase and separated by 90 degrees, which gave the inner cylinder banks 30 degrees of separation.
As V-1710 development progressed, Allison was able to offer the V-3420 with 2,300 hp (1,715 kW) for takeoff. At 2,300 lb (1,043 kg), the engine would only weigh 140 lb (64 kg) more than the single crankshaft X-3420, but it would produce an additional 700 hp (522 kW). In May 1937, the AAC contracted Allison to build the V-3420 engine prototype.
A large aluminum crankcase sat at the center of the 24-cylinder V-3420 engine. Attached to the crankcase were four cylinder banks. Each cylinder bank consisted of six steel cylinder barrels shrink fitted to a one-piece aluminum cylinder head. Each cylinder barrel was surrounded by an aluminum water jacket. A single overhead camshaft actuated two intake and two exhaust valves for each cylinder. Each cylinder had a 5.5 in (140 mm) bore and a 6.0 in (152 mm) stroke. The engine displaced 3,421 cu in (56.1 L) and had a compression ratio of 6.65 to 1. At the rear of the engine was a supercharger driven by the right crankshaft, and all accessories were driven by the left crankshaft. The engine was also intended to be used with a General Electric turbosupercharger.
There were only 340 parts unique to the V-3420 engine, and those accounted for 930 pieces of the 11,630 that made up the engine. Initially, the V-3420 had a takeoff rating of 2,300 hp (1,715 kW) at 3,000 rpm, a maximum rating of 2,000 hp (1,491 kW) at 2,600 rpm, and a cruise rating of 1,500 hp (1,119 kW) at 2,280 rpm. The basic 24-cylinder engine was 97.7 in (2.48 m) long, 60.0 in (1.52 m) wide, and 38.7 in (.98 m) tall. The engine weighed 2,665 lb (1,209 kg)—365 lb (166 kg) more than the original estimate.
In January 1938, Allison was authorized to release V-3420 engine specifications to aircraft manufacturers and airlines. This resulted in a number of aircraft designs incorporating the engine; however, only four V-3420-powered aircraft types were actually flown. The V-3420 engine was first run in April 1938, followed by an AAC order for six engines in June 1938. An engine was also displayed in the 1939 World’s Fair in New York.
The US Navy was aware of the V-3420 engine and asked Allison if it could be converted for marine use. Allison responded with the appropriate designs. In December 1939, the Navy ordered two V-3420 marine engines for installation in a new, aluminum-hulled Patrol Torpedo boat designated PT-8. The two V-3420 marine engines were delivered to the Navy, and the PT-8 boat started trials in November 1940. The PT-8 was tested through 1941, but no further boats or V-3420 marine engines were ordered. The sole PT-8 was later re-engined and still exists as of 2017.
For aircraft use, the V-3420 required further development, which was slow due to Allison’s ongoing commitments to the V-1710 engine as well as the AAC’s preoccupation with vastly expanding its resources for the coming war. In late 1940, Allison focused on two major models of the V-3420 engine: -A and -B. The V-3420-A had crankshafts that rotated the same direction—either clockwise or counterclockwise, depending on the desired rotation of the propeller. The -A engine used a single-rotation propeller with either an attached or remote gear reduction, but most commonly with an attached gear reduction. The V-3420-B had crankshafts that rotated in opposite directions and was used with contra-rotating propellers. Different versions of the -B engine could accommodate either an attached or remote gear reduction, which allowed a number of propeller shaft configurations, including right-angle drives. The -B engine almost always had a remote gear reduction. The two crankshafts of the V-3420-B were kept in sync by idler gears at the front of the engine. The idler gears also balanced power loads from the crankshafts to the contra-rotating propeller shafts.
In September 1940, Allison’s V-1710 commitments became overwhelming, and development of the V-3420 engine was put on hold. As a result, the XB-19 had four 2,000 hp (1,491 kW) Wright R-3350 18-cylinder radial engines installed in place of the V-3420s. However, the R-3350 was encountering its own extensive developmental issues that put its use in the Boeing B-29 Superfortress in question. In February 1941, the AAC requested that Allison restart development of the V-3420-A with an output of 3,000 hp (2,237 kW) as a possible replacement for the Wright R-3350. The B-29 bomber was too important for its fate to be tied to one engine.
A V-3420 engine was delivered to Wright Field in October 1941, but with the bombing of Pearl Harbor in December, the V-3420 program was again put on hold so that Allison could focus on the V-1710 engine. History repeated itself in mid-1942 when the suitability of the R-3350 engine was again in question. Allison was instructed by the Army Air Force (AAF—the AAC was renamed in June 1941) to prepare the V-3420 for installation in a B-29, which was redesignated XB-39. Nine engines were built and delivered by October 1942. On 1 October 1942, the AAF ordered two Fisher XP-75 Eagle fighter prototypes that were powered by the V-3420-B engine. This was followed by an order placed on 28 October for 500 V-3420-A engines for installation in 100 production B-39 aircraft.
As the aircraft projects were underway, continued development of the V-3420 engine increased its output to a takeoff rating of 2,600 hp (1,939 kW) at 3,000 rpm with 8 psi (.55 bar) of boost, a normal rating of 2,100 hp (1,566 kW) at 2,600 rpm at 25,000 ft (7,620 m), and a cruise rating of 1,575 hp (1,175 kW) at 2,300 rpm at 25,000 ft (7,620 m). However, the engine could be overboosted in emergency situations to 3,000 hp (2,237 kW) at 3,000 rpm with 10.2 psi of boost (.70 bar).
The first aircraft to fly with the V-3420 was the Fisher XP-75. Developed by the Fisher Body Division of General Motors, the XP-75 was a long-range escort fighter. Through 1943, the AAF felt a desperate need for such an aircraft and ordered six additional XP-75 prototypes, bringing the total to eight. In addition, the AAF expressed its intent to purchase 2,500 P-75s if the prototypes met their performance estimates. The V-3420-B engine for the P-75 had a two-stage, variable speed supercharger (and no turbosupercharger) that was hydraulically coupled to the right crankshaft. The engine alone weighed 2,750 lb (1,247 kg), and its weight increased to 3,275 lb (1,486 kg) with its 3.5 in (89 mm) diameter extension shafts and remote gear reduction.
The XP-75 first flew on 17 November 1943, and the aircraft almost immediately ran into issues. Its V-3420-B engine was not entirely trouble free either; unequal fuel distribution was a continuing problem for the V-3420. The issue was mostly solved by having each alternate engine section fire every 30 degrees of rotation, rather than both engine sections firing every 60 degrees of rotation. The aircraft was redesigned to correct its deficiencies and was given the new designation of P-75A. The AAF ordered 2,500 P-75As on 7 June 1944, and production started immediately. However, the entire P-75 program was cancelled four months later, in October 1944. The P-75A did not live up to expectations, it was outmatched by aircraft already in service, and the end of the war was in sight. Eight XP-75 and six P-75A aircraft were built, but three of the aircraft crashed during testing. One P-75A was preserved and is on display in the National Museum of the US Air Force. The rest of the surviving aircraft were scrapped.
Actual work to install V-3420-A engines in the XB-19 started in November 1942 at Fisher. The aircraft was redesignated XB-19A and flew for the first time with its V-3420 engines in January 1944. The V-3420 installation served as a test for the engine’s use in the XB-39. With the exception of range, the XB-19A’s performance increased across the board: maximum speed increased by 40 mph (64 km/h); cruising speed increased by 50 mph (80 km/h); service ceiling increased by 16,000 ft (4,877 m), but normal range decreased by 1,000 miles (1,609 km). The XB-19A was strictly an experimental aircraft and was never intended to enter production.
In February 1943, V-3420-A engines were selected to power the Lockheed XP-58 Chain Lightning. The V-3420 was not Lockheed’s first choice, or second, or third. The XP-58 heavy fighter program was initiated in 1940 but was beset with constant design and role changes, which were made worse by developmental issues of the aircraft’s previously selected engines. By the time it was completed, the XP-58 was oversized, overweight, underpowered, and not needed. First flown on 6 June 1944, the aircraft’s lackluster performance matched Lockheed and the AAF’s enthusiasm for the project. Only one prototype was built, and the XP-58 program was cancelled in May 1945.
Even though it helped spur the V-3420 engine program, the V-3420-powered B-29 was the last aircraft to take flight with the engine. A B-29 (actually a YB-29, the first pre-production aircraft) was delivered to Fisher for conversion to an XB-39 with V-3420-A engines. Work on the XB-39 was slow because Fisher’s main focus was the XP-75. The XB-39 finally flew on 9 December 1944. Performance of the XB-39 was superior to that of the B-29: its top speed was 50 mph (80 km/h) faster, and it had a 3,000 ft (914 m) higher service ceiling. However, standard B-29s were proving to be more than adequate, and it was not worth the time or trouble to convert any other airframes to V-3420-power.
To meet the power needs for extremely large aircraft designs during World War II, Allison proposed the DV-6840. The DV-6840 consisted of two V-3420s driving a common remote gearbox for contra-rotating propellers. A gearbox for the DV-6840 was completed in 1946, but no information has been found regarding it being tested. Allison had also planned a further development of the V-3420. This fuel-injected V-3420-C engine had a forecasted emergency output of 4,800 hp (3,579 kW) and a takeoff/military rating of 4,000 hp (2,983 kW)—both ratings at 3,200 rpm with water injection. However, the V-3420-C was never built.
The Allison V-3420 was not a trouble-free engine, but it did work well in its few applications once initial issues were resolved. The engine held a lot of potential, but that potential faded as its development languished. At the start of 1944, only 33 V-3420 engines had been delivered, and two of those were marine engines. Had the AAC committed to the engine in 1936 and provided Allison with the resources needed to develop the engine, the V-3420 very well could have powered the B-29 and various post-war aircraft. The four aircraft projects that used the V-3420 did not fail because of the engine. By the time the V-3420 program was in order in 1944, other engines were adequately fulfilling the 3,000 hp (2,237 kW) role.
Allison built a total of 157 V-3420 engines: 37 -A engines (including the two marine engines) and 120 -B engines. A number of V-3420s were sold as surplus after the war. Some eventually made their way into museums, while other engines were used in a hydroplane (Henry J. Kaiser’s Scooter Too driven by Jack Regas) and a tractor puller (E. J. Potter’s Double Ugly). However, none of the V-3420 engines took flight again.
– Vees For Victory!: The Story of the Allison V-1710 Aircraft Engine 1929-1948 by Dan Whitney (1998)
– The Allison Engine Catalog 1915-2007 by John M. Leonard (2008)
– Jim Allison’s Machine Shop: The First 30 Years by John M. Leonard (2016)
– Aircraft Engines of the World 1946 by Paul H. Wilkinson (1946)
– Allied Aircraft Piston Engines of World War II by Graham White (1995)
– US Army Air Force Fighters Part 2 by William Green and Gordon Swanborough (1978)
– McDonnell Douglas Aircraft since 1920: Volume I by Rene J. Francillon (1988)
– Lockheed Aircraft since 1913 by Rene J. Francillon (1982/1987)
– Boeing Aircraft since 1916 by Peter M. Bowers (1966/1989)
Hello Mr. Pearce,
Why is the exhaust manifold open facing forward on the P-75 installation?
Very good question; I wondered the same thing when I saw that photo. While I am not certain, I think the manifolds are backward V-1710 manifolds.
The XP-75 installation used a very odd exhaust manifold that had three cylinders per each outlet. Since the engine was buried in the fuselage, the exhaust manifolds were jacketed with an air space between the manifold and the jacket for cooling. Air was brought in, flowed through the jacket, and was expelled around the two-per-bank exhaust stacks.
What you see on the image appears (to me) to be normal Allison V-1710 exhaust manifolds as installed on a P-38 (example 1, example 2, example 3). The V-1710 manifold bolt pattern allows for a normal or backward installation.
While the manifolds on the XB-19A are different, those V-1710-style manifolds could have been used for the V-3420 installed in the XP-58 or XB-39. So, I cannot say exactly what the manifolds are, but I can say they are not XP-75 manifolds and that they look very similar to V-1710 manifolds.
Some info I found on the web: “The Kaisers would not achieve racing victory until 1954, when the three-point hydroplane Scooter (U-12) powered by a 1750 horsepower V-12 Allison engine driven by Kaiser Industries welder Jack Regas won the Mapes Trophy unlimited class at Lake Tahoe. She ran second, first, and second and posted the fastest lap at 88.748 miles per hour. Henry immediately retired the boat and built a second, the Scooter Too with a 3,420 cubic inch 24-cylinder Allison engine, producing a staggering estimated 4,000 horsepower – but she never won a race…..”
Thank you for the info and link. You reminded me that I forgot to link images of Scooter Too and Double Ugly to the article. I’ve updated that now.
At the 2010 Reno Air Races I got to see a V-3420B up close. I never thought I’d get to see one at all. Your article on this engine is extremely well written and very informative. Thanks very much for it, and all the best,
I’m glad you liked the article, and thank you for the kind words. I also saw that engine at Reno. I believe Dan Whitney arranged for the engine to be displayed and that it came from the Aerospace Museum of California in Sacramento. Quite the beast.
You are correct, I brought the V-3420B engine up from the Aerospace Museum of California, McClellan Park, CA, and that is where it resides today.
Very comprehensive article, and I agree, well written.
Thank you for your comment, and I’m glad you liked the article. Of course, it would not have been possible without your incredibly well researched book, Vee’s for Victory.
I have a 3420B right now, I’m looking for someone I can take the starter to and have it rebuilt. The starter is in pieces right now because my uncle was repairing it but his health turned for the worse and he passed away. Honestly i would like to find a complete starter, I know these were very limited production run so an actual 3420 starter may not exist, question I have, is the 1710 starter the sane in any fashion?. The engine is currently in a speed record attempt car. Future plans are undecided. I’m just trying to get it running again. Any help would be greatly appreciated.
I have a guy who is retired, having rebuilt aircraft starters all his life. He has rebuilt several aircraft starters for me and all have been excellent. If you can send me all the info off the data plate, manufacturer, model, spec.s, and a couple of photo’s of what you have in parts, I will show them to him and see what he thinks.
I was lucky enough to get to see and touch one of the Allison W-24 engines. An old friend had one. He was able to get half of the engine running. Evidently, the other half had a magneto problem. I wish I could find out what happened to the engine. Budd is gone now, so I can’t ask him.
Was the crankshaft and master rod the reason the X-3420 would have been limited to much less power than the V-3420? Any ideas what the crankshaft/crankcase design would have looked like? After reading your great description of the Packard X engine I wonder how much bearing can be squeezed into a crankshaft originally sized for half the pistons. The Volkswagen VR and WR engines have rather narrow connecting rods and crankshaft webs. As always I am grateful for your scholarship.
Hello Jiro – Glad you liked the article. The AAC wanted 1,600 hp and a single crankshaft, which developed into the X-3420. This configuration would require development of many new parts, such as the crankshaft and connecting rods. Allison felt they could create a 2,000 hp engine with the double-crankshaft V-3420. This engine would utilize the same crankshaft and connecting rods as used by the V-1710. I think it was more of Allison saying they could deliver a more powerful engine quicker if they could switch to two crankshafts and use parts already in production. I don’t think the power difference was tied to the number of crankshafts at that time. However, as has been the case with just about all single-crankshaft X engines, the crankshaft becomes the limiting factor in power output and reliability. In Dan Whitney’s Vee’s for Victory, there is a photo of the X-3420 mock-up. The crankcase appears similar to that of the V-3420. I have no idea about the crankshaft.
You have written a very good article on the V-3420, for which I thank you.
Regarding the number of crankshafts, the Army did want a single crank, as well as fuel injection. At the same time the small Allison engineering staff was being hard pressed to develop the V-1710 and get it through its Type Test (early 1930s). When Ron Hazen came on as Chief Engineer, replacing Harold Camaniz, he met with the Army and said they had to rethink their strategies. He told them that a redesign of both the V-1710 and X-3420 was needed, particularly as aircraft designers were requesting new V-1710 features, including reversible crank rotation, extension shafts, remote reduction gears and improved supercharging. He suggested that their requirement for fuel injection be eliminated on both engines and that by going to two crankshafts in a common crankcase the design, construction and manufacturing of the engines could be expedited, and at the same time their power and reliability improved. This resulted in the V-3420 design, which utilized a new (and reversible) crankshaft, improved cylinder banks and induction manifolds, along with a simple spur gear type reduction gear.
Using these components in an improved V-1710 crankcase resulted in the V-1710-E/F series engines, which accommodated the various airframe companies unique design needs. The V-3420-A was running in 1938, but by then too late for the Army’s Long Range Bomber program. The US Navy was the first to purchase the engine, as the V-3420-A8R/L, for their PT-8 boat program.
Thank you Dan, I appreciate the great info! To everyone else, I highly recommend Dan’s fantastic book about the V-1710 (and V-3420) Vee’s for Victory!