DOING THE HEAD
by Double H
A Twin Cam Cylinder Head
When it comes to getting the most power out of a naturally aspirated engine the key area that you must focus your attention on is the cylinder head. This is the one area that will potentially give you the greatest increase in engine power. Why? Well, as Langer explains in engine building and power basics, the key to increasing an engines horse power is to get the engine to ingest more air and be able to expel the resultant increase in exhaust gasses, in other words, getting the engine to pump more air by increasing the air-flow in and out of the engine.
On a motor car engine, there are three areas that can affect air-flow and where you can make improvements. These are:
- The intake system, which includes the air filter, plenum and the intake runners.
- The exhaust system, which includes the exhaust header, catalyst converter and the mufflers.
- The cylinder head, which includes the cylinder head ports, valve area and the camshafts.
We've discussed the intake system and the exhaust system elsewhere on this web site so now it's time for us to turn our attention to modifying the cylinder head. However, in this section we're going to discuss a little bit more than just the cylinder head, we're going to discuss cylinder head porting, gas flowing and power tuning the cylinder head, old school style! We'll also be discussing performance camshafts, cam timing, valve timing and valve overlap.
A word of warning though, cylinder head porting and gas flowing is a rather advanced form of car modification and is not for the novice or for the faint of heart. Cylinder head porting is a skill that must be developed and honed by hours and hours of practice. If you're intent on trying cylinder head porting, the first thing that you need to know is the porting always begins by trial and error so if you're going to do your own cylinder head porting, start on a cylinder head that you can afford to total, in fact, start with a couple that you don't mind loosing. Otherwise you should leave cylinder head porting up to a professional with a flow bench. The other thing to note, is that cylinder head porting requires some rather expensive tools. You'll need a high-speed extended pneumatic die-grinder with carbide and steel grinders, and a high-pressure air compressor (no, we're not talking about turbochargers here) to power the grinder. You could use an electric die-grinder rather than a die-grinder, but electric die-grinders don't operate at a high-speed like pneumatic die-grinders. You could also use an electic drill rather than a die-grinder but you won't get the same results as you would with a longer, more agile and thinner die-grinder. An electric drill also does not operate at the high-speeds that a pneumatic die-grinder does.
A Pneumatic Die Grinder
Right, if you've read all that, bought your air compressor and your die-grinder, and gotten hold of a few spare cylinder heads, despite our warnings, then we can move on and start modifying the cylinder head for extreme power. But remember that we did warn you. Right, we'll begin by looking at the camshaft before moving on to the equipment you'll require to port your cylinder head, the basics of gas flowing and cylinder head porting itself.
PERFORMANCE CAMSHAFTS
The two important aspects of a camshaft, in terms of engine performance, are camshaft duration, or cam duration, and valve lift. Both cam duration and valve lift are determined by the camshaft lobe. Cam duration is the time that at least one valve of a cylinder remains open, i.e., off its valve seat, measured in degrees rotation of the crankshaft, while valve lift is the maximum distance the valve head travels from the valve seat.
VALVE LIFT
Valve lift is somewhat related to intake valve head diameter. An engine with an intake valve head diameter of 1.400in to 1.500in will generally perform best with a valve lift of 0.395in to 0.475in; an engine with a larger intake valve head diameter of 1.750in to 1.875in will generally perform best with a valve lift of 0.425in to 0.550in; and an engine with a large intake valve head diameter of 2.000in to 2.250in will generally perform best with a valve lift of 0.475in to 0.650in. But these are just rough guidelines; ultimately you will need to take some gas flow readings on a flow bench to determine the best valve lift for your particular engine.
A number of factors influence valve lift. The most important being the gap between the intake and exhaust valves, the piston to valve clearance and the intake charge pressure. These factors also influence cam duration. Another factor influencing valve lift is valve spring compression. Obviously, once the valve springs are fully compressed, it cannot give any more and the valve cannot be pushed further down into the combustion chamber.
As I've mentioned earlier, cam duration is measured in degrees rotation of the crankshaft, rather than the camshaft, and the crankshaft completes two full rotations for every rotation of the camshaft. In other words, with a 310 degree camshaft, the valves are open for only 155 degrees of actual camshaft rotation.
A performance camshaft for a naturally aspirated engine will have a duration in the range of 270 degrees to 310 degrees or more, with a 270 degree camshaft described as a 'mild' camshaft and a 310 or more degree camshaft being described as a 'wild' race camshaft. A stock camshaft usually has a duration of around 270 degrees but what differentiates a 270 degree performance camshaft from a stock camshaft is increased valve lift and a much faster rate of valve lift. With a faster valve lift rate, the valve reaches full lift quicker and remains at full lift for longer. This increases Volumetric Efficiency (VE) as more air flow in and out of the engine is possible.
A determining factor, when choosing camshaft duration is the purpose of the vehicle. The longer the duration of the camshaft, the further up the rev range the power band shifts, and the rougher the idle. Obviously, as the power band moves higher up the rev range, bottom end power is lost. Also, as cam duration and valve overlap increases, torque is lost. Fuel efficiency also decreases and exhaust emissions increase as valve overlap increases.
High performance camshafts start at 280 degrees of duration. These camshafts have increased valve overlap but not too much so emissions and fuel economy are not severely affected. These are generally good camshafts for modified street cars and produce good power from 2,500 RPM up to 7,000 RPM but they do not have a smooth idle because of the increased valve overlap.
A 290 degree camshaft requires more cylinder head work in terms of cylinder head porting and gas flowing as they work better when the engine's Volumetric Efficiency (VE) is improved. As you'd expect, these camshafts produce a fairly rough idle. These camshafts are generally good for rally cars and produce power from 3,000 RPM up to 7,500 RPM. A 300 degree camshaft requires even higher levels of VE, reaching the physical gas flowing limitations of a two valve cylinder head with a single camshaft. These camshafts are good for modified race cars and produce good power from 4,000 RPM up to 8,000 RPM. However, they have a very rough idle.
A camshaft with a duration of more than 300 degrees is an out and out race camshaft with a power band in the 4,500 RPM to 9,000 RPM rev range. To make effective use of a 300 degree camshaft, you need to ensure that the engine has a very high VE. You also need to ensure that the engine can rev beyond the red line of most stock engines.
VALVE OVERLAP
The limit for opening the exhaust valve is approximately 80° before bottom dead center (BBDC). Opening the exhaust valve any sooner tends not to increase power production but will shift the power band higher up the rev range and will reduce low end torque as downward pressure on the piston during the power stroke is released. The same applies to closing the intake valve where 80° after bottom dead center (ABDC) is the limit for increased power production.
BASIC CYLINDER HEAD PORTING
Although it sounds quite complicated, gas flowing and cylinder head porting are actually quite simple. The main aim of both gas flowing and cylinder head porting is to improve the air-flow through the cylinder head. Understanding what is good for improving air-flow, and what is bad for air low, i.e., what restricts air-flow, will go a long way to making good power gains from your cylinder head porting and gas flow work, so let’s begin there.
AIR-FLOW
As "Bad Ass" Bre and "Langer" have mentioned else on this site, the key to engine power and car performance is good air-flow in and out of the engine. Getting more air/fuel mixture into the engine and getting the exhaust gas out efficiently after combustion will get you more power. This is what is called Volumetric Efficiency (VE).
In technical terms, Volumetric Efficiency is the ratio of the volume of fresh air/fuel mixture that is drawn into the cylinder on the intake stroke, relative to the swept volume of the cylinder. Obviously, any exhaust gas that remains in the cylinder after the exhaust stroke will occupy some of the volume that fresh air/fuel mixture should occupy, and would reduce the Volumetric Efficiency of the engine. Thus, how well the exhaust gasses flow out the exhaust system is also important. Generally speaking, a multi-valve cylinder head will have a better Volumetric Efficiency, and hence will create better power, than a two-valve cylinder head. So if you have the option of fitting a two-valve cylinder head, or a multi-valve cylinder head, I’d go with the multi-valve cylinder head.
IMPROVING AIR-FLOW
Improving the Volumetric Efficiency of your engine requires that you improve the air-flow in and out of the engine. Fortunately, there are a number of things that you can do to improve air-flow, particularly through the cylinder head. The first is to ensure that nothing obstructs or restricts the air flow to and through the cylinder head, from the moment air enters the intake system until the moment it exits out of the tail pipe. However, certain obstructions in the cylinder head ports, such as the valve stem and the valve guide boss, cannot be eliminated completely but can be minimized by narrowing the valve stem without weakening it too much and shaping the valve guide boss into a ramp.
Another way of improving air flow through the cylinder head is to form the cylinder head ports and the combustion chamber into an even, smooth and consistent shape. The key word here is consistency; constituency not only in shape and size, but also consistency from one cylinder to the other. You can achieve consistency in shape by ensuring that there are no intrusions or cavities in the port, and that the port does not widen or narrow. Air-flow can be further optimized by eliminating sharp turns and bends in the path of the air-flow.
ENLARGING THE PORT
Enlarging the cylinder head port might be a good way of improving air-flow, but it has a major effect on mean gas velocity. A small port, relative to the cylinder, will have a high mean gas velocity at low RPM but it will struggle to fill the cylinder at high RPM. Thus Volumetric Efficiency will tail off at high RPM and power will fall off quickly. Conversely, a relatively large port will have a low mean gas velocity at low RPM. To maintain fuel atomization, i.e., to keep the fuel droplets suspended in the air flow, a high mean gas velocity is required. If the mean gas velocity is too low, gravity will pull the fuel droplets out of the air stream and will form puddles of fuel on the port floor. The result will be a loss of power and economy.
Now that we've got a good understanding of air-flow, we can move on to the actual cylinder head porting.
CYLINDER HEAD PORTING
Now that we've got a good understanding of air-flow, we can move on to cylinder head porting. If you haven't yet read our article on the basics of cylinder head porting and air-flow, I'd suggest you do so now as it provides the foundation for understanding what we want to achieve with the actual cylinder head porting.
PREPARING THE CYLINDER HEAD
Before we can get started, we need to strip down the cylinder head; remove the camshafts and camshaft pedestals, then remove the valves, valve springs and valve stem seals. You should also remove all manifold studs. With everything stripped, you need to inspect the cylinder head for cracks. It's no good porting a cracked cylinder head, though a cracked cylinder head may still be good for experimenting on, so don't throw it away! The most likely areas where cracks will appear are between adjacent valve seats, and around the valve seats, especially around the exhaust valve seats. You may need to some emery cloth to remove any carbon deposits to do a thorough check.
If you don't see any cracks, have the cylinder head thoroughly cleaned in a chemical bath. You can dip a cast iron cylinder head in a hot caustic solution but don't dip an aluminum cylinder head in it! Caustic solution will react with the aluminum and give off an explosive gas! For an aluminum cylinder head you should use Trichloroethane. If you don't have access to a chemical bath, you can use engine cleaner and a stiff brush to get oil and gasket pieces off. Once the cylinder head is clean and dry, use a sand blaster or a wire brush to clean off any stubborn carbon deposits. Once that's done, do another thorough check for cracks.
If you don't see any cracks, have the valve seats replaced and the valve guides removed by a reputable engineering shop. Replacing the valve seats are not crucial as long as they're in a good condition. However, you must have the valve guides removed.
WARNING: Take care when working with a grinder. Adhere to the following safety precautions when porting cylinder heads and using a grinder in general:
- Wear eye protection when working with a grinder; goggles are advisable but a full face visor would be better.
- Wear a dust mask or a respirator; inhaling metal filings is harmful.
PORTING THE CYLINDER HEAD
We'll get to enlarging the port in a while when we discuss gas flowing; but for now we'll focus on the main aim of cylinder head porting, which is to smooth and straighten out the ports. If this is your first attempt at cylinder head porting, I'd suggest you try to master that first. Starting on the intake ports, use a flame-shaped carbide and attempt to remove any obvious bumps and crevices in the port without removing too much metal, then try to straighten the post so that it has a consistent size from the mouth to the point where it curves into the valve throat. Remember to move the carbide all the time and don't hold it on one spot as it will quickly create a hollow that will be difficult to remove! Once you're happy that you've got your first port nice and straight you can use a grinding stone to smooth it if it's a cast iron cylinder head, or a sander band if it's an aluminum cylinder head. Now try to replicate your work on the other ports. Use an inside caliper to make sure all the ports are the same size.
Now, working from the valve throat side, use an oval carbide to blend the short side radius. Again, try not to remove too much metal. Also remember that you want a smooth flow through the valve throat area and that you want a consistent port size through the length of the port. Once you have blended the short side radius, turn you attention to the long side radius where the valve guide boss is located. Use an oval carbide to flatten the valve guide boss until you have a consistent port size from the manifold face to the valve seat.
Now all that's left is to smooth the port with a flapwheel or a fan grinder; then use a vernier caliper to measure the height of the valve guide boss through the hole for the valve guide. Measure the height of the valve guide boss on its shortest side and on its longest side. Then replicate your porting work on the other ports until all the ports are identical.
Once you're happy with the intake ports, turn your attention to the exhaust ports and smooth and straighten them out in the same manner, without removing too much metal and retaining the squarish shape of the exhaust ports.
MATCHING THE MANIFOLD
The intake manifold and exhaust header are integral to the efficient air-flow in and out of the engine. Getting a smooth flow from the manifold to the cylinder head ports, especially in the case of the intake manifold, is crucial for good air flow and power. Remember that air-flow doesn't like sudden changes in direction or tube size. As "Bad Ass" Bre mentioned in designing and building performance exhaust systems, the exhaust port can be slightly smaller that he exhaust header to help prevent reversion.
Start by making a cardboard template of the intake and exhaust manifold faces, and cut out the port openings and the stud holes accurately. Fit the template to the cylinder head, taking care to match the cylinder head side of the template to the cylinder head. You will have to insert the manifold studs or manifold bolts into the cylinder head to line the template up correctly. Once the template is lined up accurately, scribe the outline of the template onto the cylinder head. You can also use a manifold gasket to mask out the port sizes if the port openings on the gasket fit the manifold accurately.
Now you can enlarge the ports gradually until the intake port matches the port openings on the intake manifold, and the exhaust port is slightly smaller than the port openings on the exhaust header. Remember that the aim of cylinder head porting is to create a smooth straight port that has a consistent port size from the manifold face through the valve throat area. Also try to keep the ports walls on rectangular exhaust ports as straight as possible. You can use an engineer's square to scribe straight lines that can serve as guides for your porting on the port walls.
VALVES AND THE VALVE TRAIN
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 VALVES
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.
CAMSHAFT TIMING
As with ignition timing and other forms of engine timing, accurate valve timing, or camshaft and cam timing as some people refer to it, is critical for achieving maximum horse power delivery from your engine. 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's quite easy to accomplish in theory, but a bit more complicated as you need to determine the exact point that full-lift is achieved and the same applies to accurately determining true TDC.
FINDING TRUE TDC
An adjustable vernier gear
for accurate cam timing
for accurate cam timing
It is easiest to start setting the cam timing before you fit the cylinder head to the engine as 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 never assume that it is accurate. Always verify that TDC is marked accurately even when it appears to be accurate when viewed with the naked eye. Remember that at the piston appears to be stationary at the end of the compression stroke for approximately 10° of crankshaft rotation. TDC is at the exact middle of this dwell period and even if it is just a few degrees out, it can have a significant effect on power delivery. Start by turning the engine to the TDC mark on the crankshaft pulley and attaching the cam timing degree wheel to the crankshaft pulley. Use a piece of stiff wire affixed to a nearby bolt and bent over the degree wheel as a temporary pointer to the engine block and set the pointer to TDC or 0° on the cam timing degree wheel. Place the base of the dial gauge on the engine block with the dial indicator or stylus resting on the top of the piston and zero the dial gauge. Rotate the engine back and forth a bit to ensure that the dial gauge is correctly zeroed. Now rotate the engine to a point before TDC where the dial gauge is at 0.2 inches or 0.5mm. Either note the reading from your pointer or mark the point on the degree wheel and then turn the engine just past TDC and stop when the dial gauge is at 0.2 inches or 0.5mm after TDC. Again, note the reading from your degree wheel or mark it on the degree wheel. TDC would be the exact center point between those two readings. If that exact point is not 0° or TDC on the degree wheel, rotate the engine until you reach that exact point on the degree wheel; then loosen the degree wheel and adjust it so that your pointer is at the TDC or zero point on the degree wheel. You can also mark that point accurately on the crankshaft pulley as this will be helpful when want to check or adjust the cam timing at a later stage, with the engine fully assembled and fitted, and will be useful when you need to set your ignition timing. Then remove the dial gauge, fit the cylinder head and install the camshaft, or camshafts if it's a twin-cam cylinder head, the vernier gear, and the camshaft timing belt or timing chain. With the engine at TDC it should be 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. In other words, the heel of the camshaft lobes for the no. 1 cylinder should be in contact with the intake and exhaust valves, and the lobes should form a "v". The engine should be completely assembled now with only the valve cover left to be attached.
Once the engine is at true TDC and you have it marked on the degree wheel and crankshaft pulley, you can remove the dial gauge and fit the cylinder head gasket and the cylinder head. Then install the camshaft, or camshafts if it's a twin-cam cylinder head, the vernier gear, and the camshaft timing belt or timing chain. With the engine at TDC it should be at the end of the compression stroke for the no. 1 cylinder, so the camshafts should be installed with the intake and exhaust valves of the no. 1 cylinder closed. In other words, the round heel of the camshaft lobes should be in contact with the intake and exhaust valves of the no. 1 cylinder, and the toe of the lobes should form a "v". The engine should be completely assembled now with only the valve cover, the intake system and the exhaust header left to be attached.
SETTING THE CAMSHAFT TIMING
The camshaft manufacturer or grinder should provide you with a valve timing diagram and a chart with the specified valve lift and the exact 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. The latter is more accurate as there is also some dwell at full-left, though not nearly as much as piston has at TDC. As indicated below, we can accurately find the point of full lift in the same way as we found true TDRC. Also, the point at which the valve lift is achieved is measured in degrees of crankshaft rotation, which is why we didn't remove the cam timing degree wheel from the crankshaft. Our next step is to attach the dial gauge to the cylinder head, with the stylus on the top of the retainer cap of intake valve of the no. 1 cylinder and zero the dial gauge. 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 loosen 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. 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.
FINDING FULL VALVE LIFT
Should the camshaft manufacturer supply a chart which uses the point of full valve lift as a reference point for setting your cam timing, you would need to find the exact point of full valve lift. However, full valve lift is not one point as the camshaft also has a dwell period as they are designed to have the valve reach full life as quickly as possible and remain open for as long as possible, which is usually a good number of degrees. This can result in inaccurate cam timing as we would need the point exactly in the center of this dwell period. We can determine this point in similar way as we determined true TDC.
Start with the engine at TDC. Then turn the crankshaft until the toe of the camshaft lobe acting on the intake valve of the no. 1 cylinder is pointing more or less upward and the heel or the rounded part of the lobe is in contact with the valve, the rocker arm, or the valve lifter. The intake valve should now be fully closed. Set up the dial gauge with the stylus on the valve retainer cap of the intake valve and zero the dial gauge. Now rotate the crankshaft until the intake valve opens and is a short distance, say 0.1 inch or 0.25 mm, past full lift. Mark this point on the degree wheel. Then turn the crankshaft and stop when the intake valve starts to close and is again at 0.1 inch or 0.25 mm from full lift. Mark this point on the degree wheel. Needless to say, the point of full lift for the intake valve would be the mid-point between these two marks on the degree wheel. This point should coincide with the valve timing diagram or the chart supplied by the crankshaft manufacturer. If not, you would need to loosen up the vernier gear and adjust it as required. You should then repeat the process to ensure that the adjustment has been made correctly. On a twin-cam engine you would need to repeat this process to find the point of full lift for the exhaust valve, do the required adjustment on the vernier gear if needed and check the accuracy of nay adjustments you may have made.
(Credit to Custom Car US)
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