What are those big humps on the cowling above the exhaust stacks of the MK 14 (and as I've seen all Griffon spits)?

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alex33

Airman
20
4
Jul 30, 2016
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mk-xiv-supermarine-spitfire-ist-einen-motor-laufen-im-stead-field-in-nevada-dgw7x4.jpg

Hello!

I just noticed that all the Griffon engined Spitfires have those bumps on the engine cowling. Can someone tell me what they are and what use they serve? Since I've seen them exclusively on Griffon engined Spitfires and not a single Merlin engined one, I guessed it has something to do with the Engine itself but I could not find anything about it.

Any help in that regard would be great!

Cheers!
 
It's the rocker arm assembly, the rocker arm assembly, the rocker arm assembly....
We don't like dirt!

To quote the Irish Rovers, in the famous Chevon commercial.

Note that the Spanish Messerschmitt has the same covers, even though it uses a Merlin, because the engine ain't upside down like the DB.

DB-601Detail-1943-1.jpg
 

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They are the covers over the valve rocker arms, just like the cover on a car engine, and were much larger on the Griffon compared to the Merlin.
In order to keep the overall upper cowling within reasonable size, the area where the covers are was bulged, rather than enlarging the compete upper cowling.
Look at the upper cowling on, for example, the Hispano 'Buchon', and you'll see a similar design.
 
alex33 said:
thx but what do those parts of the engine do? Is there a special reason why they stick out so much?
Click to expand...

They don't protrude so much but the bulging of the covers was needed because under them there are working ( moving up and down ) the mentioned above valve rocker arms,. The valve rocker arms open and close each of engine cylinder while the pistons moving during the intake stroke , compression stroke, work stroke and exhaust stroke. Here two shots of the Griffon engine cutaway that present the valve rocker arms without the covers. Also you may notice the valves and pistons in the cylinders.

Griffon_engine_(cutaway).jpg


Griffon_engine_(cutaway)_a.jpg
 
Given a free hand in the design the Griffon may have been put lower in the frame but the Spitfire was already as it was so the prop was on the same axis. Any increase in size above the prop axis is added to the height of the plane so the cowlings were moulded to the cam covers o minimise the effect.
 
The design of the later Spitfire was changed giving a wider track and longer legged undercarriage to allow a larger prop from Wiki re Mk 21 The Griffon engine drove an 11 ft (3.4 m)-diameter five-bladed propeller, some 7 in (18 cm) larger than that fitted to the Mk XIV. To ensure sufficient ground clearance for the new propeller, the undercarriage legs were lengthened by 4.5 in (11 cm). The undercarriage legs also had a 7.75 in (19.7 cm) wider track to help improve ground handling. The designers used a system of levers to shorten the undercarriage legs by about 8 in (20 cm) as they retracted, because the longer legs did not have enough space in which to retract; the levers extended the legs as they came down. The larger diameter four-spoke main wheels were strengthened to cope with the greater weights; post-war these were replaced by wider, reinforced three spoke wheels to allow Spitfires to operate from hard concrete or asphalt runways. When retracted the wheels were fully enclosed by triangular doors which were hinged to the outer edge of the wheel wells.[42]
 
The Griffon II is 2240 cu in and weighs 2090 lb. This makes it the equivalent of the DB605. At the time everyone "knew" that the Merlin was too small an engine, and in fact it was the smallest displacement front line fighter engine of the war. For example, the A6M3's engine was 1700 cu in.

Then Sir Hooker worked his magic and suddenly the Merlin was big enough, while being much lighter and slimmer than the bigger iron.
 
pbehn said:
Given a free hand in the design the Griffon may have been put lower in the frame but the Spitfire was already as it was so the prop was on the same axis. Any increase in size above the prop axis is added to the height of the plane so the cowlings were moulded to the cam covers o minimise the effect.
Click to expand...

The Griffon WAS mounted lower than the Merlin.
Supermarine Spitfire side views
Look at the level of the exhausts compared to the cockpit
This is why the oil tank could no longer be located under the nose.
The Griffon Spifires (pre Mk 21) used 10'5" propellers vs 10'9" for the Merlin Spitfires
 
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As Terry said, the bulges in the cowl are to make room for the Griffon valve covers. They are not the valve covers themselves. Like the 109, the original fuselage design of the Spitfire fit nicely around the early Merlins. As engines got bigger and bigger, these early designs faced challenges to accommodate the increased size and weight. Bulged fairings were the answer to the increased space needed.
 
Griffon development was altered when it was suggested/decided that it should be made to fit the Spitfire.

That required repackaging the accessories on the Griffon to present the smallest frontal area.

Even with that, the 2 stage Griffon was 6in (152.4mm) taller than the 2 stage Merlin, though about 0.5in (12.7mm) narrower.

Rolls-Royce Griffon - Wikipedia
Rolls-Royce Merlin - Wikipedia

For the XIV the Griffon was mounted with the nose tilted down to give extra visibility. This reduced the size of the propeller that could be used, and thus why it was a 5 blade unit.

The Spitfire XII had the single stage Griffon (II or VI) mounted at the same angle as the Merlin versions, and used a 4 blade prop of 10ft 5in.. The Spitfire XII also had an addition bump on the top of the cowl, just behind the spinner. Shown in the top picture on the wwiiaircraftperformance.com Spitfire XII page.
Spitfire Mk XII Performance Testing

This was for the magneto, which was relocated on 2 stage engines.
 
stona said:
They cover the assembly at the top of each bank of cylinders that drives the valves.
View attachment 573732
Click to expand...

That photo shows a design feature that added a little height at the forward end of the valve gear covers. According to Graham White ("Allied Aircraft Piston Engines of World War II"), Rolls-Royce moved the camshaft drive to the front in the Griffon. "The Merlin drove the camshafts and magnetos from the rear of the engine; consequently, these critical timing functions were at the mercy of the torsional twist of the crankshaft and spring drive. Cam and magneto drive on the Griffon came from the front of the engine, thus eliminating any torsional windup in the system."
 
MIflyer said:
It's the rocker arm assembly, the rocker arm assembly, the rocker arm assembly....
We don't like dirt!

To quote the Irish Rovers, in the famous Chevon commercial.

Note that the Spanish Messerschmitt has the same covers, even though it uses a Merlin, because the engine ain't upside down like the DB.

View attachment 573729
Click to expand...
I just watched that commercial! It's Mobil.
 
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