Offsetting pin bores
When laying out a new race engine, the engineer (or team of engineers) will normally never consider anything other than having the cylinder bore axes intersecting the crankshaft axis. Indeed, this is also the layout of most production engines, and has certainly been the norm since the inception of the internal combustion engine.
There are reasons why designers of new engines might wish to have the cylinder axis not intersect the crankshaft axis, and these have been used by production vehicle manufacturers in justifying and producing new engines with non-conventional layouts for many years. The two reasons for producing an engine with this layout have opposing results. The first, which does not apply to race engine design, is to improve the noise characteristics of the engine, and specifically to reduce the noise due to piston slap at top dead centre (TDC).
The second reason, which is definitely of more interest to motorsport engineers, is to improve engine output by reducing friction. By having the cylinder axis offset from the crankshaft axis, the aim is to keep rod angularity to a minimum while the cylinder pressure is at a maximum. The reasoning behind the concept is that reduced angularity leads to lower piston thrust forces, and therefore lower frictional losses during the period of maximum cylinder pressure. In practice, the concept is much more easily applied to inline engines than to vee engines. For those whose business it is to develop production-based engines, the concept is difficult, if not impossible, to apply retrospectively.
However, there is an equivalent mechanism that does allow the concept to be applied, and that is to have the pin bore offset in the piston. This allows the same crank/rod pin axis geometry as an engine designed with offset cylinder axes, but within a 'conventional' engines. For those who have pistons custom-made, the option is open to design such a piston, but for some popular engines, pistons with offset pin bores can be bought from motorsport piston manufacturers.
The amount by which the cylinder axes need to be offset depends on many variables, but the main ones are the ratio of crank throw to con rod length and the angle after TDC at which maximum cylinder pressure occurs. The design engineer may have the simple aim to have the con rod parallel with the cylinder axis at maximum pressure, or he may resort to more complex simulation in order to reduce friction over a given range of engine speed.
Depending on the angle after TDC at which maximum cylinder pressure occurs, this may preclude being able to practically apply the desired pin bore offset within the piston. In this case, the engineer will have to settle for the pragmatic approach and be satisfied with whatever gain is observed. The practical limit to moving the pin bore off centre is often the excessive moment due to the large distance between the pin bore axis and the centre of gravity of the piston.
Another point to be aware of is the effect on the engine stroke due to offsetting the pin bore. For a given crankshaft stroke, offsetting the pin bore or cylinder axes increases the stroke, and this may be enough to put the engine beyond the capacity limits of the class in which it is being raced.
Fig. 1 - This production engine piston uses pin bore offsetting
Written by Wayne Ward