A LONG OR SHORT CONNECTING

ROD? TWO DIFFERENT

PHILOSOPHIES COMPARED

Of all the changes in the 2016 homologation, one in particular highlighted the different philosophies of the manufacturers: the length of the connecting rod (from the centre off the piston-pin opening to the centre of the big-end bore). Two makes, in particular, have made opposite choices: Vortex, with its new RKZ shifter engine, has changed tack with respect to the previous approval, lengthening the connecting rod from 110 mm to 115 mm., On the other hand, for its MKZ engine Modena Engines has chosen a 106 mm connecting rod, which is very short compared to the standard measurements, such as those used by IAME, which has chosen a “traditional” length of 110 mm.

Before going into the details of the choices made by Modena Engines and Vortex, let’s take a look at the general theory concerning the length of connecting rods. At an equal crankshaft height, an engine with long connecting rods can mount a shorter and lighter piston, reducing the alternating masses to the advantage of reliability and decreased component wear. Furthermore, long connecting rods also have reduced angles during movement, generating less lateral thrust and friction (less wear) on the cylinder walls. This is why road engines favour smaller pistons at the expense of longer connecting rods.
In contrast, in high-performance engines, in which pistons are already as small as possible (at high rpm, reducing masses is essential, since they generate forces of inertia that are proportionately high), the crank drive is taken to an extreme by also shortening the connecting rod. The part has an alternating motion for a fraction of time and a rotary motion for the remaining fraction. Therefore, reducing it means reducing the masses that move in both one of the motions, as well as the other. The result is a net advantage in terms of performance, but a somewhat reduced duration and a need for expensive surface treatments on the cylinder walls to mitigate the effect of increased lateral thrust. In general, this is the case with two-stroke kart engines, on which long and short connecting rods, with the same stroke, determine changes to transfer and exhaust port opening and closing times and changes to the flows.
Giovanni Corona, a manager at Vortex, and Roland Holzner, the technical director at Modena Engines, explain the choices that led to the definition of the length of the connecting rod in their latest homologation KZ engines.