FIAT A.S.8 ENGINE.

[sibboh]

It is very hard to compare superchargers in this way. Superchargers can be measured in airflow (pounds of air per minute), Pressure ratio ( how much they compress the incoming air) and efficiency (what percentage of the power used to drive the supercharger is actually compressing the air).

When looking at engine charts you are looking at a complete engine system that has to take into account the strength of the engine (blowing cylinders off is not a good thing), the fuel used (only so much boost can be used with certain fuels) AND the desired characteristics of the engine in question.

as an example the British Mercury radial could make 830hp for take off and 890hp at 1800 meters using a 7.0:1 gear drive on the supercharger (using about 914mm) , great for a flying boat or seaplane. Change the gear ratio to 9.4:1 and high level power went to 840hp at 4250 meters but take-off fell to 725hp. (using just over 1000mm) Same supercharger intake, impeller and housing/diffuser. Much better for a fighter or light bomber.

At 4250meters the Mercury supercharger was compressing the air 2.27 times and lets face it, the Mercury supercharger wasn't exactly the latest and greatest.

Just about anybody could design a supercharger with a pressure ratio of over 2 to 1 in the 30s and by the late 30s 2.3-2.8 was fairly common. The two big problems where getting the engine to stand up to it and getting fuel that allowed you to use a boost pressure of over 4-6lbs, 38-42 absolute or the metric equivalent.

Italian radial engines were rather light for their size ( in some cases very light) which leads to suspicions of a lack of structural strength to tolerate higher RPM or over boosting.

Lack of planning in 1939-34 (or later) is a bit understandable, the fuel situation changed dramatically. France was another country that got caught with engines that were fine with 80 octane fuel, OK with 87 octane but near hopeless with 100 octane or higher. The basic engines were not strong enough to stand up to higher boost pressures with out either major redesign or starting over.

German engines used 87 octane fuels too.

 

[sibboh]

@Shortround6: could you give me any example about rather light or very light italian engines? And other engines (US, UK, Germany...) for comparison. So i can understand.

 

[Dogwalker]

1) Example please.

Both the Piaggio P.XI and the Fiat A.80 were homologated in 1937. So there was not an Italian radial in the class of 1000 hp first. In late 1939 were homologated both the 1500 hp Piaggio P.XII and the 1600 hp Alfa Romeo 135 (which was never ready for operational use, but complied the approval tests), so...

2) Number of the page please.

146.

 

[Shortround6]

@Shortround6: could you give me any example about rather light or very light italian engines? And other engines (US, UK, Germany...) for comparison. So i can understand.

Using the P W R-1830 as a 'standard' it started at about 1135lbs but jumped to about 1235lbs for the first production versions. RPM was about 2400. Then came the 1310 lb engines with a take-off rpm of 2600. these were followed bu the much more common
1403-1478lb versions (depending on prop gear ratio and accessories) that ran at 2700rpm for take off and 2550rpm max continuous. Power had gone from 800hp or a bit above to 1200hp. Displacement had stayed the same at just about 30 liters.

The Fiat A 74 was a 31.2 liter engine that went about 1257lbs.
The Piaggio P.XI was a 38.6 liter engine that went about 1433lbs.
The older French Gnome-Rhone K was about 1190lbs for 38.6 liters
The French Gnome-Rhone N was the same displacement but heavier.
British Armstrong Siddeley Tiger engine was 32.7 liters for 1290lbs, it did include a two speed supercharger.

The last four engines all used a two bearing crankshaft which made both the crankshaft and crankcase smaller and lighter but restricted the ability of the engine to use higher pressures or rpm compared to and engine with a crank bearing between the cylinder rows.

Moving to bigger engines the Americans had the;

Wright R-2600=42.7 liters at about 1965lbs
P W R-2800 = 45.9 liters at about 2,300lbs
Wright R-3350= 54.9 liters at about 2400lbs (early version).
Wright R-3350= 54.9 liters at about 2780lbs (later version)

Italians had the:

Alfa 135 = 48.2 liters at about 2094lbs
Fiat A 80 = 45.7 liters at about 1625lbs
Fiat A82 = 47.1 liters at about 1910lbs
Piaggio P.XII =53 liters at about 1874lbs
Piaggio P.XXII =60.5 liters at about 2094lbs

On a weight to displacement ratio the Italian engines were on the light side. Figures are pretty much From Wilkinson's "Aircraft engines of the World" and may not agree with Italian sources.

Fiat A80 engine was about the same displacement as the P&W R-2800 and yet weighed about 70% as much. Something had to give?

 

[sibboh]

Both the Piaggio P.XI and the Fiat A.80 were homologated in 1937. So there was not an Italian radial in the class of 1000 hp first. In late 1939 were homologated both the 1500 hp Piaggio P.XII and the 1600 hp Alfa Romeo 135 (which was never ready for operational use, but complied the approval tests), so...

146.

I thought you did not understand what I meant. I'm sorry that I have not explained well. I thought I had explained it well. Subsequent interventions Shortround6 (04-28-2013 04:28 AM) and wuzak (04-28-2013 06:13 AM) made me think that.
When i speak about "lack of improvements", i want to tell about improvement of a single engine type (for example: Merlin improvements, R-1830 impreovements, etc... during the years). But i don't want to tell about improvement in the whole industrial production. For me another engine is another engine. Piaggio P.XIX was a P.XI improvement, as the P.XI was a G-R 14Krs improvement. Certainly P.XII was a P.XI derivative. But this is another thing, for me. I'm italian and i have to translate my thoughts.
I hope I have explained it well now.

About page 146. It's that.

The controversial part says:
"NOTA - La sovrapressione d'alimentazione è sempre ottenibile durante i decolli effettuati a quota zero oppure ad altitudini che non meritano grande considerazione agli effetti della diversa pressione atmosferica esistente fra tali quote e la stessa quota zero.
Durante i decolli effettuati a quote superiori alla predetta, il valore di sovrapressione scenderà, sino a raggiungere, durante decolli effettuati da un altipiano (p es. 2000 m.}, la pressione normale d'alimentazione assicurata automaticamente dal compressore.
Decollando da quest'ultima quota presa in considerazione, la sovrapressione non sarà possibile ottenerla, poichè la farfalla del compressore — a motore funzionante a pieni gas — si aprirà automaticamente per un certo angolo che sarà sempre maggiore da quello ottenibile manovrando la leva di sovrapressione a quota zero o di non molto superiore. In quest'ultimo caso perciò il comando del congegno di sovrapressione, atto ad ottenere la maggior apertura della farfalla durante il decollo, si renderà inutile poichè la leva 187 si sposterebbe a « folle » senza imprimere l'ulteriore apertura a mano della farfalla."
TRANSLATION:
"NOTE - The supply pressure is always obtainable during takeoffs made at zero or at altitudes that do not deserve great consideration to the effects of different atmospheric pressure existing between these altitudes and the same zero level.
During takeoffs made at higher altitudes to the aforesaid, the value of pressure drop, to reach, during take-offs made by a plateau (for example, 2000 m), the normal pressure power ensured automatically by the compressor.
Taking off from the latter altitude taken into account, the overboosting will not be possible to obtain it, because the throttle of the supercharger - with the engine running at full gas - will open automatically to a certain angle that will always be greater than that obtainable by manipulating the overboosting lever at zero altitude or not much higher. In the latter case, therefore, the control of the overboosting device, able to get wider throttle opening during takeoff, it will be useless because the lever 187 would move to "neutral" without giving further handmade throttle opening."

I told (05-10-2013 04:04 PM): "I see only a problem about takeoff in plateaus up to 2000 m. But takeoff, not overboost at altitude. The power chart tell me other."
And infact the manual at page 146 tell only about takeoff. At this point I think that you are not Italian.

Another thing:
I don't know when A.80 were homologated. But i know that CMASA B.G.A. first flight was in 1936, Fiat BR.20 first flight was in 10 february 1936, Breda Ba.82 first flight was during 1937. Then the BR.20 production and deliveries began during the same 1936. It's strange that the Fiat A.80 was homologated after the first deliveries.
Infact the first 20 units (MM20305-MM20324) were produced between october 1936 and february 1937.
Piaggio "P.XI RC40" was homologated in 1936. Piaggio "P.XI RC40 bis" in 1937.

 

[sibboh]

The Fiat A 74 was a 31.2 liter engine that went about 1257lbs.
The Piaggio P.XI was a 38.6 liter engine that went about 1433lbs.

Italians had the:
Alfa 135 = 48.2 liters at about 2094lbs
Fiat A 80 = 45.7 liters at about 1625lbs
Fiat A82 = 47.1 liters at about 1910lbs
Piaggio P.XII =53 liters at about 1874lbs

On a weight to displacement ratio the Italian engines were on the light side. Figures are pretty much From Wilkinson's "Aircraft engines of the World" and may not agree with Italian sources.

Fiat A80 engine was about the same displacement as the P&W R-2800 and yet weighed about 70% as much. Something had to give?

The Fiat A 74 dry weight was (A.74 manual) 590 kg, that is 1301 lbs. And not 1257lbs.
The Piaggio P.XI dry weight was (P.XI manual) 700 kg, that is 1543 lbs. And not 1433lbs.

Alfa 135 displacement is 49.722 liters (a double Bristol Mercury) and not 48.2 liters. Its weight was 990 kg, that is 2183lbs, and not 2094lbs.
Fiat A 80 weightwas 751 kg, that is 1655lbs, and not 1625lbs.
Piaggio P.XII weight were 850 and 863 kg (the difference is between left and right engine), that are 1874 and 1903 lbs.

Fiat A80 engine was an engine with a too weak supercharger. And the rpm were very slow.

 

[sibboh]

Now i post a comparison chart between Fiat A.74 and different R-1830 ( -9, -11, -13, -35 ) versions between 1936 and 1940.
We see the R-1830 improvement. And its boost improvement.
There are some problems with different data versions. But I hope it is understood the meaning of my speech.

 

[Dogwalker]

I thought you did not understand what I meant. I'm sorry that I have not explained well. I thought I had explained it well. Subsequent interventions Shortround6 (04-28-2013 04:28 AM) and wuzak (04-28-2013 06:13 AM) made me think that.
When i speak about "lack of improvements", i want to tell about improvement of a single engine type (for example: Merlin improvements, R-1830 impreovements, etc... during the years). But i don't want to tell about improvement in the whole industrial production. For me another engine is another engine. Piaggio P.XIX was a P.XI improvement, as the P.XI was a G-R 14Krs improvement. Certainly P.XII was a P.XI derivative. But this is another thing, for me. I'm italian and i have to translate my thoughts.
I hope I have explained it well now.

In the span 1937-1940, on the DB600-601 and on the Merlin, we see mostly the tevelopement from prototypes to mass production (and, on the Merlin, those to adapt it to the 100 octane gasoline). The R-1830 had surely an evolution, but that's part of an evolution started in 1932. That's similar to the evolution of the Alfa Romeo 125-128 series (basically the same engine) from 650 to more than 950 hp. Moreover, the evolution of the Alfa was done mantaining the same gasoline, while the later modification on the R-1830, as your chart show, were meant to use a different fuel (it's true hat the power output of the r-1830 grew up even with 87 octane gasoline, but much less). In Italy, there would be useless to design an engine to run on 100 octane fuel (let alone 130). In wartime, a mass produced 100 octane italian engine simply would not have flown due to the lack of fuel. An evolution similar to that of R-1830 there would have been if it had been completed the A.76. but it was dropped in 1940 not so much because it was impossible to get, but because, with just over 1000 hp (similar to R-1830-SC3-G, which worked with 87 octane gasoline), in 1940, With the licence production of the DB601 already obtained, the next generation of fighter already ordered with it (and the Alfa Romeo 135 already homologated for 1600 hp max, if someone would want to present a radial engined fighter anyway), the engine would have been born already obsolete. It was the very same concept of "lightweight engine for fighters" to be obsolete and to have been replaced by that of "powerful engine for fighters".

The controversial part says:
"NOTA - La sovrapressione d'alimentazione è sempre ottenibile durante i decolli effettuati a quota zero oppure ad altitudini che non meritano grande considerazione agli effetti della diversa pressione atmosferica esistente fra tali quote e la stessa quota zero.
Durante i decolli effettuati a quote superiori alla predetta, il valore di sovrapressione scenderà, sino a raggiungere, durante decolli effettuati da un altipiano (p es. 2000 m.}, la pressione normale d'alimentazione assicurata automaticamente dal compressore.
Decollando da quest'ultima quota presa in considerazione, la sovrapressione non sarà possibile ottenerla, poichè la farfalla del compressore — a motore funzionante a pieni gas — si aprirà automaticamente per un certo angolo che sarà sempre maggiore da quello ottenibile manovrando la leva di sovrapressione a quota zero o di non molto superiore. In quest'ultimo caso perciò il comando del congegno di sovrapressione, atto ad ottenere la maggior apertura della farfalla durante il decollo, si renderà inutile poichè la leva 187 si sposterebbe a « folle » senza imprimere l'ulteriore apertura a mano della farfalla."
TRANSLATION:
"NOTE - The supply pressure is always obtainable during takeoffs made at zero or at altitudes that do not deserve great consideration to the effects of different atmospheric pressure existing between these altitudes and the same zero level.
During takeoffs made at higher altitudes to the aforesaid, the value of pressure drop, to reach, during take-offs made by a plateau (for example, 2000 m), the normal pressure power ensured automatically by the compressor.
Taking off from the latter altitude taken into account, the overboosting will not be possible to obtain it, because the throttle of the supercharger - with the engine running at full gas - will open automatically to a certain angle that will always be greater than that obtainable by manipulating the overboosting lever at zero altitude or not much higher. In the latter case, therefore, the control of the overboosting device, able to get wider throttle opening during takeoff, it will be useless because the lever 187 would move to "neutral" without giving further handmade throttle opening."

I told (05-10-2013 04:04 PM): "I see only a problem about takeoff in plateaus up to 2000 m. But takeoff, not overboost at altitude. The power chart tell me other."
And infact the manual at page 146 tell only about takeoff. At this point I think that you are not Italian.

A questo punto mi chiedo io se sappia leggere tu l'italiano. Quella nota dice chiaramente che la leva che comanda la sovrapressione è inutile sopra i 2000 metri, ed è perciò impossibile ottenere una sovrapressione, perché il controllo automatico (che deve garantire gli 821 mm hg), da quella quota in poi ha già aperto la valvola a farfalla più di quanto la leva possa fare.
at this point I wonder if you can read Italian. That note says clearly that the lever controlling the overpressure is useless over 2000m, and is therefore impossible to obtain an overboost, since the automatic control of pressure (which maintains the 822 mm hg), from that altitude onwards, has already opened the intake valve of the supercharger more than the lever can do.
So I asked to you if there are listed, on the manual, other systems to obtain an overboost, since the only one mentioned is useless above 2000m.
Note that do not seems to me that, on the manual, the overboost being allowed in any other flying condition than the takeoff.

 

[Bretoal]

Hello
It seems that the latest debates create confusion between different questions.

To clarify things:

1) 87 octane fuel and 100 have the same energetic power. But 100 octane fuel allows a higher compression ratio and this is the only reason for the increased thermodynamic efficiency.

2) The inlet pressure is not the only criterion affecting the power, there is also charge temperature.

3) Point 2 shows that the supercharger efficiency is an important factor, the consumed power which is not transformed into pressure is lost in the temperature rise of charge. With equal pressure, a hotter charge has less density and less energetic power. In addition, a too hot charge promotes detonation.

There is also much to say about the "development" of Piaggio's license from the Gnome-Rhone 14K. Later versions incorporate improvements developed by the French motorist for its 14N and 14P, but the French government banned the export of these two engines in 1937. But France and Italy, the improvements are the same (more cooling fins in cylinder and heads, reinforced crankshafts, more efficient superchargers with two speeds).

Regards

 

[sibboh]

In the span 1937-1940, on the DB600-601 and on the Merlin, we see mostly the tevelopement from prototypes to mass production (and, on the Merlin, those to adapt it to the 100 octane gasoline). The R-1830 had surely an evolution, but that's part of an evolution started in 1932. That's similar to the evolution of the Alfa Romeo 125-128 series (basically the same engine) from 650 to more than 950 hp. Moreover, the evolution of the Alfa was done mantaining the same gasoline, while the later modification on the R-1830, as your chart show, were meant to use a different fuel (it's true hat the power output of the r-1830 grew up even with 87 octane gasoline, but much less). In Italy, there would be useless to design an engine to run on 100 octane fuel (let alone 130). In wartime, a mass produced 100 octane italian engine simply would not have flown due to the lack of fuel. An evolution similar to that of R-1830 there would have been if it had been completed the A.76. but it was dropped in 1940 not so much because it was impossible to get, but because, with just over 1000 hp (similar to R-1830-SC3-G, which worked with 87 octane gasoline), in 1940, With the licence production of the DB601 already obtained, the next generation of fighter already ordered with it (and the Alfa Romeo 135 already homologated for 1600 hp max, if someone would want to present a radial engined fighter anyway), the engine would have been born already obsolete. It was the very same concept of "lightweight engine for fighters" to be obsolete and to have been replaced by that of "powerful engine for fighters".


A questo punto mi chiedo io se sappia leggere tu l'italiano. Quella nota dice chiaramente che la leva che comanda la sovrapressione è inutile sopra i 2000 metri, ed è perciò impossibile ottenere una sovrapressione, perché il controllo automatico (che deve garantire gli 821 mm hg), da quella quota in poi ha già aperto la valvola a farfalla più di quanto la leva possa fare.
at this point I wonder if you can read Italian. That note says clearly that the lever controlling the overpressure is useless over 2000m, and is therefore impossible to obtain an overboost, since the automatic control of pressure (which maintains the 822 mm hg), from that altitude onwards, has already opened the intake valve of the supercharger more than the lever can do.
So I asked to you if there are listed, on the manual, other systems to obtain an overboost, since the only one mentioned is useless above 2000m.
Note that do not seems to me that, on the manual, the overboost being allowed in any other flying condition than the takeoff.

For the first part i agree with you.
Per la seconda parte si parla di decollo. Specificamente decollo. Scusami, ma ti ho voluto istigare apposta. Comunque non capisco perchè il grafico segni altre potenze con altre pressioni d'alimentazione e rpm se poi il motore non può raggiungerle (non tutti i nodi sono segnati con tondini, ma solo alcuni). Io penso semplicemente che il manuale parli del decollo e della potenza normale. Desidererei avere il manuale dell'L.121, visto che in alcune pubblicazioni parlano di 1005 cv e 960 cv, probabilmente rispettivamente in emergenza e decollo. I manuali italiani (e francesi) rispetto a quelli anglosassoni e tedeschi sono profondamente differenti nei modi e negli intenti. Diciamo più scarni... Cmnq non volevo essere sgarbato con te. Solo punzecchiarti. Ma mi fa piacere discutere con te. Mi serve per vedere e capire alcuni aspetti da un'altra angolazione. E infatti ho capito molte altre cose, e su molti aspetti concordo con te (come quando non rispondo su alcune tematiche). Cmnq anche tu mi hai punzecchiato... Eheheh

 

[sibboh]

Thanks Bretoal! I am very interested in the French engines and about the arguments you're talking about.

 

[Shortround6]

Now i post a comparison chart between Fiat A.74 and different R-1830 ( -9, -11, -13, -35 ) versions between 1936 and 1940.
We see the R-1830 improvement. And its boost improvement.
There are some problems with different data versions. But I hope it is understood the meaning of my speech.

Thank you for the work.

The R-1830 does show improvement due to higher rpm and better fuel (higher boost pressure). It also shows the weight gain.

We do have "Chicken or the egg" question here. I hope that translates.
If the Italians cannot get better than 87 octane fuel there is no sense building heavy crankshafts and heavy crankcases ( and heavy cylinder bolts) to hold high cylinder pressures. Without the heavy construction you can't raise the power even if you get the fuel.
Going the high RPM route (road) has a problem too. The stress on the crankshaft, crankcase and reciprocating parts goes up with the square of the speed. 10% increase in rpm is 21 % more stress. You also have 21% more friction so crankshaft HP is a smaller fraction of the fuel burned in the cylinders. You do make more power but the engine is less efficient for fuel economy. You also make more heat so you need better finning (more fins, closer spaced and longer). Better finning can limited by foundry techniques. Both American and British companies going from cast heads to forged when the currant casting techniques could not give the desired fins (at an acceptable scrap rate). Sometimes they could switch back when better casting techniques were developed, sometimes they could not.
It does no good to design a very advanced engine if you cannot make it in quantity.
Italian engine designers were not backward, they had to design for the fuel available or likely to be available and had to design engines that could be made, in quantity, using the technology available in Italy. At times the British and Americans could not make each others engines (air cooled) because of difference in HOW they were made.

 

[Dogwalker]

Per la seconda parte si parla di decollo. Specificamente decollo.

E io ti ho chiesto se, oltre all'attivazione di quella leva, inutile oltre i 2000m, il manuale prevedesse altri modi di ottenere una sovrapressione. A me non pare. Quindi devo arguirne che la sovrapressione non è ottenibile sopra i 2000m, almeno finchè i comandi del compressore non vengano modificati. Il fatto che lo stesso manuale non preveda l'uso di una sovrapressione in altre condizioni che non siano il decollo, spiegherebbe poi il perchè di questo arrangiamento (in teoria, il compressore è in grado di offrire una sovrapressione, anche se sempre meno spinta, fino alla quota di ristabilimento meno un metro), dato che è ben difficile ipotizzare, in Italia, o in Europa, da parte di un aereo equipaggiato con un Asso XI, un decollo da oltre 1000m di altezza (non parliamo nemmeno di 2000m).
Comunque, i motori erano soggetti ad evoluzione, non a caso, il manuale del P.XII riporta chiaramente l'avvertenza che quel manuale (intendendo la copia cartacea) non sarebbe stato aggiornato (sottintendendo: in seguito a variazioni nelle caratteritiche produttive del motore stesso), versioni successive del motore, anche fatte specificatamente per il montaggio su un particolare aereo, possono aver avuto settaggi diversi.
Sono contento della dicussione comunque, anche se ci ha portato un po' lontano dall'argomento originario.

 

[sibboh]

Concordo sull'ultima tua frase.
Hai ragione non c'è altro nel testo. Solo i grafici delle potenze. In realtà non solo quelli però.
Il manuale ci dice anche a pag. 25 che la velocità di rotazione del motore massima è di 2400 rpm, e quella eccezionale (per non più di 30") è di 2590. Il decollo (potenza ottenibile per non più di 3') si ottiene a soli 2140 rpm. Le altre potenze (normale a terra e in quota) a 2250 rpm. Poi in tutto il manuale non se ne parla più. Quali sono le condizioni in cui si possono usare i 2400 rpm o i 2590 rpm? Vuol dire che non erano ottenibili certi rpm? Perchè allora l'hanno scritto? E perchè hanno inserito quei grafici? Per me la mancanza di testo non dimostra nulla, se non l'assenza di accuratezza. Il grafico della potenza del motore mostra una realtà differente da quello che sostieni tu. Sono segnate alcune potenze a 2300 e a 2400 rpm. E differenti pressioni, che però non eccedono al massimo gli 890 mmHg circa (a 2400 rpm). Tutto sommato verosimile. Il tuo dubbio comunque è legittimo. E anche io non ho certezza assoluta di ciò che sostengo. Per gli stessi motivi che ti inducono a pensarla in modo differente dal mio. Cioè mancano informazioni. In più il manuale si riferisce a versioni con differente quota di ristabilimento. E anche qui i grafici lo mostrano chiaramente. In conclusione: vi è assenza di informazioni (lo dimostra il caso degli rpm e i grafici), ma alcuni indizi mi fanno pensare che la situazione fosse in realtà più complessa e interessante.
Inoltre nel 1936 compare anche il XI RC40 "spinto". Ho alcuni dati. Purtroppo dovrei rispulciarmi tutte le riviste e libri per risalire alla fonte... lo farò. Ma al momento non ti so dire da dove li ho presi. Dovrebbero avere a che fare con i Ca.135 per la Spagna, se non sbaglio.

You're right there is nothing in the text. Only the graphs of powers. In fact not only those though.
The manual also tells us on p. 25 that the speed of rotation of the motor is the maximum of 2400 rpm, and that great (for not more than 30 ") is 2590. The takeoff (obtainable power for no more than 3 ') is obtained at only 2140 rpm. Other powers (normal to the ground and at altitude) at 2250 rpm. Then throughout the manual does not talk about it anymore. What are the conditions in which you can use 2400 rpm or 2590 rpm? Does it mean that they were not obtainable certain rpm? Why then they wrote it, and why they added those charts? For me, the lack of text does not prove anything, except the lack of accuracy. The graph of the power of the engine shows a different reality from the one that you claim you.There are marked some powers at 2300 and 2400 rpm. And different pressures, but do not exceed the maximum of about 890 mmHg (at 2400 rpm). All in all plausible. Your question, however, is legitimate. And I'm not absolutely sure of what I advocate. For the same reasons that are causing you to think in a different way from mine. That is a lack of information. Additionally, the manual refers to versions with different altitude . And even here the graphs clearly show this. In conclusion there is no infos (as demonstrated by the case of rpm and charts), but some clues make me think that the situation was actually more complex and interesting.
Also in 1936 also appears XI RC40 "pushed". I have some data. Unfortunately I should watching all the magazines and books to trace the source ... I will do it. But at the moment I can not tell from where I caught the data. They should have to do with the Ca.135 for Spain, I think.

IF Asso XI Rc 40 "spinto"
730 cv at 0 m (2250 rpm) - 785 cv (2350 rpm) - not takeoff
900 cv at 4000 m (2350 rpm) - normal continuous
580 cv - economic cruising

Pressione di alimentazione (boost)
Punto fisso (at 0 m) - 2100 rpm - 820 mmHg (1,115 ata)
Decollo (Take-off) - 2200 rpm - 920 mmHg (1,25 ata)
Salita (Climb) a 500m - 2300 rpm - 820 mmHg (1,115 ata)
Salita (Climb) a 1000m - 2300 rpm - 825 mmHg (1,12 ata)
Salita (Climb) a 2000m - 2300 rpm - 840 mmHg (1,142 ata)
Salita (Climb) a 3000m - 2300 rpm - 840 mmHg (1,142 ata)
Salita (Climb) a 4000m - 2300 rpm - 850 mmHg (1,156 ata)
Salita (Climb) a 5000m - 2300 rpm - 800 mmHg (1,0876 ata)

Non è sovralimentazione, ma sono dati leggermente differenti rispetto a quanto si può osservare per la versione non spinta descritta nel manuale.
Forse solo nell'archivio storico dell'Isotta Fraschini (mi pare presso il MILS - Museo delle Industrie e del Lavoro del Saronnese - Saronno VA) si potrebbero trovare dati utili per risolvere i molti dubbi esistenti. Ma ancora nessuno ha condotto una ricerca approfondita. Chissà. Magari un giorno o l'altro ci andrò...

It's not overboosting, but data are slightly different from what is observed for the version described in the manual.
Perhaps only in the historical archives of Isotta Fraschini (I think at the MILS - Museum of Industry and Labour of the Saronnese - Saronno VA) you might find useful data to solve many existing doubts. But still no one has conducted a thorough research. Who knows. Maybe one day or another i will go...

 

[sibboh]

Thank you for the work.

The R-1830 does show improvement due to higher rpm and better fuel (higher boost pressure). It also shows the weight gain.

We do have "Chicken or the egg" question here. I hope that translates.
If the Italians cannot get better than 87 octane fuel there is no sense building heavy crankshafts and heavy crankcases ( and heavy cylinder bolts) to hold high cylinder pressures. Without the heavy construction you can't raise the power even if you get the fuel.
Going the high RPM route (road) has a problem too. The stress on the crankshaft, crankcase and reciprocating parts goes up with the square of the speed. 10% increase in rpm is 21 % more stress. You also have 21% more friction so crankshaft HP is a smaller fraction of the fuel burned in the cylinders. You do make more power but the engine is less efficient for fuel economy. You also make more heat so you need better finning (more fins, closer spaced and longer). Better finning can limited by foundry techniques. Both American and British companies going from cast heads to forged when the currant casting techniques could not give the desired fins (at an acceptable scrap rate). Sometimes they could switch back when better casting techniques were developed, sometimes they could not.
It does no good to design a very advanced engine if you cannot make it in quantity.
Italian engine designers were not backward, they had to design for the fuel available or likely to be available and had to design engines that could be made, in quantity, using the technology available in Italy. At times the British and Americans could not make each others engines (air cooled) because of difference in HOW they were made.

OK. But Germans (and i think also Russians and Japs) had the same fuels problems.

 

[sibboh]

However early versions of the Merlin already had a high pressure and rpm, but using 87 octane fuel.
And the various italian engine projects and prototypes, some powerful, after 1940 didn't have the same high rpm and the same high boost of the first Merlin. So there was much leeway. Even without the 100 octane fuel.

 

[Dogwalker]

You're right there is nothing in the text. Only the graphs of powers. In fact not only those though.
The manual also tells us on p. 25 that the speed of rotation of the motor is the maximum of 2400 rpm, and that great (for not more than 30 ") is 2590. The takeoff (obtainable power for no more than 3 ') is obtained at only 2140 rpm. Other powers (normal to the ground and at altitude) at 2250 rpm. Then throughout the manual does not talk about it anymore. What are the conditions in which you can use 2400 rpm or 2590 rpm? Does it mean that they were not obtainable certain rpm?

Sorry, but there is not possible parallel. The possibility to reach 2400 rpm, and even 2590rpm, is clearly stated in the manual. We know how to rev up the engine, and there is not a single statement, in the manual, about the fact thath the throttle doesn't work once reached, say, 2200 rpm. So we have to assume that it works at least until 2590rpm.
On the contrary, there is one described system to overboost the engine, and is clearly stated that this system doesn't work over 2000m, and is clearly stated why it doesn't work. So, if there isn't the description of another system to overboost the engine, we have to assume that this isn't possible over 2000m, at least for the version of the engine described in the manual. There is not a "lack of text". There is a clear statement, followed by an explanation of how the device work and why it doesen't work over 2000m.
However, we can add something, due to pure physics. The possibility for the overboost to be obtained only up to 2000m is intended at takeoff rpm, so at 2140rpm. As well as the "quota di ristabilimento" (the maximum altitude at which the supercharger is able to provide 822 mm hg) is 4000m at 2250 rpm (potenza normale).
Given that the supercharger is directly geared to the cranckshaft, at higer rpm the supercharger rev more, and gave more pressure. So, the automatic barometric control has to mantain the intake valve more closed to mantain the 822 mm hg, so the overboost lever will work at an higher altitude than 2000m.
For the same reason, at higher regime than 2250 rpm, the supercharger can provide 822 mm hg at a higher altitude than 4000m.

 

[Shortround6]

However early versions of the Merlin already had a high pressure and rpm, but using 87 octane fuel.
And the various italian engine projects and prototypes, some powerful, after 1940 didn't have the same high rpm and the same high boost of the first Merlin. So there was much leeway. Even without the 100 octane fuel.

V-12s are rather different than radials. Having 7 or 9 cylinders acting on one crankpin tends to raise the stress involved.
V-12s with liquid cooling can "usually" use a bit more boost than an air-cooled engine. An engine is limited to the leanest running cylinder and/or the hottest cylinder. Liquid cooling usually limited local hot spots in a cylinder head compared to air cooling.
Smaller cylinders can "usually" use a bit more boost or compression than large ones because they cool a bit better.

The Question of fuel is a tricky one and one that has had little research done (or published) on it. Since fuel changes with age you can't do a modern test on old fuel even if you could find some. Testing was NOT standardized between countries at the time and fuel was made to certain specifications which varied a bit from country to country. Suppliers were free to use whatever blend of fuel compounds they wanted as long as the result met the official specifications. And in some cases at the time the specifications were almost written backwards because they didn't know how to test for what they wanted.
The British KNEW they wanted 'good rich mixture response' but didn't know how to test for it. They KNEW that fuel with a high aromatic content provided better rich mixture response than a fuel with low aromatic content so they specified that their fuel should have a certain aromatic content (20% ? ) while meeting the other requirements (BTUs per gallon, boiling points, vapor pressure, gum content, etc). Aromatic compounds tend to dissolve certain rubber products like gaskets and seals so other counties may specify the SAME octane rating in LEAN condition but limit the allowable aromatic compounds to 2%. The fuel acts just about exactly the same UNTIL you go to a full throttle, high boost rich mixture condition. In some cases countries knew so little about fuels that they specified which oil field the gasoline should come from. They knew there was a difference in finished product but not why. (In America California oil would give 70 octane fuel (no lead) while Pennsylvania oil would give 38-40 octane fuel using the same refining in the late 1920s)

How much improvement there was with the British fuel I don't know, nobody seems to have published much in the way of reports on pre-war fuel. Most fuel history skips over the early years (except for highlights) and starts to get detailed with the coming of 100 octane fuel. Which differed considerably.

How Italian fuel compares I have no idea.
The Japanese Fuel was 92 octane or so and came from the Dutch East Indies. The crude in that area is high in aromatics, what that did or didn't do for the Japanese fuel I haven't seen recorded.

 

[Dogwalker]

On the italian 87 octane gasoline, we can say that, with that fuel, had to be fed both the licence produced DB 601-605 than the German produced ones, delivered first than the Italian production begun (imports then paid delivering to the Germans about the same number of engines produced in Italy, which then had to be fed with German fuel). Furthermore, the aircrafts of the LW in North Africa had to be fed with fuel transported in Italian convoys, and I've never heard that the fuel for the LW was transported separately from that of the Regia Aeronautica. So, the Italian fuel had to be more or less equivalent to the B4.

 

[Shortround6]

It may have been, I don't know or have not read anything about Italian fuel. I do know that US and British fuel was different until sometime in 1941-42. Especially the 100 octane.

The B4 may not have changed. According to many people the C3 fuel did. Early and late examples show different qualities.

Some German engine development was planned around C3 fuel and had to be back pedaled or worked around with the shortage of C3 fuel. ( the use of MW 50 and Nitrous).

With engines taking 4-6 years to bring form drawing board to production the crystal ball used for future fuel availability had better be pretty clear.