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. Next you should check the wear on the valve guides. The valves should not be able to rock in the guide. If they do, and the valve stem is of the correct diameter, the valve guides needs to be replaced.
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.
Reworking the Valves
A little reworking the valve face and the valve stem, particularly on a stock valve, will result in a significant improvement of the air-flow around the valve. A simple 30° backcut applied to the stock valves can lead to a 7% to 12% improvement in air-flow into the engine at mid-lift. The backcut improves the transition from the valve seat into the valve's underside if the valve head. However, the backcut should not reduce the width of the valve seat by too much. The valve seat should still be between .075in. and .090in. wide on the intake valve and between .065in. and .080in. wide on the exhaust valve.
You can also work the intake valve stem, undercutting it by between 0.015in. and 0.175in. across the distance from the below the valve guide to the underside of the valve head. This can lead to 5% to 7% improvement in air-flow at low-lift. However, to prevent possible stress fractures and cracking of the valve stem, the undercut area must be carefully polished and smoothed into the valve head and the uncut portion of the valve stem.
The shape of the underside of the valve head should also be machined smooth. In addition, the underside intake valve head can be radiused to improve air flow around the valve head and into the combustion chamber. The ideal size of the radius will depend on the shape of the combustion chamber. On a semi-hemi combustion chamber, the radius should be between 0.24 and 0.26 times the diameter of the valve, while on a bath-tub combustion chamber it should be about 0.19 to 0.21 times the diameter of the valve.
Finally, a square corner on the edge of the intake valve between the valve margin and valve face will reduce backflow when a performance camshaft is installed. On the exhaust valve you would want a radius corner as the flow direction here is out of the combustion chamber.
The Valve Seat
The valve seat insert in the cylinder head also needs some attention. A multi-angle valve seat will improve air flow but it must also provide a perfect gas-tight seal when the valves are closed. The actual valve seat angle should be 45° to match the valve seat on the valve face. It should be 0.050in. wide on the intake valve and 0.070in. wide on the exhaust valve. The outside diameter of the seat should be 0.015in. to 0.020in. smaller than the intake valve diameter and up to 0.010in. smaller than the exhaust valve diameter. A 30° top cut should be added to blend the seat into the combustion chamber. This should be just wide enough to blend the 45° seat into the combustion chamber. A 60° throat cut can also be added to blend the valve seat into the valve throat. A fourth cut of 70° to 75° into the valve throat sometimes provides a bit of improvement in air flow though this is often not a significant improvement.
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 will 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 usually not necessary to fit bigger exhaust valves on a naturally aspirated engine, even 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 usually bigger than the exhaust valve.