Proper attention to the valves and valve train components is important when modifying a cylinder head. In fact, the cylinder head porting we discussed previously would be less effective if we do not improve the air-flow round the valves. That is what we'll be looking at in this section. We'll also be looking at fitting larger valves, improving air-flow round the valve guide, and improving the valve train components. We've already discussed the camshafts and valve timing elsewhere so we won't be repeating that here.
The first thing to do with the valves is to check them for wear. If you find any signs of wear, then you need to replace the valve. You need to check both the valve stem, and the valve face. You can check the valve stem using a micrometer, or you can gently run your index finger and thumb along the length of the valve stem and work your way right round the stem. You can visually check the valve face for wear. If you feel the slightest ridge on the valve stem or the valve face is badly pitted, replace the valves.
Most stock intake valves are made of EN52 steel while most stock exhaust valves are made of more wear resistant and stronger 21/4N Austenitic stainless steel. If you're building a modified street car, the stock valve material will be perfect, as long as the exhaust valves are made of 1/4N Austenitic stainless steel; however, if you're building a modified race car, it would be better to replace the stock intake valves with stainless steel valves, which are more wear resistant. As for the exhaust valves, you can easily verify whether they are made of 21/4N Austenitic stainless steel or EN52 steel as 21/4N Austenitic stainless steel is non-magnetic while EN52 steel is magnetic. So, if a magnet sticks to your exhaust valve head, it's EN52 steel. Some manufacturers use a bi-metal construction, with an EN52 steel valve stem micro welded to a 21/4N Austenitic stainless steel valve head; so check the valve head, not the valve stem.
Fitting Larger Valves
There are several things you need to take into account when deciding on bigger valves. The most obvious is that you need sufficient space in the combustion chamber for bigger valves. However, the valve head should be at least 2 mm from the combustion chamber and cylinder wall. Also, if you fit bigger valves, you'll need to open up the ports, which means you'll need to do more cylinder head porting to achieve the full power benefit of fitting bigger valves. But this also means that you'll have reduced the mean gas velocity at low RPM as bigger ports have lower mean gas velocity at low RPM. This translates into less bottom end power, especially on small bore engines.
It's not necessary to fit bigger exhaust valves on a naturally aspirated engine, even if it's on a heavily modified race car. This is because of the large pressure differential in the cylinder and the exhaust header. The pressure in the cylinder during the exhaust stroke, when the exhaust valve opens is usually five times higher than the pressure in the exhaust header. Air flows from a high pressure area to a low pressure area until equilibrium is reached; therefore the exhaust gasses are literally sucked out of the cylinder. The movement of the exhaust gasses is aided by the upward movement of the piston, which keeps the pressure in the cylinder while forcing even more exhaust gasses out through the exhaust valve. This also explains why the intake valve is bigger than the exhaust valve.