Ford GAA: The Biggest Petrol V8 Ever Built

Starting life before WWII as a 27 litre V12 designed for aviation purposes, the Ford GAA would eventually become 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 companies designs. It was highly likely a big market for high performance, light weight 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. To win, the engine would need to be more advanced and have more power than the Merlin.

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

After completing his design, he presented to the Air Corps a 27 litre, all aluminium 60-degree V12, with 4 cams and 48 valves, it had 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 the Merlin or the older Allison, pushing out 1800hp on its first test run. But to Ford’s dismay it was the Allison V12, due to it being around for a number of years previously, with many spare parts and men trained on repairing and maintaining it, that would win the contract.

The Ford GAA at Bovington Tank Museum

All was not lost there however, the Tank Corps approached Ford in need of a V8 to power their Sherman tank. To solve the Tank Corps’ problem, Ford removed 4 cylinders from the unused V12 casting cores to make an 18 litre V8. To speed up assembly and supply of this new engine, they simply retained the same design from the V12, keeping the 60-degree angle and its all-aluminium construction.

The Ford GAA, the largest petrol V8 in history, was born.


For the time the GAA was a marvel of engineering. Due to its military role it had no belts or chains whatsoever, everything was gear driven for durability and to reduce maintenance. There was a power divider to drive the cams, distributors and pumps.

A rare sight on engines at the time, its spark plugs were located centrally in the combustion chamber allowing for a more complete detonation of fuel. It was a true Double Overhead Cam engine, with 2 exhaust valves and 2 inlet valves per cylinder. A 90kg flywheel was used to 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.


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 of the tanks with this engine were known to remove the governor to increase rpms and allowing more 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.

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

The exposed gear-driven cam shafts. Author: Hohum CC BY 3.0

Providing a brilliant service on the battlefield the GAA V8 proved itself and soon after the war ended, the engines began ending up in civilian hands. Because of its extreme reliability and low sustained RPM 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, and 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 moving 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.

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.