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Fiat Tipo 406 “Twin Six”, 1927


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#1 blueprint2002

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Posted 05 December 2020 - 05:49

This engine needs no introduction, to members of this forum, though there may be some limitations to the information available in the public domain. I myself have only just realised a contradiction which others have probably already resolved.

The January 1951 issue of “Motor Sport” shows a longitudinal cross-section, apparently a factory drawing, in which the crankshaft and main bearing arrangement is clearly apparent. Contrary to what one might expect, there are only four main bearings (not seven as is usually the case with the in-line six): one at each end, one between cranks 2 and 3, and one between cranks 4 and 5.

Moreover, the cranks are disposed quite differently from what is the usual arrangement. Here, crank 1 is aligned to 2, 3 is aligned to 4, and 5 is aligned to 6; while each pair thus formed is at 120 degrees to the next. Compare this with the normal: 1 aligned with 6, 2 aligned with 5, and 3 aligned with 4, again with 120 degrees between each pair and the next.

Now for the contradiction I mentioned. In “The Racing Car” by Clutton, Posthumus and Jenkinson (paperback edition published 1962), it is clearly stated that the firing order is 1-5-3-6-2-4 for each of the two crankshafts; the trouble is that this (the customary sequence for an L6) is only possible with the normal crank arrangement, as described above! The nearest approach to this, with the cranks arranged as in the drawing, is 1-5-3-2-6-4.

Which one is correct? The book is written by acknowledged authorities, but the magazine article is authored by WB, who I imagine is just as dependable!

If that isn’t enough confusion, there is another drawing, also apparently contemporary and possibly a factory drawing, which schematically depicts the auxiliary drives from the two crankshafts, one view for the supercharger drive, the other for the camshaft drive. The crank arrangement shown here enforces yet other firing sequences, the nearest being 1-5-3-4-2-6. (Of course, in all arrangements, many alternative firing orders are also possible, but only within the restrictions imposed by the crank arrangements). This drawing features in “Primotipo’s” blog of 22-11-2019, and is attributed by him to LJK Setright.

So far as good mechanical balance is concerned, and hence freedom from vibration, the customary crank arrangement of an L6 engine, and consequent firing order, is as stated above. The well-known smoothness of innumerable L6 engines, for over a century, stems from this fact.

However, it is not beyond the bounds of possibility that Fiat chose the second arrangement (that shown in the “Motor Sport” article). Well known, at the time, for technical innovation, they may well have sought the several benefits of fewer crankshaft bearings, reduced length and weight being only the most obvious. If that be so, then the second arrangement makes engineering sense, because the “flying” (unsupported) crank web is not subjected to the fearsome combined bending and torsion, that the conventional arrangement would have experienced. The disadvantage, of residual unbalanced inertia moments, is neatly resolved by phasing the second crankshaft such that its moment always cancels that of the first.

It is not easy, however, to account for the third arrangement (that shown by “Primotipo”). As it happens, two-stroke L6 diesels generally use this firing order (the one I mentioned in this context), but of course with the cranks disposed at 60 degrees, not 120, so maybe this is just a coincidence.

Anyone have any information, knowledge or thoughts on this subject? Even if Fiat, at the time, deliberately set out to confuse the issue, so as to misdirect copycats such as Coatalen, by now surely the facts are known.

Firing order may seem to be a mere detail, even trivial, but it is usually the tip of an engineering iceberg, much of it rather subtle.

PS. I’d like to attach the references I’ve quoted, but the procedure is beyond me, so as usual I’m sticking to words, hopefully unambiguous.

 



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#2 Roger Clark

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Posted 07 December 2020 - 12:14

Fiat en Grand Prix, by Sebastien Faurès Fustel de Coulanges Is the best source I know for information about these cars.  Karl Ludvigsen's history of the V12 Engine also has a description  but neither answers the question.  I must admit that I can't see how the Setright/Primotipo diagrams imply any firing orderl.  The diagrams appear in Setright's book Grand Prix and in Ludvigsen's book.  William Boddy is usually an author you can rely on for his core interests but in this case I think he would have relied totally on the Antique automobile translation of the Motor Italia article.  I don't know where Cyril Posthumus got his information for The Racing Car but I am sue he would have had some reason for saying what he did.

 

I wondered whether the 6-cylinder Type 404 of 1922 might provide some sort of precedent but that engine was derived from the 8-cylinder 402 of a year earlier.  The 406 was completely different.  I suppose that, in a racing engine, Fiat would have been more interested in light weight than smoothness.  

 

I wonder whether this topic would receive more replies on The Nostalgia Forum or at least a cross reference there.



#3 blueprint2002

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Posted 09 December 2020 - 01:15

Fiat en Grand Prix, by Sebastien Faurès Fustel de Coulanges Is the best source I know for information about these cars.  Karl Ludvigsen's history of the V12 Engine also has a description  but neither answers the question.  I must admit that I can't see how the Setright/Primotipo diagrams imply any firing orderl.  The diagrams appear in Setright's book Grand Prix and in Ludvigsen's book.  William Boddy is usually an author you can rely on for his core interests but in this case I think he would have relied totally on the Antique automobile translation of the Motor Italia article.  I don't know where Cyril Posthumus got his information for The Racing Car but I am sue he would have had some reason for saying what he did.

 

I wondered whether the 6-cylinder Type 404 of 1922 might provide some sort of precedent but that engine was derived from the 8-cylinder 402 of a year earlier.  The 406 was completely different.  I suppose that, in a racing engine, Fiat would have been more interested in light weight than smoothness.  

 

I wonder whether this topic would receive more replies on The Nostalgia Forum or at least a cross reference there.

Thank you Roger for your most helpful response. 

I'll attempt an explanation for the firing order here, trying to keep it clear and unambiguous.

You will see that both crankshafts (LJKS/Primo) have cranks 1 & 4 at TDC: consider any one. Either 1 or 4 may fire now, we'll choose 1. After 120 degrees rotation, anticlockwise as shown, cranks 2 & 5 come to TDC: either may now fire but we'll choose 5 so as get a direct comparison with the "standard" firing order. Similarly, choose 3 after a further 120 degrees rotation. No more choices left,  as the crankshaft continues around another circuit, first 4, then 2 and finally 6 must fire to complete the cycle.

Of course, other firing orders are also possible, simply by making other choices on the first circuit, but the "standard" firing order of 1-5-3-6-2-4, is not one of them; the arrangement of the cranks being decisive.

Seems unlikely to me that the Tipo 404 had anything but the standard firing order, for the usual excellent reasons. The unique layout of the 406 provided the opportunity to innovate, as well as the need, because otherwise there would probably have been 14 main bearings and 14x3 camshaft bearings, with the associated space, weight etc complications.

The only other engine, that I recall, in which similar considerations seem to have ruled, is the Ferrari F1 Flat-12 of the seventies, which also had only four main bearings.

As suggested, I am also going to try the Nostalgia forum for this discussion.



#4 Kelpiecross

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Posted 09 December 2020 - 03:34

  I don't know about the firing order -  the more basic question for me is why they think this "twin crankshaft"  layout would be better than than a conventional V12?    



#5 Roger Clark

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Posted 09 December 2020 - 08:45

  I don't know about the firing order -  the more basic question for me is why they think this "twin crankshaft"  layout would be better than than a conventional V12?    

Possibly weight. Doug Nye says in Motor Racing Mavericks that the Fiat engine weighed 381lbs compared with 500lbs for the straight-eight Delage. 



#6 Roger Clark

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Posted 09 December 2020 - 09:33

Thank you Roger for your most helpful response. 

I'll attempt an explanation for the firing order here, trying to keep it clear and unambiguous.

You will see that both crankshafts (LJKS/Primo) have cranks 1 & 4 at TDC: consider any one. Either 1 or 4 may fire now, we'll choose 1. After 120 degrees rotation, anticlockwise as shown, cranks 2 & 5 come to TDC: either may now fire but we'll choose 5 so as get a direct comparison with the "standard" firing order. Similarly, choose 3 after a further 120 degrees rotation. No more choices left,  as the crankshaft continues around another circuit, first 4, then 2 and finally 6 must fire to complete the cycle.

Of course, other firing orders are also possible, simply by making other choices on the first circuit, but the "standard" firing order of 1-5-3-6-2-4, is not one of them; the arrangement of the cranks being decisive.

Seems unlikely to me that the Tipo 404 had anything but the standard firing order, for the usual excellent reasons. The unique layout of the 406 provided the opportunity to innovate, as well as the need, because otherwise there would probably have been 14 main bearings and 14x3 camshaft bearings, with the associated space, weight etc complications.

The only other engine, that I recall, in which similar considerations seem to have ruled, is the Ferrari F1 Flat-12 of the seventies, which also had only four main bearings.

As suggested, I am also going to try the Nostalgia forum for this discussion.

That’s very clear, thank you, but the drawing I think you were referring to is a head-on schematic. I don’t see how you can infer a firing order from them. 



#7 Allan Lupton

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Posted 09 December 2020 - 09:51

That’s very clear, thank you, but the drawing I think you were referring to is a head-on schematic. I don’t see how you can infer a firing order from them. 

His explanation seems straightforward to me.

What is not straightforward is that the side-view shows 3 and 4 working together but the head-on pairs 3 with 6 and 4 with 1



#8 blueprint2002

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Posted 12 December 2020 - 03:59

That’s very clear, thank you, but the drawing I think you were referring to is a head-on schematic. I don’t see how you can infer a firing order from them. 

Sorry I can't do better than I have, but I do have a suggestion.

Engineering college is a long way behind me, but I seem to recall this matter being dealt with, under the heading of "Engine Balancing", in a book called "Theory of Machines" by Thomas Bevan. If you can find a copy today, it might be helpful. 



#9 Roger Clark

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Posted 12 December 2020 - 09:34

Thank you for the suggestion but, as I said, your explanation was very clear, even to a non-engineer. However, I now see what I had missed in the Setright drawing. Apologies for that if it was clear to everybody else. 
 

However, it might be interesting to know the origin of the Setright drawing. The one in Motor Sport appears to be from Fiat but this one, or at least the annotation, is English. If I have (finally) understood, the annotations are crucial to the derivation of the firing order. Is it possible that there was a simple mistake by whoever added the annotations?



#10 blueprint2002

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Posted 14 December 2020 - 01:19

Thank you for the suggestion but, as I said, your explanation was very clear, even to a non-engineer. However, I now see what I had missed in the Setright drawing. Apologies for that if it was clear to everybody else. 
 

However, it might be interesting to know the origin of the Setright drawing. The one in Motor Sport appears to be from Fiat but this one, or at least the annotation, is English. If I have (finally) understood, the annotations are crucial to the derivation of the firing order. Is it possible that there was a simple mistake by whoever added the annotations?

 

Good point Roger, I had overlooked the English language annotations, which makes the origins of this drawing still more uncertain. 

A mistake doesn't seem so likely, given that there are two crankshafts and both have the crank numbers in the same sequence.

Be that as it may, the Motor Sport/Fiat drawing shows a crank arrangement that could not possibly have resulted in the firing order stated by Cyril Posthumus. But which is not illogical if Fiat were aiming to produce an unusually compact and potent 12, without compromising engine life and reliability (as already briefly discussed in my first post).