Over the years, I have thought a great deal about the difficulties associated with developing the Fulvia’s engine but I still do not have the full answer (or anything like it) although I do have a few ideas.
To me the key word (or rather, words) is volumetric efficiency. One clue is that whilst the 1600 would obviously provide more torque by virtue of its capacity, it seems that developers could get substantially better specific outputs from the 1300. There are two factors here: one is valve sizes – and the other, port sizes.
The standard valve size on the 1300S engine is 37mm for the inlets and 33mm for the exhausts. The 1600’s valves are only 1mm bigger. Now, consider the difference in capacity: the 1600 is 22% larger. A couple of minutes with the calculator shows that the 1600’s valve area is only 5.8% greater than the 1300’s. Now I am not suggesting that the valve area should be 22% greater for the 1600, as the greater depression will of course pull more air through a given aperture. Still, 5.8% seems a bit mean. Four-cylinder engines of similar capacity and of the high-performance type would typically have inlet valves of 41mm and exhaust valves of 38mm, perhaps more. And of course this is not the end of the story: there are the ports to consider. Apart from the fact that the internal water passages limit considerably the opening up of the ports, there is what I call the “No 4 exhaust port problem”. Straddling No 4 exhaust port are the two right-hand rear cylinder head bolts. They are just 36mm apart centre to centre. For a performance engine it is generally considered that the exhaust port diameter should be the same size – or greater than – the exhaust valve diameter, obviously impossible. On the 1600 head, about 33mm is the limit and even this can be a problem sometimes, it being quite normal to break through the port wall where the bolts are.
From the above it should be apparent that there is a built-in advantage with the 1300 engine in volumetric efficiency terms, since the ports and valves are virtually the same size as those on the 1600 – for 22% less capacity - and there are other factors too.
I have no mathematical pretensions, but I do know that good balance in an engine is influenced both by engine speed and by the weight of the reciprocating parts – the lower the better. In the case of the Fulvia, there are probably additional factors: the engine architecture (crankshaft centre line is not at the apex of the V, although this is probably necessary) bore/stroke ratio, con-rod/stroke ratio (large on the Fulvia and generally seen to be a good thing) V-angle and so on. What is undeniable is that the smaller Fulvias are definitely smoother, with a good 1200 being almost turbine-like. Perhaps this is one of the reasons why the tuners can get the 1300 to buzz up so readily?
Other influences include pumping losses (the breathing arrangements are poor) and frictional losses – due to crankcase distortion and other matters associated with the architecture. Finally, two important factors both entirely due to the strange (and actually unnecessary) idea of installing the engine at 45 degrees: the nasty curve in the inlet manifold, together with its unequal length passages, and the limited space for a proper exhaust manifold.
How it began: Fulvia berlines at Fulvia 40 in Turin in the evening