Ford GAA; One of the Biggest Petrol V8s Ever Built

The Ford GAA is one of the largest petrol V8 engines in the world. This powerful, reliable engine was incredibly advanced for the time, using techniques that wouldn’t reach civilian engines for decades. We will explore the origin of this engine, and what made it so special.

The GAA’s Origin

The Ford GAA V8 originated to before WWII, as a 27 litre V12 for aviation purposes. It would eventually becoming a world-class V8 for both military and civilian purposes.

It was originally designed as a V12 on request from Henry Ford. Anticipating the start of another world war, Henry Ford set about designing an engine better than any competing designs.

It was highly likely a big market for light weight, high performance engines to power fighter aircraft would open, and Ford wanted the contract to supply them. He started work on a V12 to beat Roll-Royce’s distinguished Merlin engine but with the same 27 litre displacement.

RAF ground crew prepare a Rolls-Royces Merlin to be fitted to a Halifax.

After completing his design, he presented the Air Corps with a 27 litre, all aluminium 60-degree V12. It had 4 cams and 48 valves, and some truly modern features for the time. So confident was Ford that he’d win the contract, he already had the tooling and casting cores ordered for its manufacture.

The prototype was much more advanced than both the Merlin and ageing Allison V12s. On its first test run, it pushed out 1800hp, far more than even the Merlin. But, to Ford’s dismay, it was the Allison V12 that would win the contract. This was because the Allison was already a common engine in US service. It had many spare parts, and maintenance crews knew how to repair and maintain it.

The Ford GAA at Bovington Tank Museum

All was not lost however.

The Tank Corps approached Ford in need of a V8 to power their Sherman tank, to which Ford gladly assisted.

The Design

To make a V8, 4 cylinders were removed from the unused V12 casting cores.

To speed up production of this new engine, the 60-degree angle and all-aluminium construction of the V12 was retained. The resulting engine was 18 litres in displacement.

The Ford GAA, one of the largest petrol V8s in history, was born.

To increase durability and reduce maintenance, everything was gear powered. No belts or chains were used whatsoever, limiting the components that could be damaged in combat.

There was a power divider to drive the cams, distributors and pumps.

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Its spark plugs were located centrally in the combustion chamber allowing for a more complete detonation of fuel, a rare sight on engines at the time. It was a true Double Overhead Cam engine, with 2 exhaust valves and 2 inlet valves per cylinder.

The exposed gear-driven cam shafts.
Image by Morio CC BY-SA 4.0

A 90kg flywheel helped smooth out the firing order of the huge engine. The GAA’s capabilities were regarded as under used powering tanks, never being properly pushed to its limits in this role.

Its Performance

The 18 litre V8 produced 500hp at 2600 rpm, and produced a monstrous 1400nm of torque at idle. It was designed to live at low rpms to keep sustained powerful output. Due to this an rpm governor had to be installed with the limit at 2600 rpm. Crews were known to remove the governor to increase rpms and power in dangerous situations.

However without the rpm limiter, a stock engine would rev to 3800 rpm before the valves would stop following the contour of the cam shaft lobe, and begin to float, known as valve float. If this happens the valves are no longer strictly controlled by the cams, and can cause a piston and valve to collide.

A Ford GAA at the Dutch Cavalry Museum

An odd feature was the two carburettors being mounted on opposite ends to each other on the engine. This made the fuel-air mixture much richer at the end cylinders compared to the central ones, often fouling the outer spark plugs quicker.

In Service

The engine was used in the M4A3 Sherman tank, and while this was probably the Shermans best engine, it couldn’t be made in enough numbers to satisfy demand. Later, the GAA powered the M26 Pershing.

An M26 Pershing next to a Ford GAA.
Image by Morio CC BY-SA 4.0

Civilian Service

The GAA V8 had proved itself on the battlefield. Soon after the war ended, the engines began ending up in civilian hands. Because of its extreme reliability and low RPM power the engine was ideal in industrial roles such as in oil fields, trucks and boats.

After people began tinkering it was discovered that the engine’s true potential was relatively untapped in its standard form. The GAA had massive capabilities for increased horsepower and torque, especially when used with a turbocharger or supercharger.

It was found the stock pistons were good up until around 1200hp at which point the rings began to fail, and the pistons themselves can crack with moderate boost pressures. However modifications such as replacing pistons and valve springs, balancing components and adding direct injection and turbocharging, can give a sustained reliable output of 2200hp.

Noticing this, the engine became popular on the drag strips, where it performed so well it was subsequently banned from competing in the 1950s. More recently, tractor pullers have managed up to 5000hp for shorter durations.

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The Ford GAA was rushed to service, but over its lifetime served many different interests, proving the adaptability of the engine. From its unequalled reliability in combat, to pushing over double its standard horsepower with stock components it really is a testament to Ford’s ability at constructing a solid engine.