This paper was originally written as a magazine article, hence its brevity. However, the magazine never got off the ground, so I published it here.

The majority of big bore kits are sold as bolt on accessories, employing the existing carb, ignition and pipe. However, what does the action of increasing cylinder capacity do to the power characteristics of the engine?

The figure below shows the simulated power curve of a hypothetical 80cc motocross engine, which has a 52mm big bore as opposed to the stock 47mm bore, making 102cc from 82cc. This is one of the largest displacement increases available, since engine capacity is increased by almost 25%, when compared to a 125cc machine going to 140cc, which is only a 13% displacement increase. 

The graph shows an increase in power of approximately 2.5Bhp or 13%. More interestingly, figure 2 shows the peak torque figure has increased by 1.6 lb/ft or 15%, but the torque peak has shifted 500-1000rpm down the rev range. This is because this simulation used the standard practise of simply fitting a bigger piston.

The carb, pipe etc remains stock, so does the porting. Now, the engine’s ‘breathing’ characteristics are slightly restrictive for the new cylinder capacity, being optimised for the original engine. Notice these effects show themselves more more on machines with less radical porting and breathing arrangements, such as trail/green lane machines rather than full on race. Most people know a motocross bike has porting only excelled by Road Race engines, and large diameter carbs to suit.  It has been said that Big Bore kits provide more of a torque rather than power increase, as these curves show.

The Figure above shows an optimised big bore engine. The optimised engine uses a race or fast circuit pipe and a 32mm carb with enlarged reed cage. These effects push peak power further up the rev range, as displayed in the figure.

Here, peak power is at 11000rpm as opposed to 10000rpm stock.  The same procedure applied to the standard 80 does produce more ultimate horsepower, but the torque curve has simply moved up the rev range, producing a peaky or expert only engine.  By contrast, the tuned 100cc engine has more torque than the stock 80cc engine almost everywhere, producing an engine which has around 4.5 Bhp or 25% more power than stock.

In summary, fitting a big bore kit generally gives a larger increase in torque rather than power. This makes a motocross engine easier to ride as demonstrated in these simulations.   Simple carb and pipe mods can increase breathing to the engine, resulting in the same power delivery as stock, only more so, these are summarised in the graph below.

Table 1 gives the full low down on the consequences of fitting a big bore kit and the possible remedies for them.

Simulations performed using Mota 2 stroke Engine Simulation Software