SHOULD the P39 have been able to handle the Zero? Was it training or performance? (1 Viewer)

[varsity07840]

Since we are talking about the P-39, lets clear up a few inaccuracies that have perpetuated over the decades:
1. The XP-39 was not ruined by deleting the turbocharger. Turbo was deleted to get the P-39 and P-40 ready in time for WWII. Turbo was new and was the main reason that the similarly turbocharged P-38 didn't enter combat until the end of 1942. And the intercooler and oil cooler arrangements were not adjustable to let in more or less air as needed so they would not have worked in combat. Allison was coming out with engines that had higher critical altitudes and even one with a second stage supercharger that ended up going into the P-63.
2. The rejection of the P-39 and P-400 (export P-39) by the British was solely political and economic. They ordered those planes in 1940 when Hitler was about to invade France expecting a long war and needing more airplane production. They cancelled the order in late 1941 after France had fallen and the Battle of Britain was over. They no longer needed these planes since their own fighter production (Spitfire and Typhoon) was adequate with no threat of Nazi invasion. But the main reason for cancellation was these original orders were hard money purchase contracts and they no longer had the money to pay for them, and in the meantime Lend Lease had been enacted that would send them all the planes they wanted/needed for free. Why spend $millions on a plane when they were now free. So the British specified much more armament and weight on these P-39s so that performance suffered and they could weasel out of the contract.

 

[Shortround6]

Couple of things, not arguing that the 1650-1 made more power than the Allison, it did because it had a larger diameter impeller (supercharger). That is a fact.
But the two figures listed in the "Engine Ratings" column (below the Takeoff rating) are for low gear (1240hp/3000rpm/11500') and high gear (1120hp/3000rpm/18500'). Takeoff was in low gear (to not overboost the engine at low altitude where the air is thicker) and then manually shifted to high gear at 11500' with critical altitude being 18500' to draw in more of the thinner air at higher altitudes.
And the single speed Allison was in no way a disadvantage. Later models (starting with the 1325hp V-1710-63) were equipped with an automatic boost control or automatic manifold pressure regulator (same thing). The maximum boost (or manifold pressure) for the engine was preset and this gizmo mounted on the carburetor limited boost to at or below that preset amount. In other words you couldn't blow up (overboost) the engine at ANY throttle setting. This eliminated the need for two speeds, manual shifting or automatic shifting. And this device apparently worked very well and was very dependable since it was used on most all the engines Allison manufactured thereafter.
So, with no throttle management the pilot could set the boost control, firewall the throttle and not overboost the engine at any altitude. This eliminated a lot of pilot workload.
Back to the two speed supercharger, the disadvantage was that in low gear power started to decline almost right away as the plane gained altitude and the air began to get thinner, so by the time you reached the shift point (11500') power was lower. Then you shifted into high and power steadily increased up to the critical altitude (18500') when it began to fall off again (just like low gear). This resulted in a sawtooth performance curve for both the engine and airplane. The Allison didn't have that problem (especially with the auto boost control) as it made 1325HP at takeoff and maintained that figure up to 8000' where it began to decline due to the thinner air.
Only real problem with the Allison was that the Army insisted on designing planes around it that just weighed too darn much. The P-39 weighed around 7650#, the P-40 weighed 8400# and the P-51A weighed about that also. 8400# divided by 1150HP is 7.3 pounds/HP. The Me109G6 weighed 6800# with a 1475HP engine or 4.6 pounds/HP. You can see the problem.

Ah, the British had the pretty much the same device on the Merlin so the need for a two speed supercharger was NOT as you state.
The Merlin as used in the P-40F shows this pretty clearly
1300hp for take-off at at 12lbs boost (54 in) and could hold 1240hp to 11500ft (including ram?)
in high gear it could hold 1120hp to 18,500ft.
The V-1710-63 engine you reference was good for 1150hp at 11,800ft with no ram.
The difference shows up with the V-1710-83 engine with the 9.60 supercharger gear. the 1150hp point was moved to 15,500ft but at the cost of lowering the take-off power to 1200hp from the 1325hp.
At a given point in time ( engine materials, cooling and fuel) the two speed supercharger allowed for more power at both sea level and at altitude without having to compromise one or the other.

 

[Shortround6]

The rejection of the P-39 and P-400 (export P-39) by the British was solely political and economic. They ordered those planes in 1940 when Hitler was about to invade France expecting a long war and needing more airplane production. They cancelled the order in late 1941 after France had fallen and the Battle of Britain was over. They no longer needed these planes since their own fighter production (Spitfire and Typhoon) was adequate with no threat of Nazi invasion. But the main reason for cancellation was these original orders were hard money purchase contracts and they no longer had the money to pay for them, and in the meantime Lend Lease had been enacted that would send them all the planes they wanted/needed for free. Why spend $millions on a plane when they were now free. So the British specified much more armament and weight on these P-39s so that performance suffered and they could weasel out of the contract.

Got any proof?
The 20mm Hispano gun weighed about 100-110lbs less than the 37mm for one thing and since 37mm ammo weighs roughly 3 times per round what 20mm ammo does even with the drum the ammo load for the 20mm gun was less than 30 rounds of 37mm ammo?
four British .303 guns weigh more than four american .30 cal guns?

 

[Stig1207]

Yes, all aircraft types. A twin engined bomber and an observation plane each counted one victory. Not all victories were against Zeros.

A 1:1 ratio against all types would not indicate a good ratio against Zeros or other fighters; most air-to-air losses are likely to be to fighters. So a 1:1 ratio for the P-39 against all types may well mean a 1:2, 1:3, 1:4....and so on, against enemy fighters.

 

[varsity07840]

I agree the P39 had flaws, but it was much faster then the Zero and could easily out dive the Zero. The F4F had nothing on the Zero except being tougher and yet it fought the Zero to about a draw while the P39 didn't do as well. How can that be explained?

The F4F had a much higher ceiling.

 

[varsity07840]

Wasn't the P38 20mm much more reliable than the other 20mm mounted on other US aircraft?

same gun.

 

[tomo pauk]

Couple of things, not arguing that the 1650-1 made more power than the Allison, it did because it had a larger diameter impeller (supercharger). That is a fact.

Indeed, the bigger impeller was one of key ingredients of Merlin's power. ANother is that compression ratio was low, some 10% lower than on the V-1710, and 10-25% lower than on the DB-601/605 - that meant the Merlin took overboosting very well, more boost = more power in linear fashion. On 100 oct fuel Merlin III was factory aprooved by factory to make up to +12 psi boost (54 in Hg, around 1.9 ata) by 1939, with some subsequent types quickly going to +18 psi (67 in Hg, ) by early 1942. By time 130 grade fuel was available (~mid 1942?), better Merlins went to + 21 psi (~73 in Hg).

But the two figures listed in the "Engine Ratings" column (below the Takeoff rating) are for low gear (1240hp/3000rpm/11500') and high gear (1120hp/3000rpm/18500'). Takeoff was in low gear (to not overboost the engine at low altitude where the air is thicker) and then manually shifted to high gear at 11500' with critical altitude being 18500' to draw in more of the thinner air at higher altitudes.
And the single speed Allison was in no way a disadvantage. Later models (starting with the 1325hp V-1710-63) were equipped with an automatic boost control or automatic manifold pressure regulator (same thing). The maximum boost (or manifold pressure) for the engine was preset and this gizmo mounted on the carburetor limited boost to at or below that preset amount. In other words you couldn't blow up (overboost) the engine at ANY throttle setting. This eliminated the need for two speeds, manual shifting or automatic shifting. And this device apparently worked very well and was very dependable since it was used on most all the engines Allison manufactured thereafter.
So, with no throttle management the pilot could set the boost control, firewall the throttle and not overboost the engine at any altitude. This eliminated a lot of pilot workload.

I'm not sure how you can say at face value that 1-speed V-1710 was not in disadvantage vs. 2-speed engine, particulary vs. V-1650-1.
Later models of V-1710 are exactly that - later models. They lagged behind the V-1650-1 by 8-10 months, meaning that engines powering the P-40s and P-39s in 1942 were making 20% less power at altitude than V-1650-1. I will say that having less power = being in disadvantage.
Automatic manifold pressure regulators were not endemic to the V-1710.

Back to the two speed supercharger, the disadvantage was that in low gear power started to decline almost right away as the plane gained altitude and the air began to get thinner, so by the time you reached the shift point (11500') power was lower. Then you shifted into high and power steadily increased up to the critical altitude (18500') when it began to fall off again (just like low gear). This resulted in a sawtooth performance curve for both the engine and airplane. The Allison didn't have that problem (especially with the auto boost control) as it made 1325HP at takeoff and maintained that figure up to 8000' where it began to decline due to the thinner air.

Please, take a look at the tables. At 11500 ft, the V-1650-1 makes 1240 HP, vs. V1710-39 making 1160-1170 there. Or, at 11800-12000 ft, the difference is some 70-80 HP, while above 15000 ft we have the V-1650-1 making 25% more power. Sawtooth or not, nowhere where mattered the V-1710-39 and similar were not matching the power of V-1650-1. The V-1650-1 was making 1325 HP up to 9000 ft. And again at ~15000 ft due to having another S/C speed and a sizable S/C, despite the thinner air.
The Merlin 20 series from 1943 on were good for 1600 HP for take off, vs. 1200 HP on the 1-stage V-1710s from same time. take a look

Only real problem with the Allison was that the Army insisted on designing planes around it that just weighed too darn much. The P-39 weighed around 7650#, the P-40 weighed 8400# and the P-51A weighed about that also. 8400# divided by 1150HP is 7.3 pounds/HP. The Me109G6 weighed 6800# with a 1475HP engine or 4.6 pounds/HP. You can see the problem.

Granted, the US fighters were over weight. However, Allison have had lagging too much with introduction of more powerful engines when compared with RR and DB, they managed by late 1942 to equal altitude performance of the Merlin III from 1937. Let's recall that every important US engine in 1944 was with 2-, 3- or variable-speed drive, unless a turbocharger was involved.
BTW - the P-51A have had 1480 HP max, the Bf 109G6 around 1550 PS (fully rated) or 1400 PS (restricted from mid-1942 to Oct 1943). Sure enough, the G6 have had better altitude power.

 

[Shortround6]

same gun.

The 20mm gun in the P-38 was mounted in a substantial cradle which helped. The gun being originally designed to mounted on an engine block,
How much more relaible I don't know.

 

[tomo pauk]

Since we are talking about the P-39, lets clear up a few inaccuracies that have perpetuated over the decades:
1. The XP-39 was not ruined by deleting the turbocharger. Turbo was deleted to get the P-39 and P-40 ready in time for WWII. Turbo was new and was the main reason that the similarly turbocharged P-38 didn't enter combat until the end of 1942. And the intercooler and oil cooler arrangements were not adjustable to let in more or less air as needed so they would not have worked in combat. Allison was coming out with engines that had higher critical altitudes and even one with a second stage supercharger that ended up going into the P-63.
2. The rejection of the P-39 and P-400 (export P-39) by the British was solely political and economic. They ordered those planes in 1940 when Hitler was about to invade France expecting a long war and needing more airplane production. They cancelled the order in late 1941 after France had fallen and the Battle of Britain was over. They no longer needed these planes since their own fighter production (Spitfire and Typhoon) was adequate with no threat of Nazi invasion. But the main reason for cancellation was these original orders were hard money purchase contracts and they no longer had the money to pay for them, and in the meantime Lend Lease had been enacted that would send them all the planes they wanted/needed for free. Why spend $millions on a plane when they were now free. So the British specified much more armament and weight on these P-39s so that performance suffered and they could weasel out of the contract.

1. Allison was lagging with engines that have had higher critical altitudes by perhaps 2 years after the RR and DB. Original XP-39 installation of turbo, intercoolers, radiators etc was aerodynamically apalling, with top speed barely exceeding 340 mph above 20000ft, without any armament, protection or radio antennae protruding. The P-43, with radial engine and guns, was faster.
2. A good idea might be not to throw mud on people that don't deserve it. It tends to fly backwards.

 

[Shortround6]

I would note that the British got 3 P-39Cs from the USAAF order of 20 and these made it to England in June/July of 1941 with test flying starting July 6th 1941, the first Aircobra I showed up July 30th. The P-39Cs had no self sealing fuel tanks even though they have armor glass behind windscreen. amount of armor in other places I don't know.
The P-400 weighed 5550lbs empty (no guns, no armor, oxygen etc) while a P-39D weighed 5523lbs and a P39D-2 weighed 5658lbs. 20-30 lbs could be the production variation, 100lbs starts to stretch it.
Basic weight (guns, armor, Oxygen, radios etc) added in has the P-400 at 6328.6lbs and the P-39D at 6290lbs and the P-39D-2 at 6431.5.
This is no fuel, no ammo, no oil except what is left in the systems and so on.

Just where did those perfidious men from Albion stick all that weight in order to get out of the contracts?
Please note that when the first AirCobras get to England the British have the Spitfire Vb in squadron service with two 20mm cannon and four .303 machineguns so asking for one 20mm, two .50s and four .303s hardly seems like they are deliberately loading down the plane.

 

[Glider]

2. The rejection of the P-39 and P-400 (export P-39) by the British was solely political and economic. They ordered those planes in 1940 when Hitler was about to invade France expecting a long war and needing more airplane production. They cancelled the order in late 1941 after France had fallen and the Battle of Britain was over. They no longer needed these planes since their own fighter production (Spitfire and Typhoon) was adequate with no threat of Nazi invasion. But the main reason for cancellation was these original orders were hard money purchase contracts and they no longer had the money to pay for them, and in the meantime Lend Lease had been enacted that would send them all the planes they wanted/needed for free. Why spend $millions on a plane when they were now free. So the British specified much more armament and weight on these P-39s so that performance suffered and they could weasel out of the contract.

I am sorry but this isn't close to being accurate. For use in Europe the P39 was no match for the Luftwaffe or as good as the RAF fighters, it was also totally unfit for combat when the RAF first tried to use them in the autumn of 1941. As for the change in weapons as others have pointed out the 20mm was lighter and better for air to air combat.

At low altitude the P39 was better but the war in Europe tended to be fought at higher altitudes. Had the cost been an issue, the RAF could simply have taken the US P39 at no cost under lend lease and made changes in the UK. Nearly all the lend lease aircraft came to the UK and had to be modified for RAF service.

 

[P-39 Expert]

1. Allison was lagging with engines that have had higher critical altitudes by perhaps 2 years after the RR and DB. Original XP-39 installation of turbo, intercoolers, radiators etc was aerodynamically apalling, with top speed barely exceeding 340 mph above 20000ft, without any armament, protection or radio antennae protruding. The P-43, with radial engine and guns, was faster.
2. A good idea might be not to throw mud on people that don't deserve it. It tends to fly backwards.

Please accept my apology if you think I threw mud on you. Message boards are for debate and everyone always doesn't agree with everyone else. I'm just trying to explain to you what the numbers on the engine chart mean.

 

[P-39 Expert]

Ah, the British had the pretty much the same device on the Merlin so the need for a two speed supercharger was NOT as you state.
The Merlin as used in the P-40F shows this pretty clearly
1300hp for take-off at at 12lbs boost (54 in) and could hold 1240hp to 11500ft (including ram?)
in high gear it could hold 1120hp to 18,500ft.
The V-1710-63 engine you reference was good for 1150hp at 11,800ft with no ram.
The difference shows up with the V-1710-83 engine with the 9.60 supercharger gear. the 1150hp point was moved to 15,500ft but at the cost of lowering the take-off power to 1200hp from the 1325hp.
At a given point in time ( engine materials, cooling and fuel) the two speed supercharger allowed for more power at both sea level and at altitude without having to compromise one or the other.

I don't see how a single stage supercharger could compromise on power, all the power the engine could generate was available at every altitude subject to governance of the auto boost control below the critical altitude. The two speed supercharger by definition limited power at takeoff/low altitude. Both systems worked effectively.

 

[P-39 Expert]

A 1:1 ratio against all types would not indicate a good ratio against Zeros or other fighters; most air-to-air losses are likely to be to fighters. So a 1:1 ratio for the P-39 against all types may well mean a 1:2, 1:3, 1:4....and so on, against enemy fighters.

Correct. Same for all combatants, your fighters are not just shooting down opposition fighters, your main goal is to get by the escorting fighters to knock down the higher value targets like bombers or cargo planes.

 

[P-39 Expert]

I would note that the British got 3 P-39Cs from the USAAF order of 20 and these made it to England in June/July of 1941 with test flying starting July 6th 1941, the first Aircobra I showed up July 30th. The P-39Cs had no self sealing fuel tanks even though they have armor glass behind windscreen. amount of armor in other places I don't know.
The P-400 weighed 5550lbs empty (no guns, no armor, oxygen etc) while a P-39D weighed 5523lbs and a P39D-2 weighed 5658lbs. 20-30 lbs could be the production variation, 100lbs starts to stretch it.
Basic weight (guns, armor, Oxygen, radios etc) added in has the P-400 at 6328.6lbs and the P-39D at 6290lbs and the P-39D-2 at 6431.5.
This is no fuel, no ammo, no oil except what is left in the systems and so on.

Just where did those perfidious men from Albion stick all that weight in order to get out of the contracts?
Please note that when the first AirCobras get to England the British have the Spitfire Vb in squadron service with two 20mm cannon and four .303 machineguns so asking for one 20mm, two .50s and four .303s hardly seems like they are deliberately loading down the plane.

All that weight was in the form of the useless 4 .30 caliber machine guns in the wings that weighed almost 400# (including gun mounts, ammo, ammo boxes and gun chargers) and and other such items as a cockpit heater that was fueled by kerosene when the P-39 already had probably the best cockpit climate control system of any American fighter that simply ducted hot air from the radiator. Reduce the weight of the P-400 by almost 500# and you have a plane very competitive with the Spitfire V.

 

[P-39 Expert]

I am sorry but this isn't close to being accurate. For use in Europe the P39 was no match for the Luftwaffe or as good as the RAF fighters, it was also totally unfit for combat when the RAF first tried to use them in the autumn of 1941. As for the change in weapons as others have pointed out the 20mm was lighter and better for air to air combat.

At low altitude the P39 was better but the war in Europe tended to be fought at higher altitudes. Had the cost been an issue, the RAF could simply have taken the US P39 at no cost under lend lease and made changes in the UK. Nearly all the lend lease aircraft came to the UK and had to be modified for RAF service.

Cost was the big issue, especially for the British. They had stood alone against Germany since the fall of France and they were dead broke. They had to order those planes before France fell not knowing how long the war would run. After they were safely past the Battle of Britain and a German invasion was no longer possible, they moved to cut their losses and weasel out of not only the P-39 contract but a similar contract for P-38s that were ordered without turbochargers. Those were hard money contracts and payment was demanded. The British knew that if the Battle of Britain had not gone their way and they needed the P-39s that they could have quickly stripped the excess weight (.30 cal MGs, heater, unnecessary radio equipment) from those planes to make them competitive.

 

[Shortround6]

You don't understand how the supercharger works.

See my post #96.

It takes power to drive the supercharger, the power needed goes up with the square of the speed. a given impeller, inlet and diffuser set up will need twice the power if driven by a set of 10.0 gears than if driven by a set of 7.0 gears. Of course the peak pressure will be much lower with the 7.0 gears. I will get back to this.

of the power going into the supercharger, a good supercharger in 1939-41 used about 70% power to actually compress the air. The extra 30% was pretty much wasted in churning the air around without actually compressing it, however this "wasted" power did not simply go away. The extra churning created friction between the air and the impeller and the housing/diffuser and this friction turned into heat. Basically if you had 100hp driving the supercharger input shaft you were doing good if you got 70hp worth of compression of the air. Now simply compressing the air will heat it, ask anybody who has grabbed a bicycle tire pump after it has seen some use. The extra 30hp turned pretty much into pure heat. Your intake charge could go from 60-70 degrees f to several hundred degrees very easily.
SInce there is a limit to the amount of boost you can use with a given fuel that also depends of the temperature of the air/fuel before it gets to the cylinder there is a problem with using a high gear at low altitude.
at sea level the air, rounded off is at 15lbs/sq in so if we want 44in of manifold pressure (7lbs of boost) we only need to compress the air 1.46 times. However at 12,000f the air pressure 9.5 lbs so if we still want 44in we need to compress the air 2.3 times. at 18,000ft we need to compress the air 2.93 times.

Now if we use a single gear set and we chose a set that will give us 44in of manifold pressure at high altitude (14-18,000ft) we are spinning way faster than we need at low altitude. we are using up power form the crankshaft that could be going to the propeller, we are over heating the intake charge at low altitude and our fancy auto boost control simply closes off the the throttle plate and restricts the air flow entering the supercharger. It doesn't solve any of these problems.

The two speed supercharger spins the impeller slower, uses less power from the crankshaft, heats the intake charge less meaning that at a give pressure the air is denser, and the lower temperature means we can use more pressure if we so desire. Of course at the slower speed the supercharger cannot compress the air enough to give the desired manifold pressure at higher altitudes.
During the 1930s many engines came with different supercharger gears for their single speed superchargers to suit them to the expected duty. Many transports/flying boats getting engines with high take-off power but low altitude performance while engines intended for fighters or fast bombers sacrificed take-off power for power at altitude (12-15,000ft)
The coming of high octane fuel allowed for higher temperatures in the intake charge and higher boost without detonation setting and wrecking the engine.

 

[P-39 Expert]

I am sorry but this isn't close to being accurate. For use in Europe the P39 was no match for the Luftwaffe or as good as the RAF fighters, it was also totally unfit for combat when the RAF first tried to use them in the autumn of 1941. As for the change in weapons as others have pointed out the 20mm was lighter and better for air to air combat.

At low altitude the P39 was better but the war in Europe tended to be fought at higher altitudes. Had the cost been an issue, the RAF could simply have taken the US P39 at no cost under lend lease and made changes in the UK. Nearly all the lend lease aircraft came to the UK and had to be modified for RAF service.

Just which Luftwaffe plane(s) were so superior to the P-39?
Regarding lend lease, how could the British take the P-39 under lend lease when they had so publicly derided them as inferior under the hard money contract? "Oh these P-39s suck, but we'll take them for free"? They no longer needed them after winning the BoB and their internal fighter production was then adequate. They didn't need them and couldn't pay for them. Bell had the last laugh though. They got a $2million cash advance from the British in the original order. Bell was a relatively new company struggling under depression era economics and although $2mil doesn't sound like much today, in 1940 dollars it was a fortune and put them in the black for the rest of the war. And the Army took over the British order and had P-39s available from 1941 on.

 

[Shortround6]

All that weight was in the form of the useless 4 .30 caliber machine guns in the wings that weighed almost 400# (including gun mounts, ammo, ammo boxes and gun chargers) and and other such items as a cockpit heater that was fueled by kerosene when the P-39 already had probably the best cockpit climate control system of any American fighter that simply ducted hot air from the radiator. Reduce the weight of the P-400 by almost 500# and you have a plane very competitive with the Spitfire V.

Not even close.
I asked where the British added weight to get out off the contract. They took out over 100lbs compared to an American P-39 with 37mm gun. The P-39c had a pair of 30 cal guns in the fuselage

weight figures may be suspect. The speed sure is, British got 359mph out of the ones they got.
Basically Larry Bell sold the British and French an imaginary aircraft. When the British took delivery it would not come close to meeting the promised performance. The XP-39 when delivered to Wright field in 1939 was 10% overweight and the prototype had no guns let alone armor or self sealing tanks.
Yanking a cabin heater wasn't going to save it.
Nobody says you have to fill the ammo boxes completely full.

Spitfire Vb went a bit over 6500lbs, cutting 500lbs out of an Aircobra still leaves you over 500lbs heavy and several hundred hp short.

 

[P-39 Expert]

You don't understand how the supercharger works.

See my post #96.

It takes power to drive the supercharger, the power needed goes up with the square of the speed. a given impeller, inlet and diffuser set up will need twice the power if driven by a set of 10.0 gears than if driven by a set of 7.0 gears. Of course the peak pressure will be much lower with the 7.0 gears. I will get back to this.

of the power going into the supercharger, a good supercharger in 1939-41 used about 70% power to actually compress the air. The extra 30% was pretty much wasted in churning the air around without actually compressing it, however this "wasted" power did not simply go away. The extra churning created friction between the air and the impeller and the housing/diffuser and this friction turned into heat. Basically if you had 100hp driving the supercharger input shaft you were doing good if you got 70hp worth of compression of the air. Now simply compressing the air will heat it, ask anybody who has grabbed a bicycle tire pump after it has seen some use. The extra 30hp turned pretty much into pure heat. Your intake charge could go from 60-70 degrees f to several hundred degrees very easily.
SInce there is a limit to the amount of boost you can use with a given fuel that also depends of the temperature of the air/fuel before it gets to the cylinder there is a problem with using a high gear at low altitude.
at sea level the air, rounded off is at 15lbs/sq in so if we want 44in of manifold pressure (7lbs of boost) we only need to compress the air 1.46 times. However at 12,000f the air pressure 9.5 lbs so if we still want 44in we need to compress the air 2.3 times. at 18,000ft we need to compress the air 2.93 times.

Now if we use a single gear set and we chose a set that will give us 44in of manifold pressure at high altitude (14-18,000ft) we are spinning way faster than we need at low altitude. we are using up power form the crankshaft that could be going to the propeller, we are over heating the intake charge at low altitude and our fancy auto boost control simply closes off the the throttle plate and restricts the air flow entering the supercharger. It doesn't solve any of these problems.

The two speed supercharger spins the impeller slower, uses less power from the crankshaft, heats the intake charge less meaning that at a give pressure the air is denser, and the lower temperature means we can use more pressure if we so desire. Of course at the slower speed the supercharger cannot compress the air enough to give the desired manifold pressure at higher altitudes.
During the 1930s many engines came with different supercharger gears for their single speed superchargers to suit them to the expected duty. Many transports/flying boats getting engines with high take-off power but low altitude performance while engines intended for fighters or fast bombers sacrificed take-off power for power at altitude (12-15,000ft)
The coming of high octane fuel allowed for higher temperatures in the intake charge and higher boost without detonation setting and wrecking the engine.

Sorry I don't understand how a supercharger works. I thought it compressed intake air to provide more power to the engine. Whether the power is reduced at lower altitudes because of a lower gear or because of a lower throttle setting on a single gear seems irrelevant to me. The sole purpose of low gear is to keep from overboosting the engine at takeoff. The same protection against overboosting at takeoff in a single speed engine is provided by reduced throttle. The Allison's single speed was high gear, it didn't have a low gear. It reduced boost at takeoff and low altitude by reduced throttle. Simple operation, lighter weight, more reliable.