Performance modifications done at Squadron level.

[MIflyer]

The Americans, unlike some other air forces, did not use boost limiters in the early part of the war.

Yes, and Winkle Brown liked to mention the need to keep and eye on the manifold pressure.

I recall an article in Flypast I wished I had saved, but I gave the magazine to Kermit Weeks, since it had an article on the Sunderland. They found some still crated Hawk 81A and decided that while they were useless for combat in the ETO, they would make good aircraft for novice pilots to give bomber gunner trainees some practice at dealing with fast moving targets. They sent the first P-40 up, and the pilot just shoved the throttle forward and went roaring off, only to have the Allison blow its top soon after it got off the ground.

And I read of a P-40E pilot, surprised on the ground in the PI, who did a full panic takeoff as bombs fell from overhead, only to discover to his horror tat the manifold pressure gauge was reading only about 10 inches. He figured he was sunk, but he'd keep right on going away from the target area, gaining speed and altitude. Eventually he saw the gauge go DOWN further and finally figured out that the needle had gone right past the upper end of the gauge and started a new trip around the dial.

I believe the P-40M was the first with a manifold pressure regulator as standard equipment.

By the way they even added a manifold pressure regulator to the later model P-38's, even though they had a turbo with its own approach to regulation. For the photo recon birds this was a problem because the two regulating schemes fought each other, leading to a jerky flight path. This may have not been much of a problem for the fighters but for taking pictures it was unacceptable and the manifold regulators were removed at some recon units.

 

[andy1967]

This may sound like nit-picking and to some extent it is.
The Americans, unlike some other air forces, did not use boost limiters in the early part of the war. And many (most) of the planes sold to or given to allied air forces did not have boost limiters. The pilot was expected to keep an eye on the boost gauge and adjust the throttle accordingly.
There was nothing in the throttle linkage or even a screw or knob on the throttle body to keep a ham fisted movement of the throttle lever from going into over-boost.
Boost limits were doctrine and training (follow the manual).
Now if a pilot in a P-40 is using 44in of boost at around 13,000ft (altitude and ram dependent) and he dives without pulling back on the throttle he is going to be getting 50in of more pretty quickly. If he doesn't pull back on the throttle he could be hitting over 55in by the time he gets to around 7000ft and around 66in at 2000ft and this does not include RAM.
It was one more thing for the American (and allied pilots of American aircraft) to keep track of in combat and we can all guess that more than a few engines were over-boosted by accident.
At some points in 1942-43 American aircraft did get boost limiters although not all planes got them at the same time. American planes often just got a thin wire across the throttle to limit the movement to a 'safe' area. Push the throttle lever hard enough and wire broke and then pilot was then responsible for watching the boost gauge. By the end of the war a few American planes might have gotten a bit more sophisticated.
1940 Hurricanes and Spitfires had a knob or button that moved a spring to keep the boost within limits. 6lbs of boost 'normal' and 12lbs when the control was activated. Pilot did not have to watch the boost gauge at low altitudes to keep from going over the 12lb limit. The control device would take care of it.
When later Merlins were allowed to use more boost the mechanics could adjust the control to allow for more boost. The Americans had nothing to adjust.
Americans may have trashed more engines by accident
Fortunately the Allison was pretty rugged and would tolerate a fair amount of abuse.

What really scared the crap out of the Allison company was the USAAC deciding to OK the use of higher boost as an official policy in late fall of 1942 at about the same time that they were introducing the engines with the 9.60 supercharger gears instead of the 8.80 gears. To hit 68-72in of boost with the 8.80 gears you needed a cool day, to be flying at around 2000ft or lower and be flying pretty much straight and level at high speed to maximize RAM. With the 9.60 gears they not only could hit closer to 80in they were also operating closer to the detonation limits at all times. At 44in the engine with the higher gears had a higher manifold temperature than the engine with the 8.80 gears and when over boosted it would hit the detonation limit sooner while the higher gear made it easier to hit higher boost in many flight conditions.
Official limits for the P-40s were supposed to be
P-40D/E was 56in.
P-40K was 60in
P-40M/N was 57in
but K/M/N had different crankshafts and blocks than the early D/E. At some point in E production they started to switch over but not at the same point in time so identifying which engine in which planes had both new components was tough.
Complicating this was the fact that the engines didn't quite make the same power at the same boost pressures.

I have been ready a British report from August 1943 that confirms what you said about the ruggedness of the Allison 1710-39. Instead of using 56hg the British were using 72hg for up to 20 minutes at a time without any damage to the engine. The Bearings were failing at 1500hrs on average compared to 500-600 hrs for the Merlin.

Not all Allisons used the same supercharger gear ratio. They didn't vary a lot but there are some differences.
A higher ratio means it take more power to drive and it heats the air more for a given manifold pressure. Lower ratio means less power to drive (more power to the prop) the air is cooler. This is relative as even the air from the 7.48 gear set is going to be hundreds of degrees F.

I have no idea what anybody building a race Allison was using in the last 20 years (or earlier) for supercharger gears or exactly which engines they were using or what parts they were using.
From 1940 through 1945 Allison made 4 different crankshafts and even the last ones built would fit in the early engines. The last version had the counterweights and had a huge margin of fatigue life over the early versions. These counter weighted cranks went into the the last P-38s built, perhaps some P-63s and in the Allison powered F-82s. These late model engines were allowed to use much high boost pressure than the earlier Allisons. Most had 8.10 gears and in some cases used water injection and/or 100/150 fuel.

7.48 gears were used in the A-36 and in P-38F & G.
8.10 gears were used in the P-38H, J and L and the P-63s. These are all two stage supercharged and this ratio is only on the main supercharger.
8.80 (or 8.77) was standard supercharger gear for most P-40s and most P-39s.
9.60 was the gear set for the P-40M and N and the P-39M, N and Q.

 

[ColFord]

Mustang IA and II were being used super low, often flying right over the sea. A-36 had to fly over mountains sometimes. So i can see a difference. Generally the British cropped impellers seemed to go a little too far, they'd end up with a lot of power down low but it tapered off very quickly and they were anemic at even marginally medium altitude. This was a big problem for the Barracuda for example when they needed to fly over mountains in the Pacific.

In actual fact, the -87 fitted to the A-36A gave them their peak performance up to around 5-10,000ft then quickly tapered off after that. In part the power curve in the -87 was designed for load carrying at low altitude, and the flight envelope of the usual dive bombing attack profile flown by the A-36A.

The modified Allisons in the RAF Mustang I, IA and II gave them their best performance in a band from sea level up to around 8-10,000ft but it then tapered off to around 14,000ft. For the mission profile they were flying, it worked well as their usual altitudes for the Tac/R photography was 0-1,000ft for very low oblique photography, 1,000-2,000 low oblique, and between 4,000ft to 8,000ft for vertical photography depending on target and camera lens fitted. The unmodified -39 supercharger as originally provided produced power up to around 19,000ft, tapering off rapidly up to around 24,000ft. Basically the RAF's Allison engine modifications in their Mustangs pushed the power band down by about 5,000ft and maximised it at the the lower altitudes,where for the role it was most required and in a way it could be sustained for longer periods 'at full throttle and full boost'.

When in 1944, with the lack of new Allison engine Mustangs coming from NAA and the depletion of remaining aircraft due to losses from all causes, the RAF considered re-equipping the RAF's Tac/R Squadrons with Mustang IIIs. They looked at options to re-engine these with a version of the R-R Merlin retuned, including supercharger modifications, for better low altitude performance. That didn't progress given the high priority demand for the Merlin engined Mustang IIIs for other duties, primarily long range fighter work.

The Allisons fitted to the USAAF P-51As in the ETO that were used in the Tac/R role, from their performance and mission profiles, may have had some engine and supercharger modifications made similar to those done for the RAF Mustangs. The USAAF P-51As in the CBI, they likely retained the standard V-1710-81 supercharger arrangement given their use and mission profiles in that theatre which included medium altitude fighter duties including escort to USAAF B-25s up to around 20-25,000ft - the -81 was set up for an increase in altitude power up to around 25,000ft stretching to around 28,000ft.

 

[bada]

I think clipped wings were very common for all marks of Spitfires (except the HF variants) in North Africa and Italy by late 1942. It was generally an improvement across the board except for highest altitude bands.

Well, the A.A.E.E disagree with you

 

[ColFord]

I have been ready a British report from August 1943 that confirms what you said about the ruggedness of the Allison 1710-39. Instead of using 56hg the British were using 72hg for up to 20 minutes at a time without any damage to the engine. The Bearings were failing at 1500hrs on average compared to 500-600 hrs for the Merlin.

That would actually be a USAAF report of 26 August 1943 written by Col. C W Bunch titled "British Army Cooperation Tactical Employment of the Mustang I (P-51)" submitted to the Tactics Officer at the Northwest African Air Forces. The report summarises discussion held with RAF officers of Army Co-operation Command in the UK that took place in May 1943, including with Wing Commander P W Dudgeon DFC RAF (his name is miss-spelt in the USAAF Report) who was a Staff Officer at ACC HQ, recently completing a tour of command of an operational RAF Mustang I Squadron. Discussions also included RAF engineering and other specialist staff at HQ ACC. From that report, the USAAF made a number of informed decisions about their own employment of Allison engine Mustangs in the Tac/R role both in the MTO and in the ETO - they also launched further visits to the UK to discuss Tac/R operations as their focus shifted to the requirements for the planned invasion in France in 1944.

 

[andy1967]

That would actually be a USAAF report of 26 August 1943 written by Col. C W Bunch titled "British Army Cooperation Tactical Employment of the Mustang I (P-51)" submitted to the Tactics Officer at the Northwest African Air Forces. The report summarises discussion held with RAF officers of Army Co-operation Command in the UK that took place in May 1943, including with Wing Commander P W Dudgeon DFC RAF (his name is miss-spelt in the USAAF Report) who was a Staff Officer at ACC HQ, recently completing a tour of command of an operational RAF Mustang I Squadron. Discussions also included RAF engineering and other specialist staff at HQ ACC. From that report, the USAAF made a number of informed decisions about their own employment of Allison engine Mustangs in the Tac/R role both in the MTO and in the ETO - they also launched further visits to the UK to discuss Tac/R operations as their focus shifted to the requirements for the planned invasion in France in 1944.

That's the one I was referring to i made a mistake thanks for the correction.

 

[fannum]

In 'Nam, we found the official equipment didn't always pick up some variants of the SA-6 SAM launch radar, but the broad band Radio Shack "Fuzz Busters" police radar detector would. The fuzz busters could detect X Band CW at least enough to tell the crews an SA-5/6 was potentially coming.
We'd jigger the power supply to work off 28VDC, disable extra freqs, put velcro on the bottom to mount in the corner of our F-4 glare panel, and install/remove each hop as they were officially prohibited.

 

[SaparotRob]

Now that's a blast from the past.

 

[MIflyer]

I have been ready a British report from August 1943 that confirms what you said about the ruggedness of the Allison 1710-39. Instead of using 56hg the British were using 72hg for up to 20 minutes at a time without any damage to the engine.

A friend of mine knew a man who worked at Allison and told him that they ran some of the V-1710 on the test stand at very high manifold pressures and the biggest problem they had was finding bolts that were strong enough to keep it from flying off the stand.

 

[Engineman]

Generally the British cropped impellers seemed to go a little too far, they'd end up with a lot of power down low but it tapered off very quickly and they were anemic at even marginally medium altitude. This was a big problem for the Barracuda for example when they needed to fly over mountains in the Pacific.

Very broadly, the British were quite hard-pressed in many theatres of the Air War in 1941 to 1943. The need for better low altitude performance in some specific roles was met with
some of the mentioned modifications to the Merlin engine, specifically where the low altitude performance of the engine could be improved by making the supercharger suit low
altitude performance, to the detriment of the medium and high altitude performance. This was done to suit Royal Navy requirements needing power at low altitude and Royal Air
Force requirements for the same against the new Fw 190 A. Pilots will have noticed the reduced performance as altitude increased, but that was the pay-off for the improved low
altitude performance. I don't think any pilot would wish for reduced performance in part of the flying envelope, but in these cases, it was a matter of below par everywhere or, better in the altitude block they were expected to need the performance most to stay alive!
Being more specific, the Clipped, Cropped and Clapped Spit V versions with the Merlin 45M did have a fine low alt performance and roll-rate from 1942 against the Fw 190 but, were very niche by the time that the fully sorted Mk 9 came along with the 2-stage Merlin, hence they were not liked after 1943. Also, the Barracuda lugging 14,000 odd lb of aircraft around from small carriers on the single speed and stage Merlin 32 with 1600 hp at low level, was a lot better off than the 1300 hp it would have had if they had not optimised the Merlin 32 for the low altitude task.

Eng

 

[MIflyer]

A book I have, "Spitfire Mark V in Action" (which is not a Squadron Signal publication) provides a detailed account of that variant's combat history. I was surprised to find that the Spit LFVb was still in action at the time of D-Day. And it describes how a Spit LFVb was found to be equal in performance to a Griffon powered Spit XII up to 15,000 ft.

 

[Geoffrey Sinclair]

The Merlin 45M is an engine that dare not speak its name as far as the Ministry of Aircraft Production is concerned, none reported built nor other Merlin modified to 45M. Merlin 45 production started in January 1941 and ended in June 1942 apart from a final batch of 74 November 1942 to January 1943, total production put at 3,574 compared with 3,669 Merlin 46. There were 485 Merlin 55 reported built May to November 1943, Merlin 55M production was a batch of 134 June/July 1943, then from September 1943 to November 1944, all up 1,416 in the production reports.

Using the lists at home gives 266 Spitfire/Seafire with Merlin 45M, 1,256 with Merlin 55 and 71 with Merlin 55M. Given the engine numbers it is probably best to assume most Merlin 55 were Merlin 55M. The Spitfire list is reporting what is on the aircraft cards and many of the Merlin 45M are in aircraft built from mid 1941 on. However the evidence is the 45M was not around in 1941 based on when other Spitfires are reported to have had engine changes to the 45M or 55M, in 1941/42 the RAF was wanting performance at height, a situation which changed in 1943. Clipped wings tested in late 1942, cropped impellers and clapped airframes/engines.

Engine testing: W3228 Vb originally with M45, FF 27-5-41 R-RH 27-6-41 Merlin RM3S (MXII) and RM6S (M56) trials and gen dev of M45 M46 M50 and M55 AAEE 16-9-41 initial trials R-RH 20-10-41 AAEE 17-11-41 aero hand trials with R-R diaphragm type carburettor. R-RH 15-12-41 RM5S (M50) install AAEE 10-1-42 trials with mods to diaphragm carburettor M45M (M50 Special) with cropped supercharger impeller (9.5in) trials with same. Engine then transfer to AB167 (Vc) further similar trials. R-RH for new radiator AST 21-7-43 130Sq 6-7-44 53OTU 10-8-44 VA 4-9-44 ? 527Sq 17-5-45 to 5939M 5SoTT 18-4-46 SOC 10-2-47.

On D-Day Fighter Command had 1 fighter reconnaissance squadron with Hurricane in Scotland, plus more in a fighter flight in the Scilly Isles, Spitfires were 11 mark V, 3 mark VII, 10 mark IX, 1 mark XII, 3 mark XIV fighter squadrons plus 2 mark V fighter reconnaissance (air spotter) squadrons. All the Spitfires in 27 squadrons of the 2nd Tactical Air Force were mark IX. The 4 RN air spotter squadrons were flying Seafires and Spitfire V.

I think clipped wings were very common for all marks of Spitfires (except the HF variants) in North Africa and Italy by late 1942. It was generally an improvement across the board except for highest altitude bands.

First clipped wing Spitfire under test at AFDU on 2 October 1942, report to Fighter Command 14 October, ordered grounded except for the flight to A&AEE Boscombe Down for testing, they liked it, early November request to Supermarine to make 20 sets of parts for clipped wings to go to 91 squadron Spitfire V, 17 November Fighter Command reported a clipped wing mark IX, resulting in 19 more sets of parts suitable for V, IX and XII. How many clipped wing Spitfires in the Middle East on 31 December 1942?

The VI and VII were the high altitude pressure cabin versions but did not use the HF designation, first HF.VIII in May 1944, first HF.IX in March 1944 but early VIII were delivered with the extended wing.

Generally the British cropped impellers seemed to go a little too far, they'd end up with a lot of power down low but it tapered off very quickly and they were anemic at even marginally medium altitude. This was a big problem for the Barracuda for example when they needed to fly over mountains in the Pacific.

So when were these operations? Merlin 32 production began in June 1942, ended in April 1945, with 3,500 built, used in Barracuda. The British did not operate the Barracuda in the Pacific in WWII, there were some operations in the Indian Ocean.

Operation Cockpit 19 April 1944, HMS Illustrious and USS Saratoga, Illustrious using Barracuda, over water approach to Sabang. Operation Transom 17 May 1944 HMS Illustrious used Avengers, which became standard for the Eastern Fleet as it was a training area for the British Pacific Fleet. There are reports of RN Avengers having trouble flying over Indonesian mountains during some of the operations, the TBF/M-1 critical altitude was around 12,000 feet, the TBM-3 raised that to around 17,000 feet. The core of what became the British Pacific Fleet left end January 1945, arrived Manus island via Australia in early March 1945.

That left the Eastern Fleet in 1945 which by VJ day had 16 escort carriers.

 

[Engineman]

However the evidence is the 45M was not around in 1941 based on when other Spitfires are reported to have had engine changes to the 45M or 55M, in 1941/42 the RAF was wanting performance at height, a situation which changed in 1943. Clipped wings tested in late 1942, cropped impellers and clapped airframes/engines.

Engine testing: W3228 Vb originally with M45, FF 27-5-41 R-RH 27-6-41 Merlin RM3S (MXII) and RM6S (M56) trials and gen dev of M45 M46 M50 and M55 AAEE 16-9-41 initial trials R-RH 20-10-41 AAEE 17-11-41 aero hand trials with R-R diaphragm type carburettor. R-RH 15-12-41 RM5S (M50) install AAEE 10-1-42 trials with mods to diaphragm carburettor M45M (M50 Special) with cropped supercharger impeller (9.5in) trials with same. Engine then transfer to AB167 (Vc) further similar trials. R-RH for new radiator AST 21-7-43 130Sq 6-7-44 53OTU 10-8-44 VA 4-9-44 ? 527Sq 17-5-45 to 5939M 5SoTT 18-4-46 SOC 10-2-47.

The need for "performance at height" was always a factor, but needs more context. Early in the war, the Germans had very-high altitude developments such as the Ju 86P and there was a flurry of British developments to be able to engage these aircraft. The technology was applied to the Spitfire airframe and some Merlin engines were specially modified.
On the German side, from early in the war some fighters were modified with Nitrous Oxide injection for very-high altitude operation. These very-high altitude developments continued throughout the war, with both sides having capability, although the British were slow to understand the technology of Nitrous-Oxide and only later using it for some Mosquito night-fighters. More generally, the 2-Stage Merlins and Griffons led for the British and some versions of DB 603, DB 605 and Jumo 213 were always capable of very-high operation. But the bulk of WW2 Fortress Europe air fighting remained in the medium levels, and so the medium levels and performance at those levels remained the major factor. This was not really a change, it was more that the European air war did not transform into a very-high altitude conflict.
Comment on the "R-R diaphragm type carburettor" trials is noteworthy. This carburettor was Rolls-Royce's late attempt with SU to devise a negative-G capable carburettor, the SU AVT 40/213. It failed.
The problem for R-R was solved partly by the RAE Restrictor from 1941 onwards, by the RAE anti-G carburettor version of the RR/ SU the AVT 40/216 starting by mid 1942,
by the adoption of the Bendix Stromberg type Pressure carburettors (a version of which was used from the very start of Packard Merlin production) and eventually, by the development
of single-point fuel injection.

Cheers

Eng

 

[GregP]

Hi Colin. Joe Yancey's race engine was a G-series power section. He made adapters on a mill and mounted an F-series nose case and accessory case.

 

[ColFord]

Hi Colin. Joe Yancey's race engine was a G-series power section. He made adapters on a mill and mounted an F-series nose case and accessory case.

Hi Greg, latest engine fitted is a V-1710-111, with a few mods to suit the aircraft. New propellor as well - also a new spinner and they added representative gun bulges and troughs to the new engine upper cowlings. By all accounts, this one after resolving the cooling issues with the new radiator core, is producing more HP without trying. Certainly creating more "Full Noise". They were still chasing the cooling gremlins when I was over there at Easter this year, but its been running well since then.

 

[Geoffrey Sinclair]

A problem with altitude descriptions is there is no agreed standard. I use 0 to 10,000 feet low, 10 to 20,000 feet medium, 20 to 30,000 feet high and 30,000+ feet very high for WWII.

The British had watched as average altitudes went up during the Battle of Britain including the around 30,000 feet Bf109 fighter/fighter bomber operations in October and decided it was important to have performance above 30,000 feet in 1941, things like Spitfires able to reach over 40,000 feet but noted this needed pressure cabins. In fighter terms this lead to the first Spitfire VI appearing in January 1941 but no more until the December 1941 to November 1942 production run, the Spitfire VII September 1942 to May 1944. While there were 4 Mosquito XV December 1942 to February 1943 plus the prototype officially delivered in August 1943. The Mosquito reported a reaction to some Ju86 high altitude bombing sorties in 1942 by 14./KG6, which had 4 Ju86R on strength in August, all needing repair, then none thereafter. A fundamental problem with tracking the Ju86P and R is most to all were rebuilds, some around in mid 1940 for operations and only small numbers ever in service.

The RAF went with liquid oxygen systems, experiments started in April 1940 and effectively stayed as experiments, the Spitfire VII installation noted it gave 300 hp/27 mph more at 40,000 feet for 7 minutes at a cost of 130 pounds for the filled system. The improvements in fuels and supercharging along with what the axis air forces were actually doing meant there was less need for the allies to feed supplemental oxygen to the engines.

Without a usual need to escort B-17 and B-24 the RAF day war in Europe was generally fought at a lower level than the 8th and 15th Air Forces, though not as low as the 9th and 12th air forces. A check of reported bombing altitudes for the 15th Air Force has 14 attacks under 10,000 feet, 885 from 10 to under 20,000 feet, 3,609 from 20 to under 30,000 feet, average just under 22,000 feet.

 

[MIflyer]

The technology was applied to the Spitfire airframe and some Merlin engines were specially modified.

It was the basis for the two stage supercharging used in the Merlin 61 and related series. It was used in the high altitude Wellington, designed to emulate the Ju86P, the Spitfire Mk VII designed to intercept the Ju86P, and the twin-engined Welkin interceptor. I find it ironic that the German attempts in very high altitude bombing and recon was essentially a failure. But it inspired Stanley Hooker to develop the engine that sealed the Luftwaffe's fate with the Spitfire IX and Merlin Mustang.

 

[NZTyphoon]

The VI and VII were the high altitude pressure cabin versions but did not use the HF designation, first HF.VIII in May 1944, first HF.IX in March 1944 but early VIII were delivered with the extended wing.

In fact, the Mk VIIs were given the prefix F. or HF., depending on the model of Merlin engine that was fitted. Similarly, the Mk. VIIIs could be F., HF. or LF.
The LF. Mk. VIII was the first Spitfire to use the Merlin 66, starting with JF462 in May 1943; from JF744 on (built in late June '43), the Merlin 66 was the predominant engine.
By comparison, the first Merlin 66 LF. MK IX, MH350, was built in August 1943, with production hitting full stride, starting with MH384 on August 8 '43.

From the Pilot's Notes for Spitfire VII & VIII, issued December 1943 (F/Lt Haywood was a New Zealand pilot, who flew Spitfire VIIs on 124 Sqn, so these P/Ns are not post-war reproductions).

 

[NZTyphoon]

The development of the Two Speed, Two Stage 60 & 70 series Merlin engines: the Merlin 60 & 62, designed for the high altitude, pressurised cabin Wellington VIs, had the highest altitude ratings at the lowest boost. The Merlin 63 (that began to replace the Merlin 61 on Spitfire production lines in early 1943) through 70 series were revised and strengthened to accept higher boost ratings, with the Merlin 66 later being able to use 100/150 grade fuel @ 25 lbs boost.
It's also worth noting that the altitude ratings of the Merlin 70s were slightly lower than those of the Merlin 61, but with improved power ratings, reflecting the higher available boost pressure.

 

[Engineman]

The development of the Two Speed, Two Stage 60 & 70 series Merlin engines: the Merlin 60 & 62, designed for the high altitude, pressurised cabin Wellington VIs, had the highest altitude ratings at the lowest boost. The Merlin 63 (that began to replace the Merlin 61 on Spitfire production lines in early 1943) through 70 series were revised and strengthened to accept higher boost ratings, with the Merlin 66 later being able to use 100/150 grade fuel @ 25 lbs boost.
It's also worth noting that the altitude ratings of the Merlin 70s were slightly lower than those of the Merlin 61, but with improved power ratings, reflecting the higher available boost pressure.

View attachment 857427View attachment 857428View attachment 857429View attachment 857430
View attachment 857431

These are good tables of these Merlins, with some good detail and very useful notes.
You can see that the "musical-chairs" of engine build standard and Supercharger details is varied to achieve the power required at different altitudes and at different boost pressures. The performance details are in some ways difficult to compare because there is a huge mechanical difference from the M.60 with its limiting one-piece blocks etc to the later engines here with two-piece blocks and redesigned supercharger, different rotors, different gearing, different carburation and many detail modifications. The engine ratings reflect all this. It is a bit simplistic to say "the Merlin 60 & 62, designed for the high altitude, pressurised cabin Wellington VIs, had the highest altitude ratings at the lowest boost", the situation was that in 1941 the desired rating was a FTH of 30,000' and the two-stage supercharger was designed to achieve that with a boost limited at that time to 9lb, which it did at 3000rpm. However, the rating listed is for climb at 2850rpm. The later high altitude engines with higher limiting boost reach their rated FTH at lower altitudes (and at 3000rpm) due to later build standard, although it is interesting to wonder what their achieved power and boost at 29,000' is?

Cheers

Eng