An Allison V-1710 with a two-speed supercharger in 1942

[tomo pauk]

But the Allison V-1710 is a different beast: Selecting the -39 found in the Mustang Mk.1, the engine can only make 1,150hp (based on USAAF 150 hr TBO). And the boost control mechanism for the Allison is different from the RR one. It allows 45.5" HgA for take off = 1,150hp then boost is decrease as altitude increases to 44.6" HgA at 11,700', but the 1,150hp is maintained. (Above 11,700' the supercharge may not longer create enough boost to maintain the 1,150hp).

So, a 2 speed supercharger on a Mustang Mk. 1 buys us nothing. (It actually hurts us with extra weight and complexity). Allison needs to find the other components which are limiting power and strengthen them. Only after the engine can accept more power (While passing the USAAF 150 hr TBO test) is there an opportunity to take advantage of a 2 speed supercharger.

In 1941, the 2-speed S/C with, say, 7.48:1 and 9.60:1 drive ratios (low and high speed, respectively), will have the sibling of the -39 making, on ~44.5in Hg boost making about 1125 HP at ~15000 ft ( where the -39 makes ~1030 HP) and about 1300 HP at ~4000 ft (where the -39 makes under 1100 HP).
All values for the military power.
Take off at ~44.5in Hg and with 7.48:1 drive ratio will also give about 1300 HP.

Once the -73 is available (May 1942), with it's strengthened construction, the company-aprooved WER gives even more, 1500-1600 HP almost between SL and 10000 ft. Similar for the ad-hoc overboosting made by non-USAAF units in 1941-42.

tl;dr, a 2-speed S/C could've gained them a lot

 

[Steamed_Banana]

In 1941, the 2-speed S/C with, say, 7.48:1 and 9.60:1 drive ratios (low and high speed, respectively), will have the sibling of the -39 making, on ~44.5in Hg boost making about 1125 HP at ~15000 ft ( where the -39 makes ~1030 HP) and about 1300 HP at ~4000 ft (where the -39 makes under 1100 HP).
All values for the military power.
Take off at ~44.5in Hg and with 7.48:1 drive ratio will also give about 1300 HP.

Once the -73 is available (May 1942), with it's strengthened construction, the company-aprooved WER gives even more, 1500-1600 HP almost between SL and 10000 ft. Similar for the ad-hoc overboosting made by non-USAAF units in 1941-42.

tl;dr, a 2-speed S/C could've gained them a lot

That 'unofficial' overboosting was also being done by US fighter units from early 1942 through mid 1942 when the new settings were officially approved.

 

[Shortround6]

The V-1710-39 engine production spans several changes. The production P-40D with that engine was first delivered in June 1941 and the first production P-40E was delivered in Aug 1941.
Production Mustang Is are slightly ahead of this with first production (slow) in April/May of 1941.
US had yet to change their fuel speciation to meet the British standards so 100/100 is about as good as US fuel gets.
The P-40K with the -73 engine is first built in May 1942.
Obviously engines are built several weeks or several months in advance of completed fighters and engine testing it sometimes being done at the same time.
Allison changed the crankshaft several times in 1940-42. They went from a "plain steel"* crankshaft to one that was shot peened making it much tougher. They then went to a nitrided crankshaft which was significantly tougher than the shot peened one. It was this crankshaft (and a few other changes) that allowed for the 1325hp take-off rating and the later 60.00 WEP rating compared to the 56.0in rating on the -39 engines.
The -33 engines in the P-40B&C were never given a WEP rating from the US Army even though the British often flew them at higher pressure settings.

The whole idea of US WEP settings was that they had tested an engine on a test stand and gotten it last for 7 1/2 hours at the power setting in 5 minute spans of time alternating with 5 minute spans of time at either military power or max continuous power to cool off. The engine might survive higher power settings but each pilot was now acting as his own test pilot.
On a test stand the engineers KNEW the air temperature and actual air pressure and density. In the Dessert 70.0in in 100+ degrees could give around 93% of the power as it could at 59 degrees. Running over NW Europe with higher humidity could also help a little. Higher than normal humidity can offer a little extra cooling (slight water injection effect) which is not going to show up in the desert. What happens over New Guinea?
Test results were also with a new (or nearly so) engine and not one that had a hundred hours on it.

The difference in bearing life is interesting but since they were tearing down both types of engine for overhaul in 300-400hour range (less in desert or coral island conditions) it didn't mean a lot to service squadrons except for morale. Maybe if the engine lost a bearing there was enough damage to prohibit rebuilding the engine? Either condition of the crankshaft bearing surface or condition of the block. Both physical damage (scoring) or heat damage affecting heat treatment (blue discoloration of crankshaft near bearing?).
This may mean more to the repair/overhaul shops and supply officers than to the pilots and crew chiefs.

 

[Steamed_Banana]

The two memoirs I was referring to both mentioned the crew chiefs frequently checking for 'silver fish' - metal fragments either from the bearing or the engine, and both stated that a catastrophic failure could cause a big fire in flight that could swiftly consume the aircraft. I said this already but I suspect it was due to the use of rebuild or refurbished engines which were put back into use at the regional depot (Aboukir? Cairo?).

450 Sqn was kind of the second tier RAAF P-40 squadron in the Med, with 3 RAAF typically taking the role of escort, fighter sweep and combat air patrol until the end of 1943, while from the start 450 Sqn were usually flying fighter-bomber missions. So they may have just gotten the more worn-out engines and / or fighters, or the rebuilds as I am guessing.

3 RAAF also notably switched to the Merlin engined "Kittyhawk II" in late 1942.

Another possibility is that this was just taking place during a season when there was more dust than usual, or maybe there was a problem with the filters they were using.

 

[MiTasol]

The British got that V-1710-39 on that original Allison got up to 72" Hg, which produced well over 1600 + hp just by overboosting, presumably with high octane fuel. The 45.5" Hg limit in the original manual was apparently low-balling it quite a bit. The US manuals for P-40s, P-39s, and P-51As were later revised to allow 54, 56, 57, or 60" Hg (depending on when and the specific sub-variant) as a high setting for WEP etc.

See this war-memo on the subject

E-GEH-16

The last paragraph of that link is very enlightening.

 

[ThomasP]

FWIW

As far as I can tell, the SC on the V-1710-39 (Ø9.5" impeller with 8.8:1 gearing) was only capable of generating ~62"Hg at 3000 rpm at SL when stationary. This would have given about 1660 BHP at SL on the test rig.

Using 100/130 grade the temperature of the charge should have remained cool enough to prevent detonation (knock/rough running) and pre-detonation (damaging the SC via backfire/blowback, blowing the head gaskets, and/or blowing the intake manifold off) for a short period of time.

If the engine bits and pieces were strong enough there should have been no problem running at 62"Hg MAP continuously for short periods. I do not have access to any heat rejection over time curves for the Allison so cannot predict how long, but 20 min is not beyond the realm of reasonable.

RAM effects might have allowed about 2.5"-2.8"Hg additional MAP at 300+ mph at low altitude (upto ~3,000 ft), but this would also have added enough heat to the charge that detonation would have occurred, and most likely pre-detonation after a relatively short period of time.

72"Hg MAP is, I think, out of the picture.

Incidentally, when using 62"Hg MAP with 100/125 grade the engine would have experienced serious detonation and most likely pre-detonation after a relatively short period of time.

Also, FWIW the engine single-stage single-speed SC on the early- to mid-war V-1710 was about 90% as efficient as the SC on the Merlin XX. and Merlin 45.

I think I did my math right for the above.

 

[Steamed_Banana]

The last paragraph of that link is very enlightening.

View attachment 872641

It's a bit of a mystery to me why the P-51A didn't do better than it did. I've learned about roll rate being an issue which was fixed in the B/C/D, but why didn't they go back and fix it in the A as well? I also don't think the Americans flying them in Burma and a bit in the Med were pushing the engines as hard as the British were, in spite of this guy's memo. It seemed like a type which had more potential than it reached, and it could have been flying in improved form and in some numbers far earlier. An 'improved' P-51A NA-73 type could have been very helpful in the South Pacific in 1942 and probably into 1943. The P-40s were contributing a lot, the P-51A was faster, climbed better and had better range. So potentially could have been a better escort fighter and a better interceptor.

It's almost worth another thread of it's own...

 

[Steamed_Banana]

FWIW

As far as I can tell, the SC on the V-1710-39 (Ø9.5" impeller with 8.8:1 gearing) was only capable of generating ~62"Hg at 3000 rpm at SL when stationary. This would have given about 1660 BHP at SL on the test rig.

Using 100/130 grade the temperature of the charge should have remained cool enough to prevent detonation (knock/rough running) and pre-detonation (damaging the SC via backfire/blowback, blowing the head gaskets, and/or blowing the intake manifold off) for a short period of time.

If the engine bits and pieces were strong enough there should have been no problem running at 62"Hg MAP continuously for short periods. I do not have access to any heat rejection over time curves for the Allison so cannot predict how long, but 20 min is not beyond the realm of reasonable.

The famous Dec 1942 Alllison memo indicates that they in fact approved 60" Hg as a WEP setting for the V-1710-73, but warned that a lower rating would be needed for the new higher geared versions. They estimated 1770 HP with ram, which was apparently possible with the Mustang. 70" Hg may have been pushing it but apparently it was being done in the field.

Sorry this is cut off here but that last part is noting that the company agreed to raise the WEP boost to 60" Hg at some time prior to the writing of that memo.

We've discussed all this before, my big question is how fast were these aircraft (the units in the Middle East and Australia were flying P-40s, and of course the British were flying Mustang I and II over the Channel) going at 60", 66", or 72" Hg. But nobody ever did a test at those engine settings, at least not with the P-40s. I have seen some comments about the Mustang IIs making 400+ mph near sea level in the Channel.

The (later) manual for the P-40K has 60" Hg indicated for WEP, as does the Specific Engine Flight Chart.

RAM effects might have allowed about 2.5"-2.8"Hg additional MAP at 300+ mph at low altitude (upto ~3,000 ft), but this would also have added enough heat to the charge that detonation would have occurred, and most likely pre-detonation after a relatively short period of time.

72"Hg MAP is, I think, out of the picture.

It's pretty well documented by the British Mustangs, and so is the long duration, apparently they kept pushing it further and further.

I suspect that for the British, conditions in the Channel and North Sea zones (i.e. colder air) may have helped. But it's pretty incredible that they were also doing this in Egypt / Libya and North Australia / New Guinea. North Africa does have a winter and gets cold during that time, and it's dry almost all the time (which perhaps improves evaporative cooling effects?) so I'm tempted to think maybe this was being done more in the winter, but the time period doesn't really fit. The units in question apparently figured this out from Spring to Summer of 1942, based on the timing of the memo.

SR6 I believe also pointed out that the streamlining etc. with the Mustang it may have had easier Ram effects, it's a bit beyond me but sounds plausible.

But New Guinea and South Pacific zone around say Darwin are pretty much perpetually hot and saturation level humidity. So it's hard to understand how engines endure that. Not to mention field conditions for mechanics etc. are abysmal. I assume they must have been blowing engines but when you look at the combat conditions during this period, they were absolutely up against a wall and getting slaughtered in early 1942, and this changed dramatically by mid-year. Something clearly changed and this was apparently part of that (another change was improved tactics, for example RAAF officially switching to energy fighting tactics against the Japanese units, after a very ugly incident with the commander of 75 Sqn being publicly criticized and then dying)

Incidentally, when using 62"Hg MAP with 100/125 grade the engine would have experienced serious detonation and most likely pre-detonation after a relatively short period of time.

Also, FWIW the engine single-stage single-speed SC on the early- to mid-war V-1710 was about 90% as efficient as the SC on the Merlin XX. and Merlin 45.

I think I did my math right for the above.

The Merlin XX seemed to be the most popular and effective engine for the P-40 units, no doubt about that. And doesn't seem to have suffered much at low altitude, though anecdotally the fastest down low was the P-40K (V-1710-73).

 

[MIflyer]

Early V-1710 did not have manifold pressure regulators, which in the case of the C series in the Tomahawk I sometimes resulted in the engine blowing up shortly after takeoff when novice RAF pilots used to the manifold regulator on the Merlin just shoved in the throttle and ignored the gauges. In contrast a P-40E pilot scrambling to get off the ground when the IJN attacked the PI shoved in the throttle and was distressed to find it read only 10 inches - the gauge had gone right on past the top of the scale and started another trip around the dial. Later V-1710, such as on the P-38L, had manifold pressure limiters and on at least some F-5 recon aircraft the regulator was removed because it fought with the turbo control and led to jerky flights poor for photography.

 

[Steamed_Banana]

Early V-1710 did not have manifold pressure regulators, which in the case of the C series in the Tomahawk I sometimes resulted in the engine blowing up shortly after takeoff when novice RAF pilots used to the manifold regulator on the Merlin just shoved in the throttle and ignored the gauges. In contrast a P-40E pilot scrambling to get off the ground when the IJN attacked the PI shoved in the throttle and was distressed to find it read only 10 inches - the gauge had gone right on past the top of the scale and started another trip around the dial. Later V-1710, such as on the P-38L, had manifold pressure limiters and on at least some F-5 recon aircraft the regulator was removed because it fought with the turbo control and led to jerky flights poor for photography.

The P-40K models were the first P-40s to get the automatic boost control on the throttle, which was then in many cases apparently modified in the field to increase boost ratings from the early (apparently somewhat over-conservative) settings. P-40D and E were apparently given the boost control throttles during refits in Theater depots from mid 1942 (sometimes also given the slightly heavier and more robust V-1710-73 engines from the P-40K as well).

 

[ThomasP]

Hey Steamed Banana,

Sorry, but whoever wrote the memo about 70"Hg - 72"Hg MAP was:
1. smoking something he should not have, and/or
2. fooling himself or trying to fool Material Command, and/or
3. was being given bad information

My bet is a combination of 2 and 3.

The supercharger as it existed in the -39 and -73 was simply not capable of generating 72"Hg MAP at any engine speed under 3300 rpm (I assume that is the "considerable overspeeding" the author of the memo references). Unfortunately, when using 100/130 grade, at those settings (70"Hg - 72"Hg and 3000 rpm) the charge would pre-detonate before it finished entering the cylinders for certain, and quite possibly in the intake manifold, which I assume is why the SEFC (Specific Engine Flight Chart) as "Revised: April 25, 1944" (upper left corner) has Remark #2 "use of over 60 in. manifold pressure will result in cylinder head or some other engine part failure." (my bold). Note that the SEFC as "Revised: April 25, 1944" still lists the maximum allowable MAP as 60"Hg at 3000 rpm for 1550 BHP. The SC was still basically the same in 1944 as it was earlier in the war, and was not capable of generating significantly higher MAP without overspeeding the engine (at ridiculous rpm for 70"Hg - 72"Hg MAP), which as I stated a couple of lines up would cause pre-detonation.

NOTE that some late-war V-1710 variants were cleared for use of 150 grade fuel. The (British) chart below is a list (possibly outdated by war's end) of the V-1710 models cleared for use of 150 grade as of Jan'45. Note that 60"Hg MAP is still listed as the maximum allowed. Unfortunately it does not specify which engines it is referring to re the "P-38 installation" nor does it specify the MAP.

However, another memo that I found dated Jun'44 mentions 1 squadron of P-38J had been modified for use with 150 grade and were cleared for 65"Hg MAP. (Allison had cleared the -91 engine on a test rig as good for 75"Hg, but the engine radiator cooling and inter-coolers on the P-38J would not allow more than 65"Hg to be used without damaging engines.)

NOTE that some of the modern race planes in the unlimited category do run their V-1710 engines at upto 3400 rpm and ridiculously high MAP, but the engines are significantly modified mechanically, and they have to use special fuels that won't pre-detonate (often 180 grade or higher) along with greatly increased cooling - which usually includes ADI and sometimes spraybars in front of the radiator(s) and/or aftercoolers.

 

[Shortround6]

Trying to use pressure limits from NW Europe in North Africa or the tropics in the Pacific runs into a lot of issues.

Test houses tried to use air at 59 degrees F. Air at 100 degrees F was about 92-93% as dense if everything else was the same.
Power is going to be down about 7-8% at the same manifold pressure.
Mustangs were faster than P-40s down low and could generate a little more ram.
Higher humidity could give a little cooling effect compared to dry air. Think very low grade water injection.
But that still doesn't explain the 8-10in extra boost the Mustangs were reporting unless they were over revving the engines by 300rpm or more.

Now the hotter air in desert is not only less dense, the simple fact that it is 40+ degrees hotter means it is closer to detonation limit to begin with.
If the intake air is 40 degrees hotter the temperate after the supercharger but before going into the cylinders will be 40 degrees hotter. The peak temperatures in the cylinder will be 40 degrees hotter, the exhaust gases will be 40 degrees hotter.

as far a cooling goes, it is not only the hotter air temperature, but the less dense air does not cool as well.

The other thing is that even in the test house the 62in (?) was only at sea level. 56in was the limit at 3000ft. 52in was the limit at 5500ft.
NA desert was all over the place. Sea level (or below) to over 2,000ft in some places.

I have no idea of what the humid air the Pacific will do. Better cooling or worse?

 

[MiTasol]

I have no idea of effect of humid air in the tropics would do to piston engine cooling and have never seen any references to it but one anomaly of flying in the tropics is that icing is very severe at altitude.

 

[Steamed_Banana]

I have no idea of effect of humid air in the tropics would do to piston engine cooling and have never seen any references to it but one anomaly of flying in the tropics is that icing is very severe at altitude.

The Australians were apparently limiting their Spitfires to +9 boost for a long time in the Theater due to overheating issues.

 

[Steamed_Banana]

Hey Steamed Banana,

Sorry, but whoever wrote the memo about 70"Hg - 72"Hg MAP was:
1. smoking something he should not have, and/or
2. fooling himself or trying to fool Material Command, and/or
3. was being given bad information

My bet is a combination of 2 and 3.

No offense, because you seem to be nice and to know what you are talking about, but I am comfortable that these wartime memos are correct. The phenomenon is very well attested with the Mustang at least. These two sources, the letter from the US liaison officer and the letter by Allison aircraft company to the War Dept, aren't the only sources for this. But I'm prepared to agree to disagree!

I don't think that anyone was routinely pushing these engines to 70" or 72" Hg, especially for long periods, other than the British flying Mustangs over the channel. Any other use e.g. by P-40s or P-39s would have been due to near death experiences in combat. But Again, I feel this is pretty well attested. it's been discussed at some depth numerous times.

The supercharger as it existed in the -39 and -73 was simply not capable of generating 72"Hg MAP at any engine speed under 3300 rpm (I assume that is the "considerable overspeeding" the author of the memo references). Unfortunately, when using 100/130 grade, at those settings (70"Hg - 72"Hg and 3000 rpm) the charge would pre-detonate before it finished entering the cylinders for certain, and quite possibly in the intake manifold, which I assume is why the SEFC (Specific Engine Flight Chart) as "Revised: April 25, 1944" (upper left corner) has Remark #2 "use of over 60 in. manifold pressure will result in cylinder head or some other engine part failure." (my bold). Note that the SEFC as "Revised: April 25, 1944" still lists the maximum allowable MAP as 60"Hg at 3000 rpm for 1550 BHP. The SC was still basically the same in 1944 as it was earlier in the war, and was not capable of generating significantly higher MAP without overspeeding the engine (at ridiculous rpm for 70"Hg - 72"Hg MAP), which as I stated a couple of lines up would cause pre-detonation.

We know that in emergency situations pilots in action were exceeding even 60" Hg, (the Allison memo notes 66" Hg) though eventually a compromise was reached specifically due to the authorities increasing the official WEP rating. Likely when they did overboost even to 60" Hg there were prices to be paid, but sometimes that was the difference between life and death on a given sortie.

NOTE that some late-war V-1710 variants were cleared for use of 150 grade fuel. The (British) chart below is a list (possibly outdated by war's end) of the V-1710 models cleared for use of 150 grade as of Jan'45. Note that 60"Hg MAP is still listed as the maximum allowed. Unfortunately it does not specify which engines it is referring to re the "P-38 installation" nor does it specify the MAP.

Yes that F4R probably represents the V-1710-73 on the P-40K and some earlier types re-engined. The later P-40M and N could not be boosted so hard, at least not officially.

However, another memo that I found dated Jun'44 mentions 1 squadron of P-38J had been modified for use with 150 grade and were cleared for 65"Hg MAP. (Allison had cleared the -91 engine on a test rig as good for 75"Hg, but the engine radiator cooling and inter-coolers on the P-38J would not allow more than 65"Hg to be used without damaging engines.)
View attachment 872736


NOTE that some of the modern race planes in the unlimited category do run their V-1710 engines at upto 3400 rpm and ridiculously high MAP, but the engines are significantly modified mechanically, and they have to use special fuels that won't pre-detonate (often 180 grade or higher) along with greatly increased cooling - which usually includes ADI and sometimes spraybars in front of the radiator(s) and/or aftercoolers.

Though unlikely, and perhaps reckless, these things did in fact seem to be done in combat units.

 

[Simon Thomas]

Effect of humidity is on pdf p7&8.

 

[MIflyer]

A couple of interesting pages out of the book Vees For Victory.

 

[Shortround6]

On page 124 of the book is graph for the power/rpm/pressure of the -33 engine which was rated at either 1040hp at 14,300ft or 1090hp at 13,200ft.
If you follow the military power line (3000rpm) you get to just about 1150hp at 11,500ft (11,700ft?) and going the other way, about 850hp at 20,000ft.
The graph does say just under 1700hp at sea level at 61in.
The differences in the supercharger between the -33 and -39 were not great.
The -73s were able to get the 1150hp about 300ft higher than the -39 engines and do it using 2.6 in less boost, changes in supercharger, manifold, taking out backfire screens?
If they had pushed the engine little harder they might have been able get over 1150hp at 12,000ft but that might require beefing up the engine itself and new type test.

Unfortunately on the next page after one MIflyer posted was one of the few mistakes in the book that I can find (there may be others, but this one is obvious) when a specific engine chart is shown for the -33 engine but the caption says it is for the -73 engine and makes reference to the 1550hp WER rating.

 

[Steamed_Banana]

There are anecdotally some claims of overboosting with the Hawk 81 / Tomahawk type fighters, but obviously this would have been more risky. That said, there was probably a relatively safe zone between the military power rating of 42" and into the vicinity of 50-55" Hg. As you (SR6) said elsewhere, to figure this out individual pilots or units would have to become their own test pilots, but there were some units who seem to have done that type of thing, including the AVG and 112 Sqn RAF, 3 RAAF, as well as some of the others.


By the way, I think this Specific Engine Flight chart is for later mod P-38s?

 

[Shortround6]

Yes, that is for the P-38H and J series. Please note that some of the higher power rating at low altitudes is due to the 8.10 supercharger gear needing less power to drive it and heating the air a bit less.

Also note that this engine was not really rated for 100 octane fuel. They just didn't bother to write out 100/130 fuel as the old 1940 100 octane fuel had pretty much disappeared.