An Allison V-1710 with a two-speed supercharger in 1942 (4 Viewers)

[Howard Gibson]

re


The Merlin XX / V-1650-1 was a two-speed single-stage engine - with a higher altitude rating than the single-speed single-stage Merlin 45 used in the majority of the Spitfire Mk V. The better aerodynamics of the Spitfire and consequent higher speed and greater RAM effect is what allowed the Spitfire to operate at approximately the same altitudes as the Hurricane - even though the Merlin 45 was slightly inferior to the Merlin XX / V-1650-1.

Reference Spitfire Mk III to see the significant improvement in performance (predicted and found on testing) when fitted with the Merlin XX vs Merlin 45 & 46. Basically an altitude increase of about 4,000 ft over the Merlin 45, and a Vmax increase of 30 mph - with everything else being equal (ie same elliptical 'B' wing as the Spitfire Mk V, same fit/finish, etc).

Spitfire Mk VB w/Merlin 45______370 mph at 20,000 ft (RM5SM, float carb, standard 242 ft2 elliptical 'B' wing)
Spitfire Mk VB w/Merlin 46______370 mph at 21,000 ft (RM6S, float carb, standard 242 ft2 elliptical 'B' wing)
Spitfire Mk VB w/Merlin 46______370 mph at 22,500 ft (RM6S, RR SU pump carb, standard 242 ft2 elliptical 'B' wing)
Spitfire Mk III w/RM2SM________385 mph at 20,000 ft (RM2SM, float carb, new smaller span 220 ft2 'B' wing)
Spitfire Mk III w/Merlin XX_______398 mph at 22,500 ft (RM3SM, float carb, new smaller span 220 ft2 'B' wing)
Spitfire Mk III w/Merlin XX_______400 mph at 24,000 ft (RM3SM, float carb, standard 242 ft2 elliptical 'B' wing)

NOTE
1. All of the above speed and altitude data is from actual flight tests. Flying weights during testing of the Spitfrire Mk III were usually about 75 lbs more than the standard Mk V. Fitting the Mk III with a 4-blade propeller was under consideration but AFIK testing was discontinued before this was accomplished.

2. The Merlin 46 - similar to the Merlin 45, but with a modified supercharger with a larger Ø10.85" diameter impeller - theoretically offered about the same altitude rating as the Merlin XX, but it was found that the Merlin 46 suffered from rough running, surging, and detonation above about 19,000 ft, whereas the Merlin XX (which used the standard Ø10.25" impeller) did not. Fitting a Tropical radiator solved some of the Merlin 46 problems, but the Australians were still experiencing the problems when they did their tests in late-43(?). The original design that became the Merlin 46 was intended to have an after-cooler, but this was not carried through to the final design for some reason. Too bad, it probably would have made the Merlin 46 an effective improvement for the Spitfire.

3. For some development information on the Merlin RM2SM see "Mustang Origins"

4. Merlin 45, 46, RM2SM, and XX rated altitudes in S gear - at +9 lbs boost, except for RM2SM which is at +9.5 lbs boost.

Merlin XX______1145 BHP at +9 lbs boost at 18,250 ft
Merlin 46______1125 BHP at +9 lbs boost at 18,250 ft
RM2SM_______1140 BHP at +9.5 lbs boost at 16,750 ft
Merlin 45______1170 BHP at +9 lbs boost at 16,250 ft

Bleh!

Are you sure about the Merlin 45? My understanding is that Merlin 45 had the same, improved impeller as the Merlin_XX, but just a single gear ratio. This gave it the same performance at high altitude, but less at low altitude.

This diagram is pulled mostly from WWII Aircraft Performance. The Seafire "curves" come from David Brown's book on Seafires. Brown shows tables with only three speeds.

The LF Seafires have Merlins with "cropped" superchargers, for low altitude performance.

 

[Steamed_Banana]

I'm not an expert on engines but I also believe that the Merlin XX, due to having two speeds and two critical altitudes (~10,000 and 20,000 ft) had more power in the lower altitude bands, while having the same or slightly better power at high altitude. Of course depending on fuel used and boost (power in both gears increased as fuel quality and boost limits
increased).

Merlin XX series seems to peak higher at lower altitudes

Compared to 45 and 45

... but performance is similar above 10,000 ft

The Merlin 45M with the cropped impeller peaked at a lower altitude of course.

I also remember, though of course correct me if I'm wrong, that the Spit III was discontinued in spite of it's very promising test results and many improvements, mainly because it was deemed slightly more complex to manufacture and that it would require more retooling, so they went to the Spit V instead basically choosing greater quantity over improved quality.

The Merlin XX was put into the Hurricane in the hopes of making it more viable and extending it's usefulness as a fighter, basically Hurricane II needed more help from a bit better engine, and because due to the size of the engine it would require some retooling to fit into a Spitfire per above.

 

[Steamed_Banana]

re


The Merlin XX / V-1650-1 was a two-speed single-stage engine - with a higher altitude rating than the single-speed single-stage Merlin 45 used in the majority of the Spitfire Mk V. The better aerodynamics of the Spitfire and consequent higher speed and greater RAM effect is what allowed the Spitfire to operate at approximately the same altitudes as the Hurricane - even though the Merlin 45 was slightly inferior to the Merlin XX / V-1650-1.

Reference Spitfire Mk III to see the significant improvement in performance (predicted and found on testing) when fitted with the Merlin XX vs Merlin 45 & 46. Basically an altitude increase of about 4,000 ft over the Merlin 45, and a Vmax increase of 30 mph - with everything else being equal (ie same elliptical 'B' wing as the Spitfire Mk V, same fit/finish, etc).

Spitfire Mk VB w/Merlin 45______370 mph at 20,000 ft (RM5SM, float carb, standard 242 ft2 elliptical 'B' wing)
Spitfire Mk VB w/Merlin 46______370 mph at 21,000 ft (RM6S, float carb, standard 242 ft2 elliptical 'B' wing)
Spitfire Mk VB w/Merlin 46______370 mph at 22,500 ft (RM6S, RR SU pump carb, standard 242 ft2 elliptical 'B' wing)
Spitfire Mk III w/RM2SM________385 mph at 20,000 ft (RM2SM, float carb, new smaller span 220 ft2 'B' wing)
Spitfire Mk III w/Merlin XX_______398 mph at 22,500 ft (RM3SM, float carb, new smaller span 220 ft2 'B' wing)
Spitfire Mk III w/Merlin XX_______400 mph at 24,000 ft (RM3SM, float carb, standard 242 ft2 elliptical 'B' wing)

NOTE
1. All of the above speed and altitude data is from actual flight tests. Flying weights during testing of the Spitfrire Mk III were usually about 75 lbs more than the standard Mk V. Fitting the Mk III with a 4-blade propeller was under consideration but AFIK testing was discontinued before this was accomplished.

2. The Merlin 46 - similar to the Merlin 45, but with a modified supercharger with a larger Ø10.85" diameter impeller - theoretically offered about the same altitude rating as the Merlin XX, but it was found that the Merlin 46 suffered from rough running, surging, and detonation above about 19,000 ft, whereas the Merlin XX (which used the standard Ø10.25" impeller) did not. Fitting a Tropical radiator solved some of the Merlin 46 problems, but the Australians were still experiencing the problems when they did their tests in late-43(?). The original design that became the Merlin 46 was intended to have an after-cooler, but this was not carried through to the final design for some reason. Too bad, it probably would have made the Merlin 46 an effective improvement for the Spitfire.

3. For some development information on the Merlin RM2SM see "Mustang Origins"

4. Merlin 45, 46, RM2SM, and XX rated altitudes in S gear - at +9 lbs boost, except for RM2SM which is at +9.5 lbs boost.

Merlin XX______1145 BHP at +9 lbs boost at 18,250 ft
Merlin 46______1125 BHP at +9 lbs boost at 18,250 ft
RM2SM_______1140 BHP at +9.5 lbs boost at 16,750 ft
Merlin 45______1170 BHP at +9 lbs boost at 16,250 ft

Bleh!

is the 220 ft2 wing the clipped wing?

 

[andy1967]

is the 220 ft2 wing the clipped wing?

The Mk 111 wing although clipped had a span of 33ft 9in compared to 32ft 2in of the MkV. Length was also greater on the Mk 111 30ft 4 in vs 29ft 11in.

 

[wuzak]

Are you sure about the Merlin 45? My understanding is that Merlin 45 had the same, improved impeller as the Merlin_XX, but just a single gear ratio. This gave it the same performance at high altitude, but less at low altitude.

The supercharger gear ratio for the Merlin 45 was 9.1:1

The supercharger gear ratios for the Merlin XX were 8.15:1 (MS) and 9:49:1 (FS).

The Merlin XX's supercharger in FS gear was spinning ~4.3% faster than the one in the 45, therefore a higher FTH for a given amount of boost.

 

[tomo pauk]

The Merlin XX / V-1650-1 was a two-speed single-stage engine - with a higher altitude rating than the single-speed single-stage Merlin 45 used in the majority of the Spitfire Mk V. The better aerodynamics of the Spitfire and consequent higher speed and greater RAM effect is what allowed the Spitfire to operate at approximately the same altitudes as the Hurricane - even though the Merlin 45 was slightly inferior to the Merlin XX / V-1650-1.

Difference in the power with both engines in FS (the only setting on the 45) was about 5%, eg. at 20000 ft, the 45 will make 1000 HP, the XX about 1050 HP. End effect on top speed might be 1-2%? The individual historical Spitfires as built will show greater difference between them.
As for the climb, the XX was heavier, that will eat out the effects of the extra power.

Reference Spitfire Mk III to see the significant improvement in performance (predicted and found on testing) when fitted with the Merlin XX vs Merlin 45 & 46. Basically an altitude increase of about 4,000 ft over the Merlin 45, and a Vmax increase of 30 mph - with everything else being equal (ie same elliptical 'B' wing as the Spitfire Mk V, same fit/finish, etc).

Mk.III was outfitted with internal BP glass, the Mk.V with the external one. Cost was 3.75mph per RAE. Retractable tailwheel + main wheel well covers - perhaps another 10 mph (difference on the Bf 109G with these mods was 14-18 km/h, ie. about 10 mph)? No guns on the Mk.III; just the two cannons, bulges and stubs will cost up to 10mph.

It was probably too bad that Mk.III was not mass produced even with the Merlin 45 in the nose.

 

[ThomasP]

re

Difference in the power with both engines in FS (the only setting on the 45) was about 5%, eg. at 20000 ft, the 45 will make 1000 HP, the XX about 1050 HP. End effect on top speed might be 1-2%? The individual historical Spitfires as built will show greater difference between them.
As for the climb, the XX was heavier, that will eat out the effects of the extra power.

??
The power outputs you refer to are the static values.

The respective BHP for the engines at their respective critical altitudes when static are as I listed below:

4. Merlin 45, 46, RM2SM, and XX rated altitudes in S gear - at +9 lbs boost, except for RM2SM which is at +9.5 lbs boost.

Merlin XX______1145 BHP at +9 lbs boost at 18,250 ft
Merlin 46______1125 BHP at +9 lbs boost at 18,250 ft
RM2SM_______1140 BHP at +9.5 lbs boost at 16,750 ft
Merlin 45______1170 BHP at +9 lbs boost at 16,250 ft

The basic Spitfire Mk V with Merlin 45 gains about 3,750 ft for the aircraft critical altitude, due to the RAM effect of the forward facing air intake at about 370 mph TAS.

The Spitfire Mk III (with standard 'B' wing) with Merlin XX gains about 5,750 ft for the aircraft critical altitude, due to the RAM effect of the forward facing air intake at about 400 mph TAS.

Basically, with RAM the Merlin 45 is still putting out around 1170 BHP at 20,000 ft, while the Merlin XX is still putting out 1145 BHP at 24,000 ft. Without RAM effect the Spitfire would reach a Vmax of about 346 mph TAS at about 16,250 ft - the thinner air at 20,000 ft allows it to reach 370 mph TAS. According to the testing the same basic effect happens at 24,000 ft with the Merlin XX, allowing the Spit Mk III to reach 400 mph TAS.*

* By the math I only get about 393 mph TAS at 24,000 ft for the Spit with a Merlin XX - using the standard Spit Mk V with Merlin 45 as a base - but hey . . . maybe the Merlin XX / Rotol prop combination was more efficient than the Merlin 45 / DH prop at that altitude & speed? Or maybe just a difference in measuring errors, or as you mention a difference between one airframe and another.

 

[tomo pauk]

The power outputs you refer to are the static values.

Of course.

The Spitfire Mk III (with standard 'B' wing) with Merlin XX gains about 5,750 ft for the aircraft critical altitude, due to the RAM effect of the forward facing air intake at about 400 mph TAS.

Basically, with RAM the Merlin 45 is still putting out around 1170 BHP at 20,000 ft, while the Merlin XX is still putting out 1145 BHP at 24,000 ft.

I'd highly doubt that the Mk.XX will do 1145 at 24Kft with all the ram effect available on 400 mph and with the best ram air intake (that was not a feature of the Spitfires until 1942-ish).
The Spitfire book by Morgan & Shacklady note that top speed of the Mk.III was 400 mph at 21000 ft.

 

[ThomasP]

re

Are you sure about the Merlin 45? My understanding is that Merlin 45 had the same, improved impeller as the Merlin_XX, but just a single gear ratio. This gave it the same performance at high altitude, but less at low altitude.

See my post above

The chart you reference has the engines at different boost levels (+12 for the Merlin XX in the Hurricane Mk IIC, and +9 lbs in the Spit Mk VA) and graphs the Vmax at the aircraft critical altitudes. For the static critical altitude for the Merlin XX +12 lbs occurs at around 15,000 ft, while the static critical altitude for the Merlin 45 is 16,250 ft - already 1,250 ft above the Merlin XX.

Everything else being equal, the increase in manifold pressure due to RAM effect is proportional to the forward airspeed squared. This is why the Hurricane Mk IIA/B with an engine critical altitude of 18,250 ft has a Vmax of about 338 mph TAS at 21,000 ft, while the Spitfire Mk V with Merlin 45 with an engine critical altitude of 16,250 ft ends up with a Vmax of 370 mph TAS at 20,000 ft. The Hurricane gains 2,750 ft in aircraft critical altitude, while the Spitfire gains 3,750 ft.

By the math, If the Spitfire aircraft critical altitude is correct, then the Hurricane aircraft critical altitude should be 21,250 ft.

Make sense?

 

[ThomasP]

re

I'd highly doubt that the Mk.XX will do 1145 at 24Kft with all the ram effect available on 400 mph and with the best ram air intake (that was not a feature of the Spitfires until 1942-ish).
The Spitfire book by Morgan & Shacklady note that top speed of the Mk.III was 400 mph at 21000 ft.

Per Rolls-Royce, the air intakes on the Hurricane Mk II series and Spitfire Mk II/III/V had an efficiency of 80%. How efficiently the supercharger handled the incoming rammed air is another matter. The test numbers for both the Hurricane Mk IIA/B and the Spitfire II/III/V fit quite well with the math (to within +/-2%) when using 80% efficiency for the intake.

Do Morgen & Shacklady mention where they got the 400 mph at 20,000 ft numbers, and for which variant of the Mk III?

 

[tomo pauk]

Per Rolls-Royce, the air intakes on the Hurricane Mk II series and Spitfire Mk II/III/V had an efficiency of 80%.

The ram air intake on a Spitfire with the stone guard or the ice guard will not have the same efficiency like the one without those appendices. Cost in speed on the Spit V was 8 mph for the ice guard, per RAE Note #1273.

Do Morgen & Shacklady mention where they got the 400 mph at 20,000 ft numbers, and for which variant of the Mk III?

They stipulate that the Merlin X (RM1SM) powered Mk.III was supposed to do 400 mph at 21000 ft, but that aircraft never flew.
As for the sources - no source; that book is awful on sources and footnotes. Not making a distinction between the calculated/advertised figures and the ones really flown really grinds my gears in the Mk.III section of the book, too. No facsimiles of the flight tests is also very bad for a book that is supposed to be a bible on the topic.

 

[wuzak]

The chart you reference has the engines at different boost levels (+12 for the Merlin XX in the Hurricane Mk IIC, and +9 lbs in the Spit Mk VA) and graphs the Vmax at the aircraft critical altitudes. For the static critical altitude for the Merlin XX +12 lbs occurs at around 15,000 ft, while the static critical altitude for the Merlin 45 is 16,250 ft - already 1,250 ft above the Merlin XX.

Just to clarify FTH/critical altitude (without ram):
Merlin XX: +12psi - 15,000ft
Merlin XX: +9psi - 18,250ft (from one of your earlier posts)
Merlin 45: +9psi - 16,250ft

So while the FTH of the Merlin XX at +12psi boost is lower than the Merlin 45 at +9psi, the Merlin XX still has +9psi boost at 18,250ft, 2,000ft higher than the 45 at the same boost.

 

[Steamed_Banana]

So can we say we have a consensus that a Merlin XX powered Spit V would be a better all around performer?

Also, from what I understand the Spit VIII is more or less the realization of what the Spit III was supposed to be, in many if not all respects, but with the exception of having the two stage Merlin 60 series instead of the XX, is that also true?

 

[Geoffrey Sinclair]

Far be it from me to contradict

Good, progress can be made, the simple comparisons between US fighter groups needs to take into account 1 with Spitfires spending time in the Coastal Air Force and 1 with P-40 in the Strategic Air Force, the date of majority P-38 missions out of Africa was later than stated, the location of 15th Air Force fighter combats was different to stated, the number of Luftwaffe losses was much higher than stated as starters.

the very long winded and strongly opinionated,

Effectiveness of the P-38

I really feel like we are talking past each other, but I'm just going to say this. I'm not trying to change the official score of any particular pilot (the case about Boyington is interesting and worth discussing, but not for the reasons you seem to assign to me) or any particular unit, or...

ww2aircraft.net

the promise in message 380

Insults are a form of engagement, I have been advised to treat the smiley as an insult, though of course we know the strongly opinionated are the ones most likely to laugh at data that does not fit.

That leaves can of worms known as the various Merlin altitude performance listings,

Reference Spitfire Mk III to see the significant improvement in performance (predicted and found on testing) when fitted with the Merlin XX vs Merlin 45 & 46.

Interesting, as far as I know the mark III tests did not include fitting a Merlin 45 or 46, apart from the clipped wing the mark III included a number of detail changes, canopy, radiator, wheel fairings, ducting, retractable tail wheel, looks like it had about the same tare weight as the mark V. The changes gave it a better top speed before the engine change. Agreed it seems standard 1940/41 Spitfire propellers were in use.

1945 Janes (second speed for Merlin XX)
Merlin XX 1,280 HP at 3,000 RPM Take off, 1,175 HP at 2,850 RPM at 17,500 feet International, 1,480 HP at 3,000 RPM at 12,250 feet maximum power
Merlin 45 1,185 HP at 3,000 RPM Take off, 1,200 HP at 2,850 RPM at 16,000 feet International, 1,470 HP at 3,000 RPM at 9,250 feet maximum power
Merlin 46 1,100 HP at 3,000 RPM Take off, 1,115 HP at 2,850 RPM at 19,000 feet International, 1,415 HP at 3,000 RPM at 14,000 feet maximum power
Merlin 50 and 55 considered equivalent of Merlin 45.

Various tables seemed to imply the Merlin 45 etc. gained more altitude performance with more boost than the XX did. Which reinforced the idea the Merlin 40 series was a deliberate choice for performance in the 20 to 25,000 foot band, better than the XX and why it helped the Fw190 be so dangerous for the Spitfire V below 20,000 feet.

Merlin 45 production began in January 1941, finished with 4 in June 1942 (plus another 74 end 1942), Merlin 46 production began in October 1941, finished with 8 in March 1943. Merlin 50 series production began in November 1942. In simple terms for new production the Merlin 45 was "1941" the Merlin 46 "1942", the Merlin 50 series "1943".

Ignoring various exotic engines, from the online Spitfire listings, engines fitted to production mark V

Mark

M45

M45M

M45MA

M46

M47

M50

M50A

M55

M55M

M55MA

M56

Total

Va

92​

1​

93​

Vb

2341​

261​

1​

1310​

3​

2​

3918​

Vc

218​

2​

1554​

2​

472​

33​

119​

12​

17​

2429​

Total

2651​

264​

1​

2864​

2​

475​

33​

119​

12​

2​

17​

6440​

RAF Performance figures, showing the propeller can effect things and a direct comparison Hurricane and Spitfire, Merlin III.

Performance	Tomahawk	Hurricane I DH	Hurricane I Rotol	Hurricane IIA	Hurricane IIC	Spitfire I	Spitfire II	Spitfire Vb
Maximum Speed (m.p.h)	345​	320​	316​	335​	332​	355​	363​	375​
Max Speed Height (Feet)	16,000​	17,500​	17,750​	22,000​	22,000​	19,000​	17,000​	20,250​

If I understand things correctly, ignoring the airframes and therefore airspeed effects, the Merlin XX had better above 20,000 feet performance than the Merlin 45 and just better than the Merlin 46, instead my error of less than the 45 and an even larger gap with the 46.

 

[tomo pauk]

So can we say we have a consensus that a Merlin XX powered Spit V would be a better all around performer?

Under 10K ft - probably. Above 10K ft - I'd say no.

Also, from what I understand the Spit VIII is more or less the realization of what the Spit III was supposed to be, in many if not all respects, but with the exception of having the two stage Merlin 60 series instead of the XX, is that also true?

Probably yes.

 

[tomo pauk]

Various tables seemed to imply the Merlin 45 etc. gained more altitude performance with more boost than the XX did. Which reinforced the idea the Merlin 40 series was a deliberate choice for performance in the 20 to 25,000 foot band, better than the XX and why it helped the Fw190 be so dangerous for the Spitfire V below 20,000 feet.

I'm grateful for your posts. Even if I don't like the tone in some of them, but that is probably just me.

Wrt. the M.45 vs. XX - the 'seemed to imply' construction removes credibility of point that is being made.
Merlin 40 series was what it was available for 1941 to mid/late 42, since the Merlin XX went into the Hurricane, Defiant and Beaufighter, and 2-stage Merlins as well as the Griffon were not available yet. The big-impeller Merlin 46 & 47 were an attempt to improve the performance above 20K ft.
Unfortunately, the Fw 190 was dangerous to the Spitfire V at any altitude (includes 20-25K ft band), unless the 45M or 50M engines were installed and the fight was at low level.

 

[Elan Vital]

It is unfortunate that the French Szydlowski-Planiol swirl throttle failed to get advertised in the UK (or proposed by French Air Ministry Services to the UK) in time to integrate it in Merlin supercharger development.
A Merlin 4X with it would have had a lot more leeway to optimize critical altitude without sacrificing too much low-altitude performance, given the greater efficiency of the swirl throttle vs the common butterfly type.

 

[wuzak]

Merlin 45 production began in January 1941, finished with 4 in June 1942 (plus another 74 end 1942), Merlin 46 production began in October 1941, finished with 8 in March 1943. Merlin 50 series production began in November 1942. In simple terms for new production the Merlin 45 was "1941" the Merlin 46 "1942", the Merlin 50 series "1943".

Increased boost tended to lower the FTH.

An example is the numbers given previously.
Merlin XX: +9psi - 18,250ft
Merlin XX: +12psi - 15,000ft

When the XX was cleared for +12psi at 15,000ft, it still made +9psi at 18,250ft.

Between 0ft and 15,000ft the uprated version had 3psi higher boost. From 15,000ft to 18,250ft the uprated version had more boost, but the advantage decreases with altitude. At 17,000ft, for example, it should have around +10.2psi boost.

 

[tomo pauk]

It is unfortunate that the French Szydlowski-Planiol swirl throttle failed to get advertised in the UK (or proposed by French Air Ministry Services to the UK) in time to integrate it in Merlin supercharger development.
A Merlin 4X with it would have had a lot more leeway to optimize critical altitude without sacrificing too much low-altitude performance, given the greater efficiency of the swirl throttle vs the common butterfly type.

That would've probably removed the need to have the separate low-altitude Merlin versions, at least for the Spitfires.
A better throttle would've also been a good addition on the other engines, including the 2-stage Merlins and Griffons, as well as on the Hercules.

 

[Shortround6]

I could be way-off but my own 'theory' is that the Merlin XX solved a problem that somewhat went way when better fuel showed up. So we have to very careful about dates, boost limits and altitudes.
In 1940 with limits of 9lbs (or so) boost the two speed supercharger offered a lot more power at lower altitudes and just enough to say so at around 17,000ft and above.
When they got 12lbs of boost the difference was almost the same but when you get to the 15-18lb area the lines get somewhat blurrier. Do we really want to argue about 1470hp at 9250ft at 16lbs from a Merlin 55 compared to 1510hp at 9250ft at 18lbs from a Merlin 24/25? Yes the 2 speed engine is good for1635hp low down at 2250ft using 18llbs of boost.

The Merlin X gave about 22.7% more power for take-off/low altitude than the Merlin III engine at 6lbs on 87 octane fuel.
Merlin XX gave about 8.0% more power for take-off/low altitude than the Merlin 45 engine at 12lbs on 100 octane fuel.
Merlin XX series engines (22 and above) made more power than the later Merlin 45 series engines (using standard impellers) but at 16-18lbs of boost the difference may have been in that under 10% range.
The Spitfire with it's low weight could afford the drop in power better than most other Merlin powered aircraft.
Most all later Merlins were limited to 9lbs of boost and 2850rpm for climbing in non-emergency conditions no matter what the combat limits were.