Although the F1 and Racing Sports versions of the M196 straight-8 engine are broadly similar, apart from the displacements, another intriguing difference is clearly indicated on the company’s heritage website.
The firing orders are radically different, as follows:
M196R: 1-4-7-6-8-5-2-3
M196S: 1-2-4-6-8-7-5-3
A minute or two with pencil and paper will show that this is the result of different crankshaft arrangements, which can be described as:
M196R: The so-called 2-4-2 arrangement, in which cranks 1,2,7 & 8 together amount to the usual 4-cylinder single-plane pattern, while cranks 3 to 6 together amount to another of the same, but at right angles to the first.
M196S: For want of a better name, I’ll call this the 1-2-2-2-1 arrangement, in which cranks 1,4,5 & 8 together amount to the usual 4-cylinder pattern, as do cranks 2,3,6 & 7, again at right angles to the first.
The question is, why did M-B do this?
As far as primary and secondary balance is concerned, both forces and moments are fully self-balancing, in both cases, so there seems to be nothing to choose between the two in this regard.
Other possible considerations include:
- Interaction between cylinders in the intake and/or the exhaust systems. This doesn’t seem likely as the system layout appears the same on both the R and S versions. Also, in 1954-55 this technology had not yet matured, as clearly shown by the exhaust layout: Cylinders 1 to 4 are all tightly connected to one manifold and tailpipe, while 5 to 8 are connected to another. Not even roughly what we know today as 4-into-1 systems, particularly when you consider the firing order.
- Torsional vibration. Remembering that both these engines were, effectively, two fours coupled end-to-end, with the power output from the middle, the difference between the two versions is even more pronounced. Maybe this was done to control torsional vibration, which would have different critical speeds in the two cases, as a result of different bore and stroke (R: 76x68.8, S:78x78).
- Main bearing loadings would likely be rather different for the two arrangements, and one that was more conducive to long life could have been selected for the S version, suitable for races from 1000 Km up to 24 hours. In which case, the question arises: why not the same for the R version? Was there some sacrifice in performance, to go with a longer life?
Of course, there could be other aspects, too. Anyone have any views, or know of an authoritative publication that might have dealt with this subject?
N.B. It may be only coincidence that the contemporary Bugatti Type 251, which also had a straight-8 engine with the drive taken from the middle, was said to have been arranged in such a manner that the two halves of the crankshaft could be coupled together in different ways, so as to alter the torque characteristics to suit different circuits. Beyond this exceedingly vague statement, I have not been able to find anything more detailed or explicit. But it does seem possible that there may have been some common reasoning applied in both these cases; could this be a clue when considered together with point #3 above?
