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Engine Valves

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Proper attention to the valves and valvetrain and other car accessories available is important when modifying a cylinder head. In fact, the cylinder head porting would be less effective if we do not improve the air-flow round the cylinder head 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.

Intake and exhaust valves

Intake and exhaust valves

There are two types of engine valves: intake valves and exhaust valves. The valve which allows mixture into the cylinder is the air intake valve; the ones through which the spent gases escape are the exhaust valves. They are designed to open and close at precise moments, to allow the engine to run efficiently at all speeds. Intake valves handle cool, low pressure, low density charges whereas exhaust valves handle hot, high pressure, high density gases. Therefore, exhaust valves are exposed to more severe operating conditions, and hence are made from much higher quality materials than intake valves. The intake valve needs to be larger than the exhaust valve to handle the same gas mass (but of low density).

Intake valve definition: a valve in the cylinder head of an internal-combustion engine that opens at the proper moment in the cycle to allow the fuel-air mixture to be drawn into the cylinder.

Exhaust valve definition: The valve through which the burned fuel charge passes on its way from the cylinder to the exhaust manifold.

Engine valves

How To Check Engine Valves for Wear

The first thing to do with engine 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.

Valves wear

Car Engine Valves Materials

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 car engine valve stem micro welded to a 21/4N Austenitic stainless steel valve head; so check the valve head, not the valve stem.

Bigger Valves

Installing bigger intake and exhaust valves may be a good idea for vehicle's performance increase, though 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.

Cylinder head valves

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.

Qick Exhaust Valves

Commonly referred to as QEV, the purpose of Quick Exhaust Valve is to increase the cycle speed of a cylinder. Depending upon your actuation requirements, installing QEV at the blind end or rod end of your cylinder will result in rapid retraction or rapid extension. When pressure is removed from the Quick Exhaust Valve's input port, back flow at the output port will unseat the valve's internal components, relieving large amounts of flow in a minimal amount of time.

Valve Timing

Stainless steel valves

As with ignition timing, accurate valve timing, or cam timing as some people refer to it, is critical for achieving maximum horse power delivery from your engine.

1. The first thing you need to accurately set your cam timing is a timing degree wheel, or a cam timing disc, that you can get from your camshaft manufacturer. You also need a dial gauge with a magnetic stand to find true top dead center (TDC) of the no. 1 cylinder and the correct valve lift, and an adjustable vernier gear. It is easiest to set the cam timing before the cylinder head is fitted to the engine. You need to accurately determine TDC using the dial gauge and accurately mark TDC on the crankshaft pulley. Usually, the car manufacturer would mark TDC on the crankshaft pulley, but you should verify that it is marked accurately as if it is even just a few degrees out, it can have a significant effect on power delivery. Accurately marking TDC on the crankshaft pulley will also be helpful when want to check or adjust the cam timing at a later stage, with the engine fully assembled and fitted.

2. The next step is to bolt the cam timing degree wheel to the crankshaft, fit a temporary pointer to the engine block and set the pointer to TDC or 0° on the cam timing degree wheel. Then fit the cylinder head and install the camshaft, or camshafts if it's a twin-cam cylinder head. The engine should be at TDC and at the end of the compression stroke on the no. 1 cylinder, so the camshafts should be installed with the intake and exhaust valves of the no. 1 cylinder closed.

Valve timing

3. The camshaft manufacturer or grinder will provide you with a specified engine valve lift and the point at which that valve lift for the intake valves and the exhaust valves should be achieved. This may be for full-lift, or a specified amount of valve lift with the valve opening. Also, the point at which the valve lift is achieved is measured in degrees of crankshaft rotation, which is why we bolted the timing degree wheel to the crankshaft. Our next step is to attach the dial gauge to the cylinder head, with the stylus on the intake valve of the no. 1 cylinder and zero the dial gauge.

4. Now rotate the crankshaft to the specified point at which the specified valve lift should be achieved and read the amount of valve lift off the dial gauge. If it is not the same as the valve lift specified by the manufacturer, then free up the vernier gear and turn the camshaft until the correct valve height is achieved. Take care not to let the valves hit the crown of the piston while you're doing this adjustment as the valves could bend quite easily.

5. With the specified valve lift of the intake valve occurring at the specified degrees of crankshaft rotation, tighten up the vernier gear. Your intake valve timing is now set. On a single-cam cylinder head you just need to verify that the exhaust valve also reaches the specified valve lift at the specified point. But on a twin-cam cylinder head you will need to set your exhaust valve timing by repeat this process for the exhaust valve of the no. 1 cylinder.