The GAA Power Plant--Part 1

I had this in my files from a prior search on the GAA but forgot where it came from, a quick search got it at www.fordgaaengine.com/ I cant see any copyright so it is posted below.

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 500-hp 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.

ENGINE OVERVIEW:

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 powering generators and 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 therefore 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.