HISTORY OF THE GAA ENGINE
There are many stories of how the GAA engine came into existance, and most likely all have some validity. The following is one version of it:
In World War 1, Ford was a manufacturer of the Liberty V12 aircraft engine for the army air corps, and as it became apparent there could be a second global conflict, Ford wanted to again get into the aircraft engine business. To this end, Rolls-Royce was contacted regarding licensed production of their new Merlin V12 engine, but no licensing argeement could be reached. In fact, the Merlin was instead licensed to Packard, and the Packard Merlin was used in P-51 Mustangs in Europe, along with other applications. With his pride wounded, Henry Ford vowed to build an engine of the exact same size, but which would be more technically advanced and with more power than the Rolls. A 1650-cid, 4-cam, 48-valve V-12 engine was designed, and Ford approached the army air corps with a production proposal. So sure was Ford that the proposal would be approved and contracts for production awarded, a matter of Ford's continuing over-optimism, that production preparations were made, including making the casting cores and ordering the tooling. How ever, the competition was the Allison 1710 cid V12, and it had already been in production for some years. Large amounts of spare parts were on hand and numbers of mechanics had been trained to work on it. So, a production contract for the Ford 1650 V-12 was not awarded.
At the same time, the army tank corps was searching for a suitable engine for the Sherman tank. Initially, the Sherman used a 9-cyl radial aircooled engine, but this engine was in critical short supply for aircraft uses. As well, the engine tended to foul spark plugs on the bottom cylinders, and seemingly at the worst times, such as when jousting with German panzers. The tank corps tried using a Chrysler "multi bank" engine, which was really five inline 6-cylinder flatheads bolted to a common crankcase. This was unsatisfactory because of the nightmare of servicing a 30-cylinder engine, and because total output was only 375 hp, not nearly enough for a 35-ton tank going cross-country. Shermans with twin GMC 6-71 diesels were produced for the Marines, mostly to lessen the threat of fuel fires on landing vessels, but since the 6-71 itself was still being developed, another alternative was desired. Therefore the army went to Ford and in effect said, "Look, we don't need another aircraft V12, but we do need a good tank V8." Ford then took the casting cores for the V12, chopped off the rear 4 cylinders, and the GAA V8 was born. To facilitate a quick production startup on the engine, it was decided to keep the existing design, which meant that the engine stayed as a 60-degree V-engine, and because all the design, engineering and coring was for alumunum, this feature was kept as well. Thus we have the Ford GAA, an "accidental" engine of the finest magnitude, likely the biggest light alloy, water cooled V8 engine ever built.
It is NOT a Merlin copy. The Rolls Royce Merlin was a single overhead cam engine with rocker followers, whereas the GAA is a true double OHC engine with the cam lobes directly acting on mechanical buckets. The Merlin's spark plugs were located on the exhaust port side of the head just below the exhaust ports, whereas the GAA had its plugs centrally located in the combustion chamber, as per modern practice. The GAA featured a patented "power divider" gearbox which drove all the cams, pumps and distributors, where-as the Merlin used a more conventional setup. Finally, the GAA featured a one-piece block casting, but the Merlin featured a split block casting. The confusion between the GAA and the Merlin arose because they both featured the same bore and stroke (5.4" x 6.0"), and because later in the war Ford of Britain did produce 400 Merlins per week at a factory in Urmston, England.
There were a little over 28,000 GAA and derivative models built. Of these, about 14,000 were factory installed into Sherman tanks when they were built, and the remaining engines were retrofitted into earlier model tanks when they came due for major overhauls. The GAA itself was used in model M4A3 Sherman tanks, and came with a dual-disc 17.5" clutch setup. The GAF model was used as the engine for all M26 Pershing heavy tanks, and was setup for use with an automatic transmission. The GAN version was used for other applications. Some of the actual V-12 engines were fitted to a few prototype tanks, where much more power was needed, and were given the GAC model designation. Most of the production in the GA-series was for the GAA model. The army tank corps wanted a 500-hp engine, and at its governed speed of 2600 rpms, the dyno sheets show this power level. The dyno sheets do go to 2800 rpms, and at this rpm the engine put out 525 hp. Despite the presence of a governor, in actual practice many tank crews were said to disable it in order to increase speed at critical times, such as when engaging German Panther and Tiger tanks. It is said that with stock springs the engine would rev to about 3800 rpms before valve float.
Ford had pioneered the one-piece V-engine block with its early flathead V8s. Up to that time V-block engines were of a split-case design. The GAA engines continued this innovation. The block itself is of a contemporary short-skirt design, with a full length horizontal reinforcing rib. It is a side-oiler block, no doubt being an inspiration for later 427s, among others. The initial 12,070 engines also had 4-bolt mains with a double splay stud arrangement. Later engines changed back to 2-bolts per main cap due to machining complexity. There are no chains or belts in the engine - everything is shaft or gear driven. At the front there is an 8-way accessory drive (power divider) unit, which powers two cam drive shafts, two distributors, oil and water pumps, and two heavy duty power takeoffs for fans, generators and other items. Housed in the protruding "snout" at the accessory end of the engine, the drive is a machinist's pride but an accountant's nightmare.
The GAA is a 60-degree V8, the inverse of the Buick 90-degree V6. Both presented their designers with the dilemma of either having an uneven-fire engine with "normal" rod journals on the crankshaft, or of having split journals for the rods, which would weaken the crank but provide for even-firing. Ford chose to go with the uneven-fire design, which resulted in the crank being a 180-degree (flat) design much like a 4-cylinder engine. It has two 4-cylinder distributors, one per cylinder bank. The uneven firing pulses were dampened out with an enormous 150-lb flywheel and a 75-lb twin-disc floater plate. The crank has hollow journals on both rods and mains, both as a lightening effort for it, and also as safety backup oil reservoirs for the bearings. The crank still weighs 135 lbs. Tapered wrist pins, thought by many to be a recent innovation, were used on all GAAs. The pins themselves were full floating with bronze inserts. Of interest is that the rod bearings were full floating also, which is to say they were two rods width and spun against both the crank journals and the rod big ends. The pistons were only 7.5:1 compression, which was all an engine could tolerate given the 80-octane gas of the day. Bore is 5.402" and stroke is 6.0". A dry sleeve engine with removable liners, there are 6 studs per cylinder.
Good cylinder heads are what turn any ordinary engine into a performance engine, and the GAA was definitely set up for good airflow. These are DOHC, 4-valve heads, with each intake valve's port being 1.90" x 2.1875", for a total inlet port area of 1.90" x 4.375" "per cylinder." No records are available as to any flow tests on the ports. Intake valves are 2.12" and exhaust valves are 1.90". Cam lift is .500" and duration is 240-degrees, using the old "advertised" method of calculation. The cams act directly on the valves, so resetting valve lash between jousts in the field with panzers was not an option. The cams themselves are driven by shaft-powered worm gears. No belts or chains to stretch or break.
The dual Stromberg aircraft 2-bbl carburetors were located outboard of the ends of the heads, a questionable choice for their place-ment. Airflow was about 480-cfm each, for 960-cfm total. Each carburetor barrel fed two cylinders, basically acting as fuel dumpers, with the result being the end cylinders ran richer and the middle cylinders ran leaner. Fuel was supplied by a mechanical pump whose flange pattern is identical to today's big block Chevy.
Due to the odd-fire layout of the engine, each cylinder bank was given its own 4-cylinder magneto. This was not all bad, because if one magneto became disabled, the other provided a measure of redundancy. Sparkplugs had a fixed .017" gap, an indication of the period's ignition system development. The lubrication system consisted of a 28-qt oil pan and a partial filtration system.
Aluminum engines are lighter than cast iron engines. However, "light" is relative. An aluminum 1,100 cubic inch engine is not going to be as light as an aluminum small block Chevy 350. In this instance, "light weight" is listed in the GAA overhaul manual as being 1,440 lbs "with all accessories and ready to run." This means including the generators and fans, their drives, the enormously heavy flywheel and twin-disc clutch, etc. As set up with modern day components and without all the combat related items, a typical GAA weighs around 900-950 lbs ready to run. A complete modern day lower engine with starter and lightened flywheel weighs around 675 lbs. The bare block itself weighs 220 lbs.
Displacement Increase: The GAA V8 engine already has a 5.402" bore and a 6" stroke, for a displacement of 1,100 cubic inches (18 liters). Any person who finds that the job cannot get done with an engine of this size should find another hobby.
Which brings up the topic of horsepower and torque. Horsepower is calculated as: torque x rpm, divided by 5252. The GAA is just not a high-revving engine, and so the "rpm" part of the equation will always dampen "braggin rights." However, on the street in actual driving, it is "torque" that does the work, and a modernized, streetable GAA is capable of putting out around 1,350 lbs / ft of torque between 2-3000 rpms. Off-idle (i.e. "the light turns green") torque is over 1,000 lbs / ft. The smallest transmission rated to handle this level of torque is the Allison MD-3000 6-spd. If, perchance we were able to get this level of torque past a differential, there are no street tires that can handle the power. Adding dual 4-inch turbos would bring over 2,000 streetable horsepower, all nice and mild -tempered so grandma wouldn't be intimidated by the idle sounds, but there's no way whatsoever to get that sort of low-end power to the pavement. This engine is the hotrodder's dream, the ultimate "no replacement for displacement", but it is also a quagmire in that there's already so much torque that not much further can be done to it, its just not possible to get the power to the ground. Nothing sophisticated, just a big-ol' 1,100 cubic inch gorilla on the streets with his equally big sledge hammer.
In 1945, the GAA engine put out, with all its faults and limitations, 175 hp and 1,000 lbs / ft of torque at 1,000 rpms. It carried this 1,000 lbs (or more) of torque through the usable rpm range, up to 2,800 rpms.
No Replacement For Displacement: One Ford GAA engine is bigger than: (3) 350 Chevys or 360 MoPars. Bigger than 2 1/2 MoPar 426s, (2) Dodge V10 Ram trucks, 6 Toyota Camry V6s, or 8 Volkswagen New Beetle 4s. When parked at a local burger palace on a weekend night with a GAA-powered Ford truck, should the trucker in the next slot mention that his Chebby has a 502 crate motor, the Ford man can respond with, "good deal, and if you had 48 more cubic inches, you'd be half as big as my Ford." End of story.
Thanks for visiting the website. Please visit the picture section for some pics of GAA internals and of some GAAs as modernized.
Final version of the Sherman tank was the M4A3E8, which featured horizontal volute suspension,
upgraded armor, upgraded turret, and a high velocity 76 mm gun, which could stop a German
Panther or Tiger tank. (click on thumbnail for full-sized image).
Military-use restoration of a GAA (actually a GAF variant) as used in an M-26 Pershing tank.
(click on thumbnail for full-sized image).
Engineering drawing of a GAA engine, showing valve layout and internals, accessory-end view.
Note the 60-degree "V", which results in the engine only being 33.25" wide, including exhaust
manifolds. Engine is 36" high, from the bottom of the pan to the top of the valve covers. (click on
thumbnail for full-sized image).
Engineering drawing of GAA engine's bottom end (late model engine shown). Note the side-oiler
main gallery, which was .750" diameter. The open "snout" housed the accessory drive, which
ran the cam drives, oil and water pumps, distributor drives and accessory drives. Late engines
used 2-bolt mains, which were easier and cheaper to machine. Early engines used 4-bolt mains
with a double splay on the main caps. (click on thumbnail for full-sized image).
GAA display engine as restored for military use. (click on thumbnail for full-sized image).
European Smidje tractor pulling team is a top threat at every event. Team uses twin GAAs with
GM-style 14-71 blowers and fuel injection. Note the big overdrive pulley ratios. Upgrade plans
include changeover to twin turbos per engine. (click on thumbnail for full-sized image).
European Dirty Herrie mini puller depends on a full-house GAA with GM 14-71 blower for power.
The short wheelbase on the mini pullers guarantees a wild ride. (click on thumbnail for full-sized
US-based Zimmermann pulling team relies on a twin Holley Dominator setup for its GAA engine.
(click on thumbnail for full-sized image).
Webmaster's Ford F350-based street truck, when completed, will feature this carbureted engine
for starters. Engine features (3) Demon 4-bbl carbs and twin MSD billet distributors. Future plans
include changeover to Ford EEC-4 based twin throttle body EFI. (click on thumbnails for full-sized
Kevin Heidrich's GAA tractor puller engine features twin GM 6-71 blowers and fuel injection. Future
updates include conversion to twin 4" turbos with mucho boost dialed in. (click on thumbnail for
Kevin Heidrich's GAA-powered '70 Mustang tube chassis street cruiser under construction. Enderle
fuel injection is shown, but engine may be converted to more economical dual Predator carbs. (click
on thumbnail for full-sized image).
Ron DeVoll with his restored GAA engine. Ron enjoys restoring historic military engines as his hobby
and has several other engines either restored or undergoing restoration. "Back in the day" Ron and his
father ran several GAAs on the family's Texas farm on irrigation pumps non-stop for months at a time
and recalls that they were as reliable as could be. My thanks to Ron for his good "I was there" info
about these engines during their life after tank duty. (click on thumbnail for full-sized image).
Brent Mullins has recently restored a Sherman tank and included a GAA engine as a part of the project.
Pic #1 shows the big Ford all put together and awaiting installation into the tank. Pic #2 shows the big
V8 all snugged down into the engine bay. In the second, overhead pic, note the
location of the carburetors and also the "cast-in" intake manifolds and hot water return manifolds. Brent
Mullins Jeep Parts in College Station, TX, specializes in all sorts of parts for military vehicles, from jeeps
to tanks, and most everything inbetween. If you need something for your military restoration, give him a call.
(click on thumbnail for full-sized image).
GAA Screensavers: Here are a few GAA pics as reprocessed to be suitable for screensavers. They are
Hi-Def 1024 x 768 JPEG format. (click on thumbnail for full-sized images).
Many more pics to come.
If you have a GAA-based vehicle, or are building one and would like to have it featured on this site, please
send some good quality pictures of it in JPEG format to: email@example.com. Any comments or questions,
please also send to the above e-mail address.
"NO", I don't know of any GAA engines for sale at this time, or where to locate one. The best place to look is in an agricultural area, because most of the GAA engines from scrapped-out Sherman and Pershing tanks were placed on skids and sold to farmers for irrigation pumps or generators in rural areas. To illustrate that one of these engines could turn up anywhere, a man in Brazil just
located two new GAAs in crates at a depot in his country. He's now busy building a street rod with one of the engines, and it will be featured here when completed. Two others turned up on e-bay recently as well, so keep a good lookout on sources like this.