Rolls-Royce Griffon Engine

Reprinted from

Key Specifications (Prior to racing modifications)

Model Griffon 58 (Griffon 74's were primarily used during 1980-1985)
Original Application Avro Shackleton reconnaissance plane
Engine Type V-12 Piston Aero Engine, 60 "V" (angle between banks)
Material Aluminum crankcase, cylinder banks and heads; Alloy pistons; Steel liners, rods, cranks, etc.
Bore x Stroke 6.0" x 6.6"
Compression Ratio 6.0:1
Supercharger Type Two-speed, single-stage centrifugal 13.4" impeller
Supercharger Ratio Medium Speed (MS) - 6.615:1, Full Speed (FS) 7.70:1
Boost Control Automatic via Rolls-Royce auto boost regulator, w/increased boost when ADI active
Fuel System Speed/Density type Fuel Injection (injected into eye of supercharger)
Weight 2,000 lbs
Power 2,450 HP takeoff at 2,750 RPM at SL with 25 psig boost using 115/145 PN fuel
Performance 0.82 lbs/hp, 314 psi BMEP, 0.42 lbs fuel/hp/hr SFC
Max. Piston Speed 3,025 ft/min
Total Manufactured Approximately 8,000


Development of the Griffon Engine began at Derby, England 1939 when Harry Cantrill was assigned to develop a conventional V-12 scaled up from the 1650 cu-in Rolls-Royce Merlin. The engine was intended to produce more than 1,500 HP at low altitudes for naval torpedo bombers. For maximum utility, it was decided to keep the engine as compact as possible so it could replace the Merlin in some applications. The resulting design had approximately the same frontal area as the Merlin and was actually shorter. The bore and stroke was the same as the Rolls-Royce "R" Schneider Cup race engines of 1929 and 1931, which were direct ancestors to the Griffon, resulting in an displacement of 2,240 cubic inches.. The Merlin proportions of the Griffon were partially achieved by moving the camshaft drives and magneto to the front of the engine.

While the displacement of the Griffon increased by 36% over the Merlin, power did not increase proportionally. This was because the piston speed of both engines is limited to about the same value (3,000 ft/min) reducing the maximum RPM of the Griffon compared to the Merlin, and also because the Griffon's larger cylinder size did not allow as much boost pressure without destructive detonation as the smaller Merlin. The Griffon 57's 2,450 HP is limited by the maximum permissible boost of 25 psig (pounds per square inch gauge, equivalent to 39.7 psi or 81" hg absolute) -vs- 2,270 HP for the V1650-11 Merlin at 29.5 psig. The Griffon turns at 2,750 RPM produces 23 power strokes/sec, and the brake mean effective cylinder pressure (BMEP) is 315 psi -vs- the Merlin's 3,000 RPM producing 25 power strokes/sec with a 360 psi BMEP. Please note that US and British boost specifications use different conventions with respect to absolute or gauge pressure, so we append a g for gauge or an a for absolute to applicable pressure measurements.

The Griffon began with a single-stage supercharger which could run at two different speeds -- the medium-speed (MS) ratio was used at sea-level, and the full-speed (FS) ratio at altitude. As the Griffon evolved two-stage supercharging was introduced, implemented with a clever compact design which combined both impellers in a single partitioned housing -- this was dramatically smaller than the "auxiliary" second stages used for both Merlin and Allison V-1710 engines which consisted were essentially stand-alone units linked by a drive-shaft to the engine. Eventually two-stage three-speed superchargers were used for prototype altitude-rated engines which provided high performance over a wide range of operating altitudes.

Griffon-powered Hydroplanes

Three Unlimited teams have used the Rolls-Royce Griffon engine for power. J. Gordon Thompson's CA-1 Supertest team was first, using the Griffon in the Miss Supertest II & III from 1954 to 1961. Bill Harrah converted from Allison to Griffon power in 1968 for his U-3 Harrah's club boat in what was to be his team's final season of competition. Harrah's Griffon engines became part of the Budweiser team's engine pool -- the third team to use the Griffon. Three hulls used Griffon power -- the 1979 hull was destroyed at the end of the season during a mile speed record attempt, the replacement boat which campaigned from 1980-1985 (now owned by Unlimited Excitement), and the 1985/1986 "Bubble" Bud -- the first boat to feature a cockpit canopy.

The original Budweiser used a variety of engines generally configured as a Griffon 74, which featured the two-stage two-speed supercharger with ratios of 5.16 MS and 6.79 FS. It used impellers of 13.4" for the first stage and 11.3" for the second stage. In 1980, the team employed a ratio of 5.81:1 for the supercharger. By modifying the crankshaft lubrication system and bearings, the team was able to increase the peak RPM to 4,000, producing peak piston speeds of 4,400 ft/min and 33 power strokes/second. By using high octane racing fuel with a rich PN of about 150, combined with ADI injection and nitrous oxide (N2O) increased the BMEP to over 360 psi. The high BMEP combined with increased power strokes per second raised increased power to over 4,000 HP.

Hydroplanes place tremendous demands on engines. Race engines face dramatically varying loads as the propeller unloads and suddenly loads again, resulting in tremendous internal stress as rotating components accelerate and decelerate. The load from a propeller is relatively constant, even in severe turbulence (it is very difficult to accelerate or decelerate the airplanes propeller because of its large diameter and thus large rotational inertia). A hydroplane, on the other hand, subjects an engine to large accelerations and decelerations as the prop-riding boat's relatively small diameter propeller (with small rotational inertia) is loaded and unloaded. The Griffon supercharger takes the most stress as its 13.5" aluminum impeller spinning at almost 25,000 RPM suddenly needs to accelerate and decelerate. A closely related problem is overspeed when the engine is unloaded at full power, power thus absorbed by accelerating the rotating components.

The engine's acceleration and deceleration is brutal on the impeller, the supercharger gear-drive, the crankshaft and other rotating components, and especially on a component all hard-core enthusiasts have heard of -- the spring shaft. In the Griffon the spring shaft is about 1 foot long and 1" in diameter, slipping into a spline on the auxiliary end of the crankshaft on one end and the supercharger drive gears on the other. It is intended to flex to absorb harmonic vibration, but when the engine decelerates or accelerates too fast it tends to shear from the resulting momentary overload (because of the gearing, it sees the rotational inertia of the supercharger multiplied by the supercharger gear ratio -- a very difficult load indeed to accelerate or decelerate). The Budweiser team overcame this problem by having the Gear Works of Seattle fabricate improved quill shafts, by limiting running time on engines and regularly (and frequently) replacing quill shafts. Following the Budweiser era, the expense of running the boat due to the inherent vulnerability of the quill shaft (and the stress put on related components) was one of the major factors in engineering the turbocharged induction system which replaced the supercharger.

The crew of the Budweiser faced another challenges adapting the Griffon to hydroplane use. The propeller reduction drive could not be adapted to drive the boat propeller which needs to turn at high RPM. A boat traveling at 170 mph (fairly typical straight-away speed) is moving 250 ft/sec, so a propeller with a pitch of 1 ft/rev needs to turn at the incredible speed of 15,000 RPM! The Budweiser was designed to use a propeller pitch of about 1.5 ft/rev and a peak prop speed of over 12,000 RPM, which is still incredible. It's easy to understand why the rear of a boat is torn apart when a prop is thrown -- 20 - 30 lbs of stainless steel turning at that speed has a tremendous amount of energy.

The reduction gears also connect the magneto, camshafts, and starter to the crankshaft, so they could not be altered or eliminated. Since the propeller reduction drive could not be adapted, a means of coupling the crankshaft to an external gearbox (custom manufactured again by the Gear Works of Seattle) needed to be found. While it sounds simple, the Griffon employees an "end-to-end" crankshaft lubrication system, which means the main bearings are lubricated by the crankshaft via a hollow passage passing through the crank, with oiling accomplished on the propeller end by a coverplate and oil tube which fits into a hole in the end of the crankshaft. The propeller reduction drive gear fits slips over the outside of the crankshaft. To connect to the crankshaft, it was necessary to eliminate the oil feed, modify the reduction gear housing to allow a coupling to be connected to the crankshaft, and engineer an appropriate crankshaft oiling system which not only replaces the previous design, but improves it to increase maximum RPM. The answer was to revert to lubricating the main bearings through the bearing webs, feeding high pressure oil into holes drilled through the bottom of the "Vee" through the main webs and through modified main bearings drilled to align with the main webs.

From 1986-1989 the Budweiser generally used Griffon 58 engines. These engines originally used single-stage two-speed superchargers which generated slightly more power than the Griffon 74 at sea-level (the single-stage supercharger was able to produce as much usable boost with slightly less horsepower) and employed a Rolls-Royce speed/density injection system. Because of the difficulty and expense involved in keeping the supercharged engines running (especially coping with the vulnerable quill shaft), the supercharger was replaced by twin turbochargers located at the supercharger end of the engine. The wheelcase and supercharger housing were replaced by an adapter mounting an oil-pump driven by the springshaft. The pump delivered pressure oil to both the main bearings and the turbochargers. Later the AirResearch turbochargers were moved to the rear of the boat, occupying space previously used by the nitrous oxide bottles. The nitrous oxide bottles were moved to just behind the cockpit, the dry sump being shifted rearward toward the space previously occupied by the supercharger. Induction was by through a huge Bendix-Stromberg PR100B4 injection carburetor, originally used on the Pratt and Whitney R-4360 radial engines that powered planes like the B-36 bomber. The carburetor measured the air entering the engine, and provided metered fuel which went to fuel injectors directed into the eye's of the intake impellers. ADI injectors were also co-located with the fuel injectors. When the original three turbochargers were found to create excessive boost, the number was reduced to two. Two wastegates were used to control exhaust manifold pressure, limiting peak boost pressure.

Griffon Description

This description applies to the engine as originally specified for aircraft use. Hydroplane use requires a number of modifications to the usual aircraft practices. Among these are modifications to the lubrication system, including main-bearing oiling as described above and oil cooling by water heat exchanger. The cooling system also differs, with an "open" system replacing the pressurized closed ethylene glycol/water to air through a radiator system. The open system is a total loss system, with a ram-rudder water pickup supplying cold water to the engine cylinder banks (usually first traveling through the oil-water heat exchanger) which cool the cylinder banks banks, the heated water then is discharged overboard. The ram water pressure is sufficient to eliminate the need for a water pump. The water pump, header tank, radiators and automatic shutters and associated plumbing are thus eliminated in the hydroplane application. The propeller shaft was not used, so it was removed and the opening sealed. The crankshaft drove the gearbox directly, so the end lubrication feed was replaced with an oil seal and a splined adapter fitted to the crankshaft. The main bearings were then fed oil by external passages drilled from outside the crankcase from between the cylinder banks instead of from the end-to-end crankshaft. The same external passages also supplied oil to the connecting rods bearings through the end-to-end crankshaft oil passages.

Type: Supercharged 12 cylinder 60 Vee liquid cooled, monoblock construction with seperate cylinder banks and crankcase.

Cylinders: Bore 6.0" (152.4 mm), Stroke 6.6" (167.6 mm), swept volume 2,239 cu in (36.7 liters). Two blocks of cylinders are mounted at 60 to each other on inclined upperface of 2-piece crankcase. Each block comprises a light alloy (aluminum) skirt with a separate light alloy cylinder-head. Separate cylinder liners in high carbon steel, having flanges a their upper ends, are fitted in the light alloy skirts, the flanges of the liners being sandwiched between the head and skirt making the liner practically unstressed in the static condition, reducing distortion. Another advantage of this arrangement is elimination of coolant leaks. Gas tightness is ensured by the use of soft aluminum-alloy jointing rings. A coolant seal on each liner at the base of the skirt is made by rubber collars located between external ribs on the liner. The cylinder assemblies are each retained to the crankcase by fourteen long studs in chrome-vandium steel which pass through tubes in the cylinder skirt and head, these tubes being sealed against coolant leaks by rubber rings. A further series of small studs form a secondary tie between head and skirt. The heads carry renewable valve seats in Silchrome. Inlet and exhaust valve guides are made in cast iron and phosphor bronze respectively.

Pistons: Machined from close forgings of Rolls-Royce 59 alloy (aluminum). The piston carries two compression rings and an oil scraper ring above the piston pin and another oil scraper ring below it. The oil ring groves are drilled to return oil to the crankcase. The fully floating piston pins of hardened steel are captured by spring wire circlips.

Connecting Rods: Nickel steel machined H-section forgings. Each rod assembly consists of a plain rod and a forked rod, the plain rod surrounding a split thin-shell lead-bronze bearing on the outside diameter of the forked rod and the rod cap retained by two bolts, the forked rod retaining a split thin-shell bearing which surrounds the crankshaft journal, the forked cap being retained by four bolts. The plain rod and forked rod intersect such that they rotate about the crankshaft in the same plane, so the cylinders and pistons align across the Vee. The small end of the rods incorporate full-floating bronze bushings.

Crankshaft: Clockwise rotation viewed from the rear (supercharger end). One piece balanced, six-throw machined forging of nitrogen-hardened chrome-molybdenum steel. Crankpins and journals are bored and fitted with oil retaining caps and the webs are drilled to allow oil to be fed axially from each end of the crank to to the main journal and connecting-rod bearings. Drive to the reduction gear pinion is from a serrated flange bolted to the front end of the crankshaft. The rear end of the crankshaft is connected by a flexible torsion shaft (spring drive) to the supercharger driving gear and also provides drives to the auxiliary gearbox, oil pumps, coolant pumps, fuel pump, tachometer and propeller constant-speed unit. Angular movement of this spring shaft is limited by stops attached to the crankshaft.

Crankcase: Composed of two halves. Both castings of aluminum-alloy. Upper portion carries cylinders and crankshaft main bearings. The front of the crankcase forms integrally the rear housing the propeller reduction gear and also contains the camshaft and starter motor drives. The lower portion forms the engine sump and contains the oil pump assembly consisting of the main pressure pump, supercharger change-speed operating pump and two scavenge pumps; also the main coolant pump which is driven through the same train of gears as the oil pumps. The main bearings, of which there are seven, consist of split steel shells lined with lead-bronze alloy, which fit into semi-circular recesses machined in the top half crankcase, and are held in position by forged light alloy bearing caps and nickel-steel studs. In addition to these studs sixteen bolts pass transversely through the caps and the whole width of the crankcase, to give great rigidity but at the same time allowing withdrawal of the lower half crankcase without disturbing the crankshaft.

Wheelcase (Auxiliary Gearbox): Aluminum-alloy casting secured by studs at rear end of crankcase. Supercharger unit is in turn bolted onto the back of the wheelcase. The wheelcase houses the two-speed supercharger drive, drives the remote gearbox coupling, engine speed indicator, propeller constant-speed unit, fuel (injection) pump, and also provides a drive to the oil and coolant pumps situated in the lower half crankcase.

Valve Gear: There are two inlet and two exhaust valves per cylinder. Inlet and exhaust vales are prepared from forgings of KE965 steel, a protective layer of "Brightray" covering the whole of the combustion face and seat of the exhaust valve and the seat only of the inlet valve. Sodium-cooled exhaust valves. Dual concentric coil springs control each valve via a steel retainer and split bronze locks. Each head features a single central camshaft located in seven pedestal brackets fixed to top of the head operates both intake and exhaust valves through rocker arms fitted with spherical-headed adjustable tappet screws. The camshafts are similar for both cylinder blocks, being driven by spur gears, bevel gears and inclined shafts from the reduction gear wheel. Valve timing of 28 overlap with 248 duration.

Induction: Injection unit is mounted to supercharger and auxiliary gearbox, consists of an updraft throttle body, injection pump, primer, accelerator pump, automatic boost control, ADI control and injection, and fuel injectors. The throttle body incorporates a Corliss throttle, which consist of a truncated rectangular cylinder which when closed completely blocks the inlet, and when fully opened is recessed into a pocket in the side of the intake throat, providing completely unobstructed airflow. Engine scavenge oil circulates through the throttle to prevent icing. The injection pump is the heart of the fuel injection system -- it performs fuel metering based upon engine speed, intake charge temperature, and intake boost pressure. It mounts to the auxiliary gearbox. Pressurized fuel connects to intake manifold primer nozzles thorough a primer solenoid. An accelerator pump mounted on the throttle body eliminates lag when changing throttle positions. The automatic boost control consists of an aneroid to measure boost which controls a servo inserted the throttle linkage, the servo closing the throttle as maximum boost is reached. A control increases maximum boost when ADI is active. The automatic boost control also incorporates an additional control which energizes ADI when a predetermined boost pressure is reached. The ADI injector is incorporated into the throttle body, ADI being directed into the eye of the supercharger. Finally, an injector incorporated in the throttle body directs fuel into the eye of the supercharger. A single induction pipe distributes the fuel/air charge to the intake manifolds, and then to the cylinders. Each intake manifold incorporates two flame-traps, each flame trap supporting 3 cylinders each. The flame traps reduced backfire, a catastrophic event if uncontrolled at high boost when the plenum is filled with volatile pressurized air/fuel and the resulting explosion (caused by the charge in the plenum being ignited by burning inside cylinder when the intake valve opens) rips the plenum apart causing loss of all power and probably an engine fire, or at least tear blades off the supercharger which will likely lead to rapid engine failure.

Supercharger: Two-speed one-stage (58) or two-stage (74) of centrifugal type, the change-over mechanism operated automatically by electro-hydraulic system controlled by an atmospherically-controlled aneroid. The hydraulic oil pressure for operating the centifugally-loaded clutches of the two speed mechanism is supplied by a special high-pressure pump previously mentioned in the crankcase section. Design of the clutches is such that slip is permitted under acceleration conditions to avoid overloading of gearing and also to damp out, in conjunction with the spring-drive, torsional oscillation from the crankshaft. The crankshaft drives a ring gear through the spring shaft, the ring gear driving the accessories associated with the wheelcase and the pinion which drives the two clutches that engage the spur gears which in turn drive the supercharger. The springshaft is concentric and inside the spring drive ring gear, which in turn concentric (inside) the supercharger drive tube. The boost pressure of the supercharger is controlled by an automatic servo mechanism coupled through a differential by an automatic servo mechanism coupled through a differential linkage to the throttle so that a constant boost-pressure is maintained at altitude up to full throttle conditions for a fixed position of the pilot's lever.

Ignition: Ignition is by two twelve-cylinder magnetos combined together in one unit and mounted in the vee directly behind the reduction-gear housing. Driven by bevel gears and an inclined shaft from the port camshaft drive. Incorporates two separate circuits which are electrically independent of each other. The timing of the two magneto circuits relative to each other is fixed, but an advance and retard range is obtained by differential action in the inclined drive-shaft to the magneto. This differential action is controlled by an automatic servo mechanism coupled to the throttle lever by suitable linkage. Four metal conduits coupled with metal braiding to the magneto housing carry the ignition leads to the spark plugs via short metal braid connections, this making the system fully screened.

Lubrication: Dry-sump system. One pressure and two scavenge pumps of the gear-type driven from the wheelcase. The pressure pump delivers oil from the aircraft tank to two relief valves in one unit which controls oil pressure to a high and low pressure system. Any excess oil is spilled back directly into the crankcase. The high pressure system feeds the crankshaft journal bearings, connecting-rod bearings and constant-speed unit. The oil to the constant-speed unit is further increased in pressure by the unit for operation of the variable pitch propeller. High pressure oil is also from the delivery side of the main pressure pump through a precision gear-type pump of low capacity, where its pressure is further increased for the purpose of operating the change speed mechanism of the two-speed supercharger drive. The low pressure system is used for feeding oil to the camshaft and rocker mechanism, oil jets feeding the propeller reduction gears, supercharger drive gears, and various other bearings throughout the engine. Used oil drains back to the lower half crankcase, where it passes through filters to two scavenge pumps, one servicing each end of the lower half, and thence back to the aircraft tank via the oil radiator. Automatic oil coolers (air-to-oil with temperature controlled air shutters) are typically employed.

Coolant: The coolant employed is a mixture of 70% water and 30% ethylene glycol. The coolant in the closed loop pressurized system is circulated by a centrifugal-type pump to the cylinder blocks and from the cylinder blocks to a small-capacity header tank and from the header tank via a radiator to the coolant-pump inlet. The flow of coolant air through the radiator is controlled, whether manually or automatically, through the medium of a temperature-sensitive device. The header tank, which incorporates features to ensure the efficient separation of steam and coolant, is provided with a loaded relief valve which seals the whole coolant system up to a predetermined pressure. This pressurizing of the system raises the boiling point of the coolant and permits the use of smaller radiators. The header tank relief valve maintains the pressure in the system and also incorporates a suction-operated valve which admits air, if for any reason the pressure falls below atmospheric.

Starting: The early starting system was of the combustion type (Coffman starter) using a multiple breech containing five cartridges (resembling big shot-gun shells and loaded with solid-fuel grains) which were indexed mechanically from the cockpit and fired electrically. The combustion gasses at pressures over 2,000 psi were piped to a starter-unit bolted to the rear face of the propeller reduction-gear housing, which drove the engine through a dog geared to the propeller. The starter unit used a piston driving a ball-jack to convert the piston's linear motion into rotary motion to drive the engine. The resulting power pulse was brutal -- while it powered the engine for less than a full revolution, the speed and energy were high enough that engine inertial would continue to turn the engine for several revolutions. If an engine failed to start after all five cartridges were fired, a detailed inspection was require to check for damage, so a specific starting regiment was followed which addressed priming, clearing and other starting procedures. The cartridge system suited military applications since it was reliable (aside from damage it caused) and could be used in remote locations where support was unavailable, thus eliminating the problem of dead batteries with no means to recharge them. Later models replaced the combustion starter with a heavier but much smoother direct cranking electric starter featuring a high torque series wound starter motor (like an automobile) with an integral multi-stage planetary reduction gear of over 100:1 driving a dog geared to the propeller reduction gear.

Auxiliaries: All aircraft auxiliaries, such as generators, vacuum pumps, auxiliary oil pump, and cabin superchargers are located on a firewall mounted remote gearbox driven a drive-shaft connected to a power takeoff from the wheelcase. This facilitates engine removal eliminating the need to disconnect auxiliary plumbing from the engine -- only the driveshaft needs to be removed. The remote gearbox has its own independent lubrication system and supply.

Propeller Drive: The Griffon 74 uses a left hand tractor propeller driven through a single spur reduction gear housed partly in a casing formed integrally with the crankcase and for the remainder, in a casing bolted to the to the front end of the crankcase. The hollow driving pinion mounted in two roller bearings concentric with, and driven by a hollow coupling shaft serrated at both ends. One end engages with a serrated driving ring on the crankshaft and the forward end with the integral serrations on the driving pinion. The coupling insulates the reduction gear unit from crankshaft torsional vibrations. The hollow propeller shaft has an integral flange which is bolted to the ring gear driven by the pinion, and is mounted in roller bearings, axial thrust being taken in both directions by a ball-thrust bearing. A hydraulically-operated variable-pitch propeller is centered upon cones at each end when fitted to the propeller shaft. High pressure oil from the constant-speed (mounted on the wheelcase) unit is supplied to the rear half of the reduction-gear housing from where it is transferred to two concentric oil tubes secured within, and rotating with the propeller shaft and so to the pitch-operating double-acting pistons of the propeller. For the purpose of valve and ignition timing the pinion has timing marks incorporated on the beveled face at the front end and a pointer is fitted on the pinion cover viewed by removal of an inspection cover.

The Griffon 57 used a contra-rotating tractor drive turning two three-blade variable pitch propellers of 13' diameter on the Shackleton. The drive was similar to the single propeller, an idler gear reversing the rotation of the crankshaft to drive a second ring gear placed forward of the left hand drive, the resulting right hand rotation being transferred by a concentric propeller shaft surrounding the first. The size of the shafts and gears are truly impressive! Similar constant-speed control and actuators as those described above are used with the contraprops.

Turbocharging the Griffon: Rolls-Royce carried our research into a turbocharged Griffon under a contract from the British Air-ministry in 1948. The RGT.30.SM Griffon used a GEC turbo-supercharger, and was intended for a projected Supermarine long-range flying boat and as an upgrade for the Avro Shackleton. At sea-level the engine had similar performance to the Griffon 57/58, since a wastegate was partially opened, limiting SL boost to 25 psig when used with ADI and preventing detonation. Altitude the wastegate closed however, routing all exhaust through the turbine. The interesting thing about this engine was the turbo was not staged with the supercharger, but instead the air pumped was used to cool the exhaust pipes and turbine, with the turbine also driving the supercharger through a free-wheeling (one-way) reduction drive. This engine configuration is really a compound engine -- the turbine is used to recover exhaust heat energy and deliver that power to the crankshaft. It was not used to increase the pressure ratio of the induction system.

After the Griffon Budweiser boat became the Frank Kenney Toyota-Volvo in 1986, turbochargers were added to the induction system. These were first located at the front of the engine (the accessory end immediately behind the cockpit), but were later located behind the gearbox at the rear of the engine in order to provide enough room for the turob cooling and lubrication systems and plenums. This also made it easier to use the huge PR100 carburetor. 

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