Why didn't Allison quickly develop a one-stage 2 speed Supercharger for the P-40

[swampyankee]

Allison was totally dependent on USN and USAAF business; Curtiss-Wright and Pratt & Whitney had large commercial customer bases. One effect of this was that both companies had larger engineering staffs and likely better field service operations and broader access to vendors.

It is possible that Allison could have developed those, but their corporate master at GM did not seem interested in spending the money to do so.

 

[tomo pauk]

Allison was totally dependent on USN and USAAF business; Curtiss-Wright and Pratt & Whitney had large commercial customer bases. One effect of this was that both companies had larger engineering staffs and likely better field service operations and broader access to vendors.

It is possible that Allison could have developed those, but their corporate master at GM did not seem interested in spending the money to do so.

Allison didn't sold a singe engine to the USN, IIRC, so dependance on that service didn't existed. Allison certainly developed a two-stage supercharged versions of the V-1710 (a year or more before Jumo and Daimler-Benz did it for their service engines), where 1st S/C stage was driven via variable-speed drive. More than 1500 of 2-stage V-1710s were produced until end of 1944.
Unfortunately, while the USN materially supported P&W (and Wright?) to develop 2-stage versions of their engines much before the US entered the war, the AAC/AAF didn't saw fit to do the same with even basis V-1710, let alone it's 2-stage version. Rather, they burned a lot of money into hi-per engines' money pits in the late 1930s.

 

[tomo pauk]

Single stage Merlins had automatic boost control. They tended to have less power than a V-1710 at low altitudes, but more at high altitudes.

The 2 speed Merlin XX had more power down low than the similar single speed 45, and had more power up high. And the Merlin XX had automatic boost control too.

Depending on a version, there were 1-speed Merlin versions, in production and service, with a bit more power than V-1710s down low. Eg. Mk. 32 and 34 with 1640 HP at 2000 ft; 1625 HP for take off.
Re. Merlin XX and 45 hi-alt performance, the difference was in a few HP?

 

[Shortround6]

The main purpose of low gear in a two speed engine is to keep the pilot from overboosting the engine at takeoff.
Allison accomplished this in their single speed engine by throttling the engine at TO and gradually opening the throttle as altitude was gained and air became thinner.
By mid '42 the automatic boost control (automatic manifold pressure regulator) automatically prevented the engine from exceeding a set manifold pressure at any altitude. So the pilot could select full throttle and not overboost the engine at any altitude.
This single speed arrangement provided plenty of power at lower altitudes (peak WEP was under 9500') and adequate power at higher altitudes since the single speed was in effect high gear. It was also lighter and simpler than a two speed arrangement.
The auto boost and variable speeds of the turbo and mechanical auxiliary stage rendered a two speed Allison unnecessary.

You have brought this up before, it was wrong then, it is wrong now and it will be wrong regardless of the number of times you bring it it up.
Please see post #3 in this thread for an explanation of 2 speed superchargers.
Please note the Merlin III was using an automatic boost limiter well before the Allison ever made it into mass production as were a number of other European engines (like the DB 601).
WEP was APPROVED in the Fall of 1942, a bit late to be sure, but way to late too have any effect on planning of engines/superchargers and accessories.

The Germans had built engines in WW I that required a restricted throttle opening at low altitudes. No supercharger but high compression ratio in the cylinders.
Throttle had to be restricted for the same reason, to keep engine from destroying itself at low altitude. as the plane climbed into thinner air the throttle could be fully opened and the engine mad more power than an equivalent engine with a lower compression ratio. I am not trying to take anything away from the people at Allison but I have never seen any claim that they invented automatic boost control or even promoted for the use you are claiming.

I would note that on some engines (like Wright R-1820s) the two speed drive weighed a whopping 30lbs. To me 30lbs for an extra 100hp (or more) for take-off is a pretty good trade.

 

[Shortround6]

Allison didn't sold a singe engine to the USN, IIRC, so dependance on that service didn't existed.

Allison may have sold 2 engines to the Navy for airship use but the Airships crashed/were removed from service before delivery could be made. I would note that Allison had designed and built some drive systems for Navy airships earlier. That is extensions and angle drives.

It is possible that Allison could have developed those, but their corporate master at GM did not seem interested in spending the money to do so.

The Corporate Master at GM was interested in making money, much like the Corporate Master/s at Continental and Lycoming. Allison during the 30s was making a nice profit with engine bearing division, they may have been making money (or at least not losing much) doing specialty engineering. However the V-1710 project was a money pit that GM had already funded to the tune of over 1/2 million dollars with no pay-off insight until the fighter order of April 1939. The Army was over $900,000 in arrears for work already done at this point and GM was considering shutting the whole project down. Not increasing company funding for new variations no matter how forward looking they may be.

 

[P-39 Expert]

You have brought this up before, it was wrong then, it is wrong now and it will be wrong regardless of the number of times you bring it it up.
Please see post #3 in this thread for an explanation of 2 speed superchargers.
Please note the Merlin III was using an automatic boost limiter well before the Allison ever made it into mass production as were a number of other European engines (like the DB 601).
WEP was APPROVED in the Fall of 1942, a bit late to be sure, but way to late too have any effect on planning of engines/superchargers and accessories.

The Germans had built engines in WW I that required a restricted throttle opening at low altitudes. No supercharger but high compression ratio in the cylinders.
Throttle had to be restricted for the same reason, to keep engine from destroying itself at low altitude. as the plane climbed into thinner air the throttle could be fully opened and the engine mad more power than an equivalent engine with a lower compression ratio. I am not trying to take anything away from the people at Allison but I have never seen any claim that they invented automatic boost control or even promoted for the use you are claiming.

I would note that on some engines (like Wright R-1820s) the two speed drive weighed a whopping 30lbs. To me 30lbs for an extra 100hp (or more) for take-off is a pretty good trade.

If I'm wrong then the AAF would have directed Allison to produce a two speed engine. They didn't. Allison estimated a low gear power increase of around 100hp which wasn't viewed as being worth the effort.

 

[Shortround6]

Do you have any documentation?

AS has been pointed out, the Merlin was using an automatic boost limiter in 1939 if not before and yet they built two speed engines with automatic boost limiters
Bristol was using autotatic boost limiters in 1939.
The First Zero's used a single speed engine with an automatic boost limiter and yet went to a two speed engine, still with an automatic boost limiter.

The list goes on and on. The US was simply late to the game with automatic boost limiters, it wasn't a break through in technology.

Allison and the USAAF got lucky. They started with 100 octane fuel and didn't have to put up with the limits that 87 or 91 octane fuel imposes. The luck continued when the development of 100/130 fuel meant that they could use 30% higher pressure in the cylinders for WEP (or slightly higher military power than before) without having to resort to a more sophisticated supercharger design.

Difference between a Merlin III and a Merlin X using the same supercharger design except for the superdrive and gears was close to 200hp when using 87 octane fuel.
The P & W R-1830 picked up 150hp for take-off using low gear over what it could make if using the the high gear only.

To give an idea of what was going consider the comparison of the V-1710-33 (long nose in P-40) and the V-1710-27/29 used in the YP-38s and some of the other early P-38s. The -33 used 8.77 supercharger gears and the -27/29 used 6.44 gears.

......................V-1710-33...............V-1710-27/29
BHP.................1040..........................1150
RPM................3000..........................3000
Map@70*..........41.9..........................39.7
Mixture temp.... 196...........................112
Friction HP*.......290..........................240
Indicated HP......1330........................1390

The Map is at 70 degrees F inlet temperature going into the carb.
The mixture temperature is the temp in fahrenheit in the manifold after the supercharger.
Friction hp includes the power needed to drive the supercharger.
Indicated HP is the power produced in the cylinders.

Please note the increase power needed to drive the supercharger with the 8.77 gears. Please also note the decreased mixture temperature.
Of course a plane using 6.44 gears and no turbo would have had truly atrocious altitude performance.

I would note that later pilot's manuals for the P-38's with -49/53 engines show 1240hp for take off using 44.5in of MAP. compared to the 1150hp at about the same pressure in the -39 engine. The -49/53 engines used 7.48 gears. Bumping the pressure up to 47in gave 1325hp which the -39 engine (and other 8.80 supercharger ger engines ) needed 51in to match.
Again the lower gear ratio engine cannot maintain it's better power as high as the 8.80 gear engine.
Please note the the later Allisons used stronger construction and were able to withstand the higher boost pressures of WEP ratings much better.

The Automatic boost control did not make 2 speed superchargers obsolete. Allison got "lucky" in that better fuel allowed higher boost pressures at low altitudes and stronger construction (better materials/techniques) allowed the engine to survive the increased power.

Remember, each and every use of WEP required notation in log books, extra maintenance (even if just checking for metal in the oil and more frequent spark plug changes) and if used enough, pulling the engine before normal life expectancy for overhaul.

Automatic boost control DID NOT,
Lower intake mixture temperatures.
Require less power to drive the supercharger.
Reduce cooling load.

Allisons were never used in a production bomber or transport where long hard take-offs and climb outs would be the normal operating procedure.
Weirdly enough, despite the "wonders" of the automatic boost control, at the end of the war Allison built 8 two speed, single stage engines to power a planned version of the C-54 transport. They used a new 10.25in impeller supercharger and were rated at 1600hp/3200rpm/SL/61.7in in low gear for take-off and military power of 1220hp/3000rpm/15,500ft. Low gear was 7.48:1 and high gear 9.60:1.

 

[wuzak]

AS has been pointed out, the Merlin was using an automatic boost limiter in 1939 if not before and yet they built two speed engines with automatic boost limiters

At some point the 2 speed engines also got automatic gear change.

Difference between a Merlin III and a Merlin XX using the same supercharger design except for the superdrive and gears was close to 200hp when using 87 octane fuel.

Do you mean Merlin X?

 

[wuzak]

If I'm wrong then the AAF would have directed Allison to produce a two speed engine. They didn't. Allison estimated a low gear power increase of around 100hp which wasn't viewed as being worth the effort.

The AAF were still very keen on the turbo to provide the altitude performance for the V-1710.

IIRC, Curtiss had to request an altitude rated engine (ie one that wasn't using a turbo). But the altitude rated engines didn't have every high critical altitudes. In that sense the 2 speed drive may not have made much difference.

Gaining altitude performance in a 1 speed supercharged engine led to a loss of low altitude performance. So if Allison could have produced a better, higher altitude supercharger, the 2 speed drive would have been beneficial.

 

[Shortround6]

Do you mean Merlin X?

yes, thank you, I will edit the post.

 

[Shortround6]

The AAF were still very keen on the turbo to provide the altitude performance for the V-1710.

IIRC, Curtiss had to request an altitude rated engine (ie one that wasn't using a turbo). But the altitude rated engines didn't have every high critical altitudes. In that sense the 2 speed drive may not have made much difference.

Gaining altitude performance in a 1 speed supercharged engine led to a loss of low altitude performance. So if Allison could have produced a better, higher altitude supercharger, the 2 speed drive would have been beneficial.

Timing is everything.
The Allison was very bit as good at altitude as the DB 601 in 1939/40 (The DB 601 got a bit better later) and every bit as good or better than any mass produced Hispano V-12 or derivative in 1939/40/41. It was also just as good as the American radial engines, the R-1820 and R-1830, and just as good or better than the Italian radials and Japanese radials of 1939-40-41. Unfortunately the game changed from late 1939 and into 1940. Planes got heavier as more "stuff" was added.
Hooker changed the game with the Merlin. The Germans revamped the 109 and cut drag a lot while gaining very little weight.

In 1939 and for most of 1940 Nobody had an engine with a higher critical altitude than around 14,000ft (correction welcome) aside from the Merlin.
And the Merlin III as you say, payed a price for it's altitude performance in poor take-off power.

 

[P-39 Expert]

Do you have any documentation?

AS has been pointed out, the Merlin was using an automatic boost limiter in 1939 if not before and yet they built two speed engines with automatic boost limiters
Bristol was using autotatic boost limiters in 1939.
The First Zero's used a single speed engine with an automatic boost limiter and yet went to a two speed engine, still with an automatic boost limiter.

The list goes on and on. The US was simply late to the game with automatic boost limiters, it wasn't a break through in technology.

Allison and the USAAF got lucky. They started with 100 octane fuel and didn't have to put up with the limits that 87 or 91 octane fuel imposes. The luck continued when the development of 100/130 fuel meant that they could use 30% higher pressure in the cylinders for WEP (or slightly higher military power than before) without having to resort to a more sophisticated supercharger design.

Difference between a Merlin III and a Merlin X using the same supercharger design except for the superdrive and gears was close to 200hp when using 87 octane fuel.
The P & W R-1830 picked up 150hp for take-off using low gear over what it could make if using the the high gear only.

To give an idea of what was going consider the comparison of the V-1710-33 (long nose in P-40) and the V-1710-27/29 used in the YP-38s and some of the other early P-38s. The -33 used 8.77 supercharger gears and the -27/29 used 6.44 gears.

......................V-1710-33...............V-1710-27/29
BHP.................1040..........................1150
RPM................3000..........................3000
Map@70*..........41.9..........................39.7
Mixture temp.... 196...........................112
Friction HP*.......290..........................240
Indicated HP......1330........................1390

The Map is at 70 degrees F inlet temperature going into the carb.
The mixture temperature is the temp in fahrenheit in the manifold after the supercharger.
Friction hp includes the power needed to drive the supercharger.
Indicated HP is the power produced in the cylinders.

Please note the increase power needed to drive the supercharger with the 8.77 gears. Please also note the decreased mixture temperature.
Of course a plane using 6.44 gears and no turbo would have had truly atrocious altitude performance.

I would note that later pilot's manuals for the P-38's with -49/53 engines show 1240hp for take off using 44.5in of MAP. compared to the 1150hp at about the same pressure in the -39 engine. The -49/53 engines used 7.48 gears. Bumping the pressure up to 47in gave 1325hp which the -39 engine (and other 8.80 supercharger ger engines ) needed 51in to match.
Again the lower gear ratio engine cannot maintain it's better power as high as the 8.80 gear engine.
Please note the the later Allisons used stronger construction and were able to withstand the higher boost pressures of WEP ratings much better.

The Automatic boost control did not make 2 speed superchargers obsolete. Allison got "lucky" in that better fuel allowed higher boost pressures at low altitudes and stronger construction (better materials/techniques) allowed the engine to survive the increased power.

Remember, each and every use of WEP required notation in log books, extra maintenance (even if just checking for metal in the oil and more frequent spark plug changes) and if used enough, pulling the engine before normal life expectancy for overhaul.

Automatic boost control DID NOT,
Lower intake mixture temperatures.
Require less power to drive the supercharger.
Reduce cooling load.

Allisons were never used in a production bomber or transport where long hard take-offs and climb outs would be the normal operating procedure.
Weirdly enough, despite the "wonders" of the automatic boost control, at the end of the war Allison built 8 two speed, single stage engines to power a planned version of the C-54 transport. They used a new 10.25in impeller supercharger and were rated at 1600hp/3200rpm/SL/61.7in in low gear for take-off and military power of 1220hp/3000rpm/15,500ft. Low gear was 7.48:1 and high gear 9.60:1.

Documentation was from Vees for Victory/Whitney. Only about 100HP increase at SL/Low Alt. Not worth the time and trouble.

 

[tomo pauk]

Documentation was from Vees for Victory/Whitney. Only about 100HP increase at SL/Low Alt. Not worth the time and trouble.

What page(s) of the book would've that be? What versions are compared?

 

[tomo pauk]

...
Unfortunately the game changed from late 1939 and into 1940. Planes got heavier as more "stuff" was added.
Hooker changed the game with the Merlin. The Germans revamped the 109 and cut drag a lot while gaining very little weight.

In 1939 and for most of 1940 Nobody had an engine with a higher critical altitude than around 14,000ft (correction welcome) aside from the Merlin.
...

Germans also quickly improved the DB 601 series, 1st with clearance of the 601A for 2600 rpm*, together with introduction of the 601N (not a jewel of reliability, but then neither the V-1710 was without problems), then in mid-1941 introduction of 601E, as well as uprating the 601A and 601N to 2800 rpm*. Plus better intake and prop with each engine?
For the V-1710 in non-turbo application, it was just barely a service-worthy engine in 1939, unlike the DB 601A, let alone the Merlin III (while the RR already has in production the 2-speed Melin X).

*above the rated altitude, ie. direct improvement of altitude performance.

 

[GregP]

Regarding post#7, Tomo, if you can find copies of the contracts for V-1710 development, especially the early ones, you will see the government defacto retained the rights to the design, and was to specifically authorize any changes they wanted to fund. If Allison themselves made improvements, these improvements were part of the design, but Allison retained the right to use improvements on other Allison-owned and/or developed projects.

The government specifically retained the right to purchase by specific part number, and they did. When they contracted for, say, V-1710-89/91 engines, Allison was constrained to produce -89/-91 engines without modifications, unless called out specifically. You can call that whatever you want. I call it design ownership. The government also retained rights to approve or deny V-1710 exports and use on other-than-government projects, and mandated cooperative production allocation so production could be assured for each project. Effectively, the government had ownership of that particular design, but Allison's name was on it.

Allison wisely insisted on only Allison in-house manufacture, but was constrained otherwise as to what they could sell from existing production lines. Allison was free to experiment on in-house prototype engines, but they didn't have many resources for that or money from GM to expand development staff. So, development was fraught with delays. A small staff restricts the number of projects that make progress.

In hindsight, Allison could have done things differently, but may also have perished in the great depression of 1929 had not the Navy been interested enough to fund development of the V-1710. So, the Navy found a buyer's market and Allison accepted the contracts eagerly, little realizing how they would be constrained later by these same contract clauses.

Live and learn, is the old saying.

 

[Schweik]

was based on the assumption that a single stage, 2-speed Supercharger might have been quickly developed for the P-40 that would have incrementally improved its high altitude performance in the time-frame of 1941-42. Instead, the USAAF fielded the P-40K in the Summer/Fall of 1942, which did nothing to improve the aircraft's high altitude performance but did provide additional performance for the overweight aircraft to get off the ground.

That's pretty funny that a P-40 is overweight but the P-38 and P-47 aren't?

P-40K used the low altitude power, especially when overboosting to 60" (which was secretly agreed to by Allison before Dec 1942) gave them ~1,500 + hp at 2,000 feet so that was quite useful in Theaters like the Russian Front (which is why so many Soviet pilots did so well with the P-40E and the P-40K in particular) and also in North Africa, and to some extent Burma.

In the Pacific, like at Darwin when the Japanese level bombers were sometimes coming in at 30,000 feet the altitude limit on the P-40s was a major problem, though they did work out effective tactics anyway. It did a lot better against tactical aircraft like dive and torpedo bombers such as at Milne Bay.

As Shortround noted, the V-1710-73 in the P-40K had a strengthened (first peened, then tempered) camshaft which allowed overboosting at higher levels and for longer periods. The gasoline quality also made a big difference though I don't understand precisely by what mechanism. From reading Allison memos it sounds like they are saying the better fuel didn't run as hot which seems counter-intuitive.

The later P-40 engines with the 9.6-1 gear ratio such as the V-1710-81 on the P-40M etc. had a bit higher altitude rating but were riskier to run at high manifold pressure.

SR6 has provided the rationale for why it wasn't pursued: little benefit to be achieved, and of course the fact that at about the same time, the single staged, two-speed variant of the Packard-Merlin engined P-40(F) was coming on line which was perhaps a far simpler solution to the problem than a dedicated Allison development.

The Packard Merlin P-40F raised the effective ceiling of the P-40 to 20k feet, which helped somewhat in North Africa but was still generally insufficient in terms of altitude performance. tey did use them to escort medium bombers like the A-20s, B-25s and B-26s, but escorting the heavy bombers was left to P-38s. Interestingly Packard strengthened the V-1650-1 / Merlin 28 used for the P-40F/L by coating the bearings with super hard iridium, and these too were overboosted quite a bit in the field.

P-38s however, while they found a niche early on in the Pacific, and were the best high altitude escort available in the Med and Europe until the P-51, more generally simply did not live up to the hopes of USAAF planners in the Med which is one of the reasons why the P-40 continued in use for so long. That along with the range limitation on Spitfires and the almost complete failure of P-39 units in the Med forced the USAAF and RAF to keep equipping units with P-40s until 1944 (and 1945 for RAF and in the Pacific / CBI Theater). The P-47 was what ultimately replaced the P-40 in the Med. As a radial engined plane it with a heavier bomb load it was a better fighter-bomber in many respects and it also had the altitude capacity to escort heavy bombers in theory, though in practice they still used the P-38s for that mostly I think until the P-51s were available.

The decision to use the single stage Merlin XX as the basis of the P-40's "Merlin 28" instead of something more like the Merlin 60 series is allegedly why Don Berlin quit Curtis aircraft company. I do think it was a major mistake though it's basically a question of quantity vs quality. Merlin 61 were not being produced by Rolls Royce until March 42 and I'm not sure when the V-1650-7 became available from Packard I'm guessing a bit later that year. 1942 was a very tough year for the Allies and the V-1650-1 powered P-40, for all it's limitations, was definitely needed and played a crucial role in the capture of Tunisia, the reduction of Pantellerina, the invasion of Sicily and the invasion of Italy through Anzio. The Spit V was a better interceptor but didn't have the legs for most escort missions. The Spit IX was the best air superiority fighter they had by far but there weren't enough of them, and all Spits were a bit too fragile for ground attack.

I think the Russians actually preferred the P-40K though with the Allison engine because they were fighting down low for the most part. They got about 100 P-40Fs but sent them right to PVO units (air defense). Allison engine P-40s seem to also have been preferred in the CBI.

One question I have is why weren't the Allison engined P-51s a bit more widely used in the Med? They did have them but were using them as dive bombers and recon planes mostly.

It seems like one answer to the 'supercharger dilemma' would have been to produce some V-1650-1 / P-40Fs while also diverting at least some of the V-1650-7s to P-40s along with the P-51s. The P-51s were better in many ways but a P-40 with a V-1650-7 probably would have been a pretty dangerous opponent for the Luftwaffe. I guess it was basically a question of prioritizing Strategic bombing over the (stalled out) Tactical campaign in Italy.

S

 

[Shortround6]

That's pretty funny that a P-40 is overweight but the P-38 and P-47 aren't?

It seems like one answer to the 'supercharger dilemma' would have been to produce some V-1650-1 / P-40Fs while also diverting at least some of the V-1650-7s to P-40s along with the P-51s. The P-51s were better in many ways but a P-40 with a V-1650-7 probably would have been a pretty dangerous opponent for the Luftwaffe. I guess it was basically a question of prioritizing Strategic bombing over the (stalled out) Tactical campaign in Italy.

Power to weight is what makes an aircraft "0ver weight".
Once you get a Allison P-40 to 15,000ft or so you have an 1150hp engine in an 8,000lb plane and as you go higher it gets worse.
P-47 had 2000hp for a 12,500lb plane and had that power o weigh ratio at 25,000ft and a bit above.
P-38 likewise a had a much better power to weight ratio at altitude due to the turbos. A late model P-38 had four times the power at 25,000ft that a P-40N did. It sure didn't weigh four times as much.

A P-40 with a V-1650-7 might have been a very dangerous opponent but it would have required quite a number of changes evenif you could actually get your hands on a quantity of V-1650-7s that weren't needed for Mustangs. You need the 6-70 heavier propeller, the engine weighs about 150lbs more than a V-1650-3 and the cooling system (includes intercooler for the 2 stage supercharger) weighs about 350-360lbs more than the cooling system in the P-40F.
given enough time they could have sorted everything out but why jump through all those hoops to build an inferior airplane? Just build more P-51Ds.
By the spring of 1944 P-40 production was winding down and they were building roughly twice as many P-51s per month as they were P-40s.

 

[wuzak]

The decision to use the single stage Merlin XX as the basis of the P-40's "Merlin 28" instead of something more like the Merlin 60 series is allegedly why Don Berlin quit Curtis aircraft company. I do think it was a major mistake though it's basically a question of quantity vs quality. Merlin 61 were not being produced by Rolls Royce until March 42 and I'm not sure when the V-1650-7 became available from Packard I'm guessing a bit later that year. 1942 was a very tough year for the Allies and the V-1650-1 powered P-40, for all it's limitations, was definitely needed and played a crucial role in the capture of Tunisia, the reduction of Pantellerina, the invasion of Sicily and the invasion of Italy through Anzio.

The V-1650-1 started production in 1941, ramping up into 1942. Note that most of the single stage Merlins built by Packard (known as Merlin 28) went to the British for Lancasters and Hurricanes (built in Canada).

The V-1650-3 may have started production in late 1942, but was well into 1943 before they began to appear in quantity. I believe this is what held up production of the P-51B.

The V-1650-7 appeared later in 1943.

Why the V-1650-3 and V-1650-7 didn't appear in P-40s is simple - they were spoken for.

Berlin had left Curtiss long before the V-1650-3 was in production. Possibly before the V-1650-1 was in production.

 

[wuzak]

Interestingly Packard strengthened the V-1650-1 / Merlin 28 used for the P-40F/L by coating the bearings with super hard iridium, and these too were overboosted quite a bit in the field.

The bearing materials were standard US practice at the time, so it made sense to use them instead of starting another process to make the same spec as Rolls-Royce were using. The main reason for the iridium seems to be that they prevented corrosion, though they did have slightly better strength.

 

[Milosh]

The Spit IX was the best air superiority fighter they had by far but there weren't enough of them, and all Spits were a bit too fragile for ground attack.

The Spit IX and later the Spit XVI were the main a/c used by the 2TAF in NE Europe. They could be dived up 60 degrees.