I would like to continue to show why, the way we go after power has changed, with the introduction of four valve engines. It will explain why the things that we used, to modify a two valve, and what most hot-up books have taught us to do, no longer are the way.
When we worked a two valve, we used the biggest inlet valve that we could fit in the head. The biggest lift that the valve gear could operate, and the most duration that would allow the engine to start and idle. To assist cylinder filling we used the energy in the inertia of the inlet and exhaust gas. At the end of the inlet stroke, we relied on the inertia of the incoming gas to keep it flowing into the cylinder even though the piston is rising, forcing more gas into the cylinder. We used the inertia of the exhaust gas to create a vacuum in the cylinder, so the inlet gas would be sucked into the cylinder, when the inlet valve open, on valve overlap. To get the engine to operate at high rpms, long duration's were used, inlet valves opened at 60* before TDC, and the exhaust didn't close till after 50* after TDC, valve overlaps in the 100* to 140* were used. Even with all this, it was difficult to get the engine to produce power over 6000 rpm, and still have it with a useable low rpm torque.
The reason for this was in the type of energy we used to perform the cylinder filling. The gas inertia was too slow to allow the filling as the rpm increased. With the coming of the four valve, there was no need for the long duration's, and high valve lifts, as the four valves provided the breathing to run into the 6 to 7000 rpms, without the need for it. As we sort more HP at the higher rpms, we had to use a faster energy source to assist cylinder filling. Sonic pressure waves took the place of the gas inertia. We use a tuned length, resonate inlet tract, that develops negative pressures in the cylinder to draw more gas in.
Instead of a vacuum to start the inlet flowing at valve overlap. We use the sonic pressure waves developed in the exhaust pipe to develop a negative pressure in the cylinders combustion chamber, to deliver an intense start to the sonic pressure wave that will travel the tuned inlet tract, to produce the negative inlet pressures, that will fill the cylinder.
The change from inertia energy, to sonic pressure, has allowed us to rev the engine as hard as we like, as instead of the energy decreasing as the rpm rises, the sonic pressures become more intense as the speed rises. This changes the way we build exhaust systems, as they are the providers of cylinder filling. It is more important to create an intense start to the inlet pressure wave, than to use valve over lap to draw the air in.
This is why the modern four valve engine is so different to the older two valve, in its tuning, but that is progress, and we have got to move with it or fall behind.
Harvey.
