Advanced French Fighters vs 1942/1943 contemporaries (2 Viewers)

Elan Vital said:

So are the French stuck with using the 12Z and GR-14R as domestic powerplants for fighters?

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Probably yes (as long as we're talking about domestic engines, see post by B Bretoal2 for the other argument), and I'd agree it's a blessing in disguise. One of the takeaways from Calum's book was that the war winners (or in any case, workhorses) were engines that already existed, with a somewhat conservative but sound basic design, and that just had a huge amount of elbow grease applied to them in countless incremental improvements. The Merlin arguably being the poster child for this argument. Conversely all the exotic concepts were largely failures, and even when they managed to enter service before the conflict ended they gave no end of trouble, such as the Sabre.

There are maybe two exceptions to this rule of thumb. First, the R-2800 that first flew after the war was already well underway in Europe, and still managed to be produced in the tens of thousands, producing not only a lot of oomph but was also well regarded wrt reliability. Secondly, concepts that were so radical and promising that even when immature they could overcome the elbow grease being applied to established engine families, like the jet engine.

z42 said:

Probably yes (as long as we're talking about domestic engines, see post by B Bretoal2 for the other argument), and I'd agree it's a blessing in disguise. One of the takeaways from Calum's book was that the war winners (or in any case, workhorses) were engines that already existed, with a somewhat conservative but sound basic design, and that just had a huge amount of elbow grease applied to them in countless incremental improvements. The Merlin arguably being the poster child for this argument. Conversely all the exotic concepts were largely failures, and even when they managed to enter service before the conflict ended they gave no end of trouble, such as the Sabre.

There are maybe two exceptions to this rule of thumb. First, the R-2800 that first flew after the war was already well underway in Europe, and still managed to be produced in the tens of thousands, producing not only a lot of oomph but was also well regarded wrt reliability. Secondly, concepts that were so radical and promising that even when immature they could overcome the elbow grease being applied to established engine families, like the jet engine.

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Yes, that's correct. But we must not forget that the development of very high-power piston engines was seriously disrupted by the advent of jet, which very quickly rendered all piston projects obsolete.

We don't know what Sabre, R-4360, and others, would have become, (plus all similar projects that major engine manufacturers had on their drawing boards), if jet engine development had been delayed by five or ten years.

Elan Vital said:

The GR-18R is at least a fairly straightforward path to a R-3350 analogue for bombers but might be too large for fighters.

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How big was the 18R exactly? Information on it seems to be relatively scarce. The 18L was similar size to the Centaurus, and the Centaurus was successfully fitted to several single-engined fighters. If the 18R is close to the 18L in terms of size, it wouldn't be infeasible to fit it on a larger French fighter - perhaps some kind of development of the MB.157?

TM06 said:

How big was the 18R exactly? Information on it seems to be relatively scarce. The 18L was similar size to the Centaurus, and the Centaurus was successfully fitted to several single-engined fighters. If the 18R is close to the 18L in terms of size, it wouldn't be infeasible to fit it on a larger French fighter - perhaps some kind of development of the MB.157?

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18 R 04 is said to be 1.406 mm in dia (about 100 mm more than 14R) , 2.141 mm length and 1.050 kg dry (about 220 kg more than 14R).

Rated power was 1.850 hp (metric) at 3.300 m. (1st gear) and 1.690 hp at 7.700 m. (2nd gear), both at 2.400 rpm and 950 mm Hg boost.

Overboost - approximately equivalent to US WEP - gave 2.100 hp at 900 m. (1st gear) and 1.760 hp at 5.600 m. (2nd gear), both at 2.500 rpm and 1.200 mm Hg manifold pressure.

Elan Vital said:

So are the French stuck with using the 12Z and GR-14R as domestic powerplants for fighters?

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No, not at all.
It is far better to have a very small number of types of engines (that are produced en masse), than to have half a dozen of so-so engines. The working 12Z and 14R can serve more than good the French between 1941 and 1943, offering the capabilities of fully-rated DB 601E/605A and BMW 801D, respectively.
The 2-stage supercharged versions of the 12Z and/or 14N would not be a long shot with France surviving healthily through these years.

We can also recall that French are not fighting alone, British will also add their weight against the Germans, and French will be also buying the US stuff.

Ironically enough, surviving France will probably mean that lend-lease is not implemented, and that is a huge can of worms of it's own. So is the Germans not attacking Soviet Union if West is still in the game.

z42 said:

There are maybe two exceptions to this rule of thumb. First, the R-2800 that first flew after the war was already well underway in Europe,

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This is not quite correct. P&W first flew an R-2800 in Vultee Y-19 on July 19th 1939. This was about 1 year after they had completed 1000hrs of ground testing. and they had just finished 3300hrs of ground testing before the first flight.
Granted they did not fly the R-2800 in a "war plane" until May 19th 1940. The XF4U needed a lot of modifications
History of the R-2800 can be illuminating. By Feb 12th 1940 they had reached 5000 hrs running time on the ground engines and the Vultee Y-19 engine/s.
By the time of the July 1st type test they had 325,000 engineering man-hours invested.
P&W were running 3 18 cylinder test engines and 1 9 cylinder test engine from Jan 1938 on.

Suggestions of French, Italian, British (or others) "just" coming up with 18 cylinder engines need to be measured against P&W's history. Maybe other companies were smarter but by how much? P&W had built 3 different successful 9 cylinder engine (and one not so successful) and 3 different 14 cylinder engines. The R-2800 used the same size cylinders as the largest 14 cylinder engine (R-2180) which was discontinued as just not being powerful enough.

The GR18L was sort of old generation and the GR18P (more details needed) and the 18R was a new generation engine (adding a center bearing means a new crankcase and a new crankshaft and slew of changes in vibration problems).
The GR18L may have been a OK solution to make good power with 87 octane fuel (actual details of service use are lacking) and 1300hp at 3400m (?) is very creditable for for a 736kg engine. Problem is that it may not have had much growth potential which forces the adoption of major changes (or a whole new engine).
Use the same size bore as the GR14s and the Bristol Mercury and Pegasus. Cost savings both for tooling and engineering. They knew how the 146mm cylinders cooled, mostly.
Problems for the GR18s was they used a longer stroke, about 1/2 way between the GR14s/Mercury and the Pegasus. Easy gain in displacement but higher piston speed.
Lack of cooling really limits potential gains in power with better fuel. And even with more cooling fins the light weight limits increases in boost/chamber pressures.
Just using a two speed supercharger doesn't gain much. Wiki figures are from 1936(?)
figures from a 1938 edition of Jane's are bit better but need a bit of interpretation.
GR18L-01 rated at 1300hp at 1600m at 2150rpm.
Take-off was 1400hp at 2150rpm
Max power was 1360hp at 2800m at 2150rpm

GR18L-03 was rated at 1300hp at 3700m at 2170rpm
No take off rating given
Max power was 1360hp at 4900m at 2170rpm.

An increase of 20rpm is not enough to explain the increased altitude, misprint?
Just for comparison an early R-2800 was making 1500hp at 2600rpm at 4270m using 100/100 and weighed about 50% more.

The -03 engine may have had a better supercharger (?) but I have my doubts about take off power. Any single speed supercharger than peaked at 4900 meters with 87 octane lost hundreds of horsepower at sea level.
Numbers just seem to be too good for the engines weight.
Even the post war GR18R-12 seems a little too good 100kg lighter than an R-2800C series commercial engine and 350kg lighter than an R-3350 BD series engine.
American engineers were that bad or French engineers were that good? or French salesmen tended to exaggerate at a sales dinner after a few bottles of wine?
Or French engines needed replacement/overhaul sooner?

FWIW the French showed considerable interest in the P&W 2800 and ordered a few examples to start figuring out the most compatible domestic airframes for it - or design a new airframe.

As part of the orders in America to supplement French production until it could catch up, the French ordered the V-1710, the P&W 1830 and Wright 1820, slightly smaller radials, and the Wright R-2600. The latter would match that of the latest US aircrafts on French orders like the Martin 187 while also being a step up from the GR-14N and a supplement to 14R in French bombers.

The French factories that had originally been meant to produce Merlins and Hercules had been repurposed to focus on domestic designs, though it was hoped that in 1941 and 1942 they could produce the latest engine types (the Vulture had been considered since nobody knew the Merlin would progress so much, and Hercules VI). Shortly before the French defeat, the agreements for US production of the Merlin did include the French who would take part of the production.
Aside from the US Merlin which could be eventually delivered in large numbers, I think the other British types would most likely have been abandonned simply because mass production would start too late compared to the incoming US engines and reinforcing the production of domestic types.

The matter of continued US assistance (even without Lend-Lease) is complicated. French gold and dollar reserves were not infinite and had to be spent wisely, just as with the UK they are basically good enough for 1941-42 tops. The French did order ground equipment OTL at the end of May but that was in a particularly disastrous situation when major losses of French industrial capacity were expected. If things go well enough, they may not ask too much from the US in that area as long as the domestic ground production facilities are still not maxed out.
In the aero industry however, it so happened that the makeup of French orders in the US led to a virtuous division of labor. The US Martin and Douglas bombers neatly filled a gap between the light-ish Bloch 170 series and the medium B4 bombers. The H-75, H-81 and P-39 fighters were on order but total quantities are still modest compared to what will be accumulated in 1941-42 (1500-ish US fighters). It would make sense to continue buying the Martins and Douglas bombers rather than set up production of French analogues, while progressively reducing the input of US fighters.

Shortround6 said:

The GR18L was sort of old generation and the GR18P (more details needed) and the 18R was a new generation engine (adding a center bearing means a new crankcase and a new crankshaft and slew of changes in vibration problems).
The GR18L may have been a OK solution to make good power with 87 octane fuel (actual details of service use are lacking) and 1300hp at 3400m (?) is very creditable for for a 736kg engine. Problem is that it may not have had much growth potential which forces the adoption of major changes (or a whole new engine).
Use the same size bore as the GR14s and the Bristol Mercury and Pegasus. Cost savings both for tooling and engineering. They knew how the 146mm cylinders cooled, mostly.
Problems for the GR18s was they used a longer stroke, about 1/2 way between the GR14s/Mercury and the Pegasus. Easy gain in displacement but higher piston speed.
Lack of cooling really limits potential gains in power with better fuel. And even with more cooling fins the light weight limits increases in boost/chamber pressures.
Just using a two speed supercharger doesn't gain much. Wiki figures are from 1936(?)
figures from a 1938 edition of Jane's are bit better but need a bit of interpretation.
GR18L-01 rated at 1300hp at 1600m at 2150rpm.
Take-off was 1400hp at 2150rpm
Max power was 1360hp at 2800m at 2150rpm

GR18L-03 was rated at 1300hp at 3700m at 2170rpm
No take off rating given
Max power was 1360hp at 4900m at 2170rpm.

An increase of 20rpm is not enough to explain the increased altitude, misprint?
Just for comparison an early R-2800 was making 1500hp at 2600rpm at 4270m using 100/100 and weighed about 50% more.

The -03 engine may have had a better supercharger (?) but I have my doubts about take off power. Any single speed supercharger than peaked at 4900 meters with 87 octane lost hundreds of horsepower at sea level.
Numbers just seem to be too good for the engines weight.
Even the post war GR18R-12 seems a little too good 100kg lighter than an R-2800C series commercial engine and 350kg lighter than an R-3350 BD series engine.
American engineers were that bad or French engineers were that good? or French salesmen tended to exaggerate at a sales dinner after a few bottles of wine?
Or French engines needed replacement/overhaul sooner?

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Here are some figures given by official Gnome et Rhône documentation.

The 18L 00/01 (these two engines are identical but rotate in opposite directions) is said "semi-supercharged," equivalent to the British MS or moderately supercharged.

It produces 1,220 hp (metric) on ground and 1,300 hp at rated altitude of 1,600 m, with 877 mm Hg manifold pressure.

Overloaded at takeoff, 1,400 hp with 948 mm Hg.

All these values are at 2,150 rpm.

The 18L 02/03 is normally supercharged.
It produces 1,125 hp on ground and 1,300 hp at rated altitude of 3,700 m, with 865 mm Hg manifold pressure.

Overload at takeoff, 1,235 hp with 935 mm Hg.

All these values were at 2,170 rpm.

The maximum power of 1,360 hp at 4,900 m given by some sources is a theoretical figure obtained with RAM effect at 450 km/h. In other words, it's worthless. Idem for the "1,360 hp at 2,800m." for the 18L 00/01.

In fact, compared to the 18L 00/01, the 02/03 had a higher compression ratio (6.1 instead of 5.5), a new valve timing diagram, and, above all, a significantly modified supercharger : the impeller diameter was reduced from 350 to 330 mm, but it rotated at 8.42 times the crankshaft speed, compared to 5.65 times in the 18L 00/01.

But these engines were clearly condemned by their lack of a central crankshaft bearing, and their finning which would prove barely sufficient for the 14N while the cylinder individual displacement was 10% higher. In fact, the 18P and R integrated, like the 14P and 14R compared to the 14N, a very reinforced crankshaft with central bearing, extensively modified and reinforced crankcase and a much increased finning. Plus obviously a much more efficient supercharger coupled with a two-speed gear. Only these developments allowed the performance gain displayed by the 18R.

And don't worry about American engines performance comparison ; while the figures given above are most likely accurate, nothing is said about the reliability and robustness of these engines. Making them lighter and more powerful is (relatively) easy; doing so for dozens or hundreds of hours is another matter. And besides, the reliability of the R-2600 was legendary !

Thank you for the detailed reply.
The English Wiki entry is far from clear about when the center bearing was to be introduced although the increase from under 2200rpm to 2400 rpm for the 18P is a strong indicator.
Unfortunately that does put a real crimp into getting the 18P into service in 1942-43 as it is basically a new engine.
In the US it took just about 3 years to go from start of design to 5th engine delivered for new engines. Some took longer, very few were shorter. Revisions were more varied and often depended on what else was going on. P&W took 1 1/2 years (starting in May 1940) to go from the A series R-2800 (1850hp) to the B series (2000hp) and 3 1/4 years also starting in May 1940 to get to the C Series R-2800 engines used in the later P-47s and F4U-4s. Granted P&W was also busy expanding, working on improved R-1830s and introducing the R-2000 (that took 3 years of low priority to change the bore size from 140mm to 146mm).
It took Wright about 2 1/2 years to go from the A series R-2600 to the B Series R-2600 (1600hp to 1700hp) when they changed the crankcase from a
aluminum to steel. Wright also had other projects in hand.
A bit like Hispano getting side-tracked with the radials and having to come back to the V-12s. Work had not stopped but where was the effort in the mid/late 30s? Trying to play catchup in 1939-40 was going to take a lot of effort.

Shortround6 said:

Thank you for the detailed reply.
The English Wiki entry is far from clear about when the center bearing was to be introduced although the increase from under 2200rpm to 2400 rpm for the 18P is a strong indicator.
Unfortunately that does put a real crimp into getting the 18P into service in 1942-43 as it is basically a new engine.
In the US it took just about 3 years to go from start of design to 5th engine delivered for new engines. Some took longer, very few were shorter. Revisions were more varied and often depended on what else was going on. P&W took 1 1/2 years (starting in May 1940) to go from the A series R-2800 (1850hp) to the B series (2000hp) and 3 1/4 years also starting in May 1940 to get to the C Series R-2800 engines used in the later P-47s and F4U-4s. Granted P&W was also busy expanding, working on improved R-1830s and introducing the R-2000 (that took 3 years of low priority to change the bore size from 140mm to 146mm).
It took Wright about 2 1/2 years to go from the A series R-2600 to the B Series R-2600 (1600hp to 1700hp) when they changed the crankcase from a
aluminum to steel. Wright also had other projects in hand.
A bit like Hispano getting side-tracked with the radials and having to come back to the V-12s. Work had not stopped but where was the effort in the mid/late 30s? Trying to play catchup in 1939-40 was going to take a lot of effort.

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Which wiki are you talking about?

Generally speaking, there are some quality entries on Wiki, but there are also a large number of errors, approximations and untruths.

Among the most recent ones I read.... It is about the VK 105 (Soviet derivative of the Hispano 12Y): "The M-105 was the first Klimov V-12 engine design to use reverse-flow cylinder heads, forcing the induction system to be placed on the outside of the cylinder banks, with the exhaust system also exiting from the outboard side, with twin sets of "siamesed" exhaust ports adjacent to each other." This is completely stupid, as the clearing of the central V-space by grouping intake and exhaust ports outside the engine has been an unchanged feature of Hispano-Suiza V-12 engines since the early 1920s. It also existed, of course, in the M-100 and 103.

One example among many other ones !

Shortround6 said:

Thank you for the detailed reply.
The English Wiki entry is far from clear about when the center bearing was to be introduced although the increase from under 2200rpm to 2400 rpm for the 18P is a strong indicator.
Unfortunately that does put a real crimp into getting the 18P into service in 1942-43 as it is basically a new engine.
In the US it took just about 3 years to go from start of design to 5th engine delivered for new engines. Some took longer, very few were shorter. Revisions were more varied and often depended on what else was going on. P&W took 1 1/2 years (starting in May 1940) to go from the A series R-2800 (1850hp) to the B series (2000hp) and 3 1/4 years also starting in May 1940 to get to the C Series R-2800 engines used in the later P-47s and F4U-4s. Granted P&W was also busy expanding, working on improved R-1830s and introducing the R-2000 (that took 3 years of low priority to change the bore size from 140mm to 146mm).
It took Wright about 2 1/2 years to go from the A series R-2600 to the B Series R-2600 (1600hp to 1700hp) when they changed the crankcase from a
aluminum to steel. Wright also had other projects in hand.
A bit like Hispano getting side-tracked with the radials and having to come back to the V-12s. Work had not stopped but where was the effort in the mid/late 30s? Trying to play catchup in 1939-40 was going to take a lot of effort.

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If G&R can get off their asses during the mid-30's, they probably could get the 18P/R into mass production c. 1941~1942.
A possible way to free up resources would be getting rid of the small and underpowered 14M. Most of the aircraft that used the 14M were light fighters - which we're culling in their entirety in this theoretical - and the ones that weren't (Potez 630, NC.600, Br.693) could likely be easily transferred to the 14N instead.
Preventing the existence of the stillborn Hispano radials would also allot more resources to G&R since the two companies wouldn't be competing for contracts.

TM06 said:

If G&R can get off their asses during the mid-30's, they probably could get the 18P/R into mass production c. 1941~1942.
A possible way to free up resources would be getting rid of the small and underpowered 14M. Most of the aircraft that used the 14M were light fighters - which we're culling in their entirety in this theoretical - and the ones that weren't (Potez 630, NC.600, Br.693) could likely be easily transferred to the 14N instead.
Preventing the existence of the stillborn Hispano radials would also allot more resources to G&R since the two companies wouldn't be competing for contracts.

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Some people seem to think that the G&R 14R or 18R are "new" engines; this is not true. They are simply evolutions of previous models, through successive improvements, sometimes minor (various reinforcements), sometimes more significant, such as a third bearing, a two-speed S/C, etc.

Finning surface increase is undeniable, but it is achieved using a same technology, unchanged from 1930 to 1950 (casting of the cylinder heads in steel shells, not sand, according to the Bruneau frères foundry process).

Also, the first versions of the 14R (00/01 and 02/03) had the 14P previous S/C with tangential inlet; it started only with the 14R 04/05 that the engine received a new, high-efficiency central inlet S/C.

All this to say that these progressive developments made it possible to boost production rates very quickly from the moment a type definition was established.

By the summer of 1938, Hispano-Suiza had been eliminated from the air-engine race, and Gnome-Rhône was at ease on this front - the company's fears centered more on competitors such as Bristol or P&W. At the start of the conflict, new versions of the HS 14AA and 14AB with new oil circuits, Sarrazin dynamic dampers, and extensively modified finning were beginning to appear, but these engines hadn't really proven their worth, and it would undoubtedly have taken a lot of work for the Official Services to reverse their negative view of these models.

About adapting engines other than the 14M to airframes.... keep in mind that these engines were very, very small - only 96 cm in diameter, or 34 cm less than the 14N! Adapting the 14N had been attempted on the Breguet 693: the result (the Breguet 700) was a monster that was difficult to fly. In addition to visibility issues, achieving smooth and balanced controls would have required a complete redesign of the wings and engine nacelles. This went far beyond simply adapting the engines.

Another problem to solve was the installation of larger-diameter propellers, which was often impossible due to the engine center distance and the height of the landing gear. Consider the Whirlwind, condemned to remain attached to its Peregrines by simple questions of geometry!

TM06 said:

Most of the aircraft that used the 14M were light fighters - which we're culling in their entirety in this theoretical - and the ones that weren't (Potez 630, NC.600, Br.693) could likely be easily transferred to the 14N instead.

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Actually they weren't. Two light fighters (single engine) used the 14M but they only built 1 of each (?).
for some reason the French were enamored of light twins of many types. Light twin fighters, light twin recon planes, light twin attack or light bombers.
And they were truly small/light which means, as Bretoal2 has stated, they are going to be very difficult to install 14N engines in. You might as well start over.
The Potez 630 was, as far as weight goes, about a 75-80% Blenheim (Potez was under 10,000lbs loaded, Blenheim I was 12,500lbs).
Br.693

was about 10,800lbs. French were not happy with the results when they built some with P&W R-1535 engines which were larger in diameter and heavier than the 14Ms but were small compared to the 14K/N. Please note that the Engines are very close to the nose and Sticking bigger engines in is really going to screw up the CG.
This thing was about 2/3rds the weight of a Douglas DB7 with the original R-1830 engines.
If you spend a lot of time and effort you might actually get something useful at the end. You may also take a shortcut and start over.

The attraction to small aircraft was dictated by a lack of foresight. No one in France had foreseen that the mass of military aircraft would increase very rapidly at the outbreak of hostilities—and therefore the need for power.

There were also financial considerations. Smaller means cheaper, easier, and faster to build, so for a given amount of money, one could hope to have more aircraft in a shorter time. The same mistake had been made in the 1920s with the " jockey fighter " program, which was a total fiasco.

Bretoal2 said:

Yes, that's correct. But we must not forget that the development of very high-power piston engines was seriously disrupted by the advent of jet, which very quickly rendered all piston projects obsolete.

We don't know what Sabre, R-4360, and others, would have become, (plus all similar projects that major engine manufacturers had on their drawing boards), if jet engine development had been delayed by five or ten years.

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Agreed, but even with jet engines delayed I don't think these more "exotic" piston engines would have made much more of an impact during WWII than they did historically. Just not enough time to properly debug them and then deploy in numbers. Now after WWII with continued development some of them would probably have been developed into solid engines, and the Korean War might have looked very different with various "superprops" contending for air superiority instead of early jets.

z42 said:

Agreed, but even with jet engines delayed I don't think these more "exotic" piston engines would have made much more of an impact during WWII than they did historically. Just not enough time to properly debug them and then deploy in numbers. Now after WWII with continued development some of them would probably have been developed into solid engines, and the Korean War might have looked very different with various "superprops" contending for air superiority instead of early jets.

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I think the Piston engine had about reached it's limits by 1945. This assumes that the fuel stays at about 145/145.
They managed to stretch things a bit in the late 40s but the power to weight ratios of the engines didn't get much better. And things got rather complicated from a maintenance standpoint.

With 3 supercharger impellers this managed to beat the 1hp per pound using 145/145 fuel and water injection.
The P&W R-4360 28 cylinder engine managed to get about 3500hp in service but it was a 3500-3700lb engine not including turbochargers needed for high altitude flight.
Wright was using turbo compound engines for better fuel economy but a 3250hp R-3350 was about 3300lbs and that is with a 2 speed supercharger.
France was sticking four Jumo 213 cylinder banks on on crankcase to get a 24 cylinder 4000hp engine but the engine weighed just over 4000lbs.
In 1949 RR was offering the Eagle 22, a 24 cylinder sleeve valve engine offering 3500hp at low altitude using 21lbs of boost from a 3900lb engine.

You need an airframe that can hold these engines and you need the fuel capacity to feed them. Some people were working on "Superprops" but they were also running into limits on propellers. You need a really good prop to go over 500mph. The Prop blades go supersonic well before the airframe does and the efficiency of the prop drops off sharply.
Even in the 400-500mph speed range they were having troubles with props. They got contra-rotating props to work but it took several years longer than they thought.
In 1944-45 several American experimental planes were actually faster with 4 blade single props than contra-rotating props. Often only single digits but the contra-rotating props were heavier, more expensive.

Bretoal2 said:

. Consider the Whirlwind, condemned to remain attached to its Peregrines by simple questions of geometry!

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Westlands offered a variety of alternative engines from Allisons to Merlins. Even considered the Welland. Their Airships preferred Spitfires and Typhoons and would not consider any development of the Whirlwind which remained viable into 1943 when they last used them in front line service.

The two modern French bombers, Amiot 354 and Lioré-et-Olivier LeO 45, were crying out for larger, more powerful engines from the start. With 7210 and 710 sq ft wings respectfully and max gross weights just either side of 25,000lbs 1060hp (?) T-O power engines were too small. That is what they wound up with, the H-S 14AA engines they had been designed around had failed. The 14AA was a 45.2 liter (2788cu in) engine compared to the G-R 14K/N 39.7liter (2360cu in) and one should have expected the larger engine to be developed to make more power as development progressed.
Please note that both planes got at least one installation of P&W R-1830s much later (post war).
More powerful engines might have allowed for a greater fuel/bomb payload. The LeO 451 had room for a fair bomb load but then it couldn't carry much fuel. 1200-1400kg bomb loads were not enough for the main French bomber types. He 111s could carry 2000kg inside.

Perhaps (if they were reliable) the G-18 engines, even at 1300-1400hp, would have been useful.

Shortround6 said:

The two modern French bombers, Amiot 354 and Lioré-et-Olivier LeO 45, were crying out for larger, more powerful engines from the start. With 7210 and 710 sq ft wings respectfully and max gross weights just either side of 25,000lbs 1060hp (?) T-O power engines were too small. That is what they wound up with, the H-S 14AA engines they had been designed around had failed. The 14AA was a 45.2 liter (2788cu in) engine compared to the G-R 14K/N 39.7liter (2360cu in) and one should have expected the larger engine to be developed to make more power as development progressed.
Please note that both planes got at least one installation of P&W R-1830s much later (post war).
More powerful engines might have allowed for a greater fuel/bomb payload. The LeO 451 had room for a fair bomb load but then it couldn't carry much fuel. 1200-1400kg bomb loads were not enough for the main French bomber types. He 111s could carry 2000kg inside.

Perhaps (if they were reliable) the G-18 engines, even at 1300-1400hp, would have been useful.

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The Amiot 350 was originally planned to be powered by Gnome & Rhône 14Ps, of the 1200/1300 hp class. These engines were never produced aside from a few prototypes, and their certification failed. Their successor, the 14R, was just entering production when war broke out. The Amiot prototype equipped with them had not yet been flown by June 1940 (as the HS 12Z version).

Ultimately, the fastest Amiot 350 was the version powered by two Rolls-Royce Merlin X, which reached a top speed of nearly 500 km/h.

The LeO 451 was initially powered by 1,120 hp Hispano-Suiza 14AAs; Flight tests were plagued by these engines (50 flight hours in one year, with 13 engines "burnt out"...) and replaced by G&R 14N 20/21s, the most powerful versions available in 1938.

The LeO 455 with 1,320 hp 14R engines had been flying since December 1939, but its tests were very slow... the engine installation was still not perfected in June 1940.