The Motorist 03 | Page 18

PERFORMANCE CAMSHAFTS By Gerald Buys - Owner of Camtech SA ([email protected]) I am often asked how fitting a high performance camshaft, or regrinding the existing camshaft, increases the performance of the vehicle. It’s no good explaining it if the reader doesn’t know the fundamentals of how a four– stroke engine works, so let’s start there. First, with the piston somewhere near the top of its stroke (Top Dead Centre, or TDC), the inlet valve opens, and as the piston travels down the bore it sucks in a fresh charge of air-fuel mixture (or air, in the case of a direct injection engine). Then, somewhere near the bottom of the pistons stroke (Bottom Dead Centre, or BDC), the inlet valve closes, and the piston travels back up the bore, compressing the air-fuel mix. Somewhere near TDC the spark plug fires, igniting the air fuel mix, which, being compressed to something like one tenth of its original volume, is hot and full of potential energy and burns furiously, expanding rapidly and forcing the piston back down the bore. This is called the power stroke, and provides all the energy to turn the crankshaft. Somewhere near BDC the exhaust valve opens, and as the piston travels back up the bore, the burnt gases are expelled, and near the top, the exhaust valve closes, the inlet valve opens once again, and we’re back where we started. Simple really. Just ask Karl Benz. But hang on - how do we get more power? By burning more fuel. And in order to burn more fuel, we need more air, or else it won’t burn. In a non-turbocharged engine, the best way to persuade more air to enter the engine is to take advantage of the inertia of the fast moving gases, both fresh charge entering the engine, and burnt gases leaving the engine. So, if we close the exhaust valve sometime after TDC, and open the inlet valve sometime before TDC, the fast moving exhaust gases will cause a low pressure in the cylinder and “suck” the inlet gases into the cylinder way before the piston starts travelling down the bore. This is called the overlap period, and all modern engines take advantage of this effect. If we increase the overlap period, we get more air into the engine, particularly at high engine speeds, and our power goes up. Now, if we close the inlet valve sometime af-