The single biggest factor that determines an engine’s ultimate power potential is the total inlet valve area
Not all cylinder head designs have the same flow efficiency for a given valve area though – and it is the flow potential rather than the valve area itself that really determines the power potential – but valve area is much easier to measure and provides an ideal starting point for further analysis. There is no point however in having big valves if the port shape or other factors restrict the flow. To discuss this further it is best to consider engines with 2 valves per cylinder separately from 4 valve valve engines (or even 5 valve engines which are gradually appearing in road cars).
2 Valve Per Cylinder Engines
Engines with only one inlet and one exhaust valve can be further split into two main categories.
Parallel Valve Engines
In this type of design the valve stems are parallel to each other and usually, but not always, parallel to the cylinder bore axis. Examples might be the Mini, MGB, VW Golf, Peugeot 205 and Ford Crossflow engines. The total valve diameter is directly limited by the bore size because the valves open into the bore.
There obviously needs to be some clearance between the two valves and also between each valve and the adjacent bore wall simply to prevent contact. Production engines might have 3mm or 4mm for each of these clearances although this can be reduced on a race engine to around 1.5mm between the valves and 1mm to 1.5 mm between each valve and the bore wall to allow the largest possible valves to be fitted. So at best there is a limit on total valve diameter of about 3.5mm to 4.5mm less than the bore diameter.
This remaining space would normally be allocated as about 55% to 57% for the inlet valve and 43% to 45% for the exhaust valve diameter. In other words the exhaust valve would be around 80% of the diameter of the inlet valve for best power output – perhaps even a little less in some cases.
Combustion chambers can either be of the “bathtub” type where the volume is contained mainly in the cylinder head or the “Heron” design where the head face is flat and the volume is in the piston dish and between the top of the piston and the top of the bore.
Regardless of the exact design chosen there is always going to be some loss of flow potential because of shrouding between the valves and the closely adjacent bore wall or combustion chamber walls. In simple terms there is just not enough space for the airflow to get past the valve head into the cylinder cleanly. The bigger the valves and the closer they end up to an adjacent wall the greater the shrouding effect becomes and a law of diminishing returns sets in.
In some cases a smaller valve ends up producing more flow than a larger one if the required clearance space around the valve head can’t be achieved. The effect of shrouding can be to reduce the flow and power potential by around 10% compared to the same sizes valves with zero shrouding.
Early designs of this type of engine had pushrod type valve trains and the Mini and MGB engines were limited even further by their Siamese port design. Some of the more modern single overhead cam engines can rival the inclined valve type design in their power output though. A little common sense needs to be applied when evaluating these types of engine.
Inclined Valve Engines
In these designs the valves are angled both relative to each other and to the bore axis. Examples include the Ford CVH and Twin Cam engines which have large angles between the valves and the Ford Pinto engine which has a fairly small included angle. This design has two main advantages. Because the valves open away from the bore wall in towards the centre of the cylinder there is little or no shrouding of the flow.
As the valve opens further and the airflow increases, so the necessary space around the valve head increases at the same time. Secondly it enables larger valves to be fitted in a given bore diameter than the parallel valve head design. Within limits, the greater the angle between the valves the larger they can become although if the included angle is too large the inlet valve can hit the exhaust valve when they are both open during the overlap period.
Disadvantages of this design is that the combustion chamber has to be something like a hemisphere, or at least fairly domed, and this shape isn’t very compact and doesn’t burn well. The advantages of extra valve area and lack of shrouding outweigh this consideration by a large margin though.
4 Valve Per Cylinder Engines
The constraints of fitting 4 valves and their related valve trains into a cylinder head means that all 4 valve designs end up being fairly similar – at least in flow terms. The inlet valves are angled away from the exhaust valves, the spark plug ends up central in the chamber and usually twin overhead cams are used – although a few designs manage with a single cam and rockers. There is little or no shrouding with most 4 valve engines and in effect they are like multi valve versions of the inclined design of 2 valve engine.
There is a significant difference though between 4 valve and 2 valve engines in terms of flow and power potential for a given total valve area. This is because the ratio of total valve area to total valve circumference is not the same. To understand this better let’s look at an example. Copyright David Baker and Puma Race Engines
Compare an engine with two small inlet valves of 25mm diameter with a similar sized engine with one large valve of 35.36mm diameter. The total valve area is the same in both cases – about 982 square mm. So the total peak flow when the valves are fully open should be very similar. The total circumference is very different though. The two small valves have a total circumference of 157mm.
The single large valve has a circumference of only 111mm. The ratio is 1.41 to 1 – or in other words the square root of 2. This has a big effect on flow at low valve lifts. If all three valves are open by the same small amount – say 1mm – the two small valves have a flow area which is 41% bigger and consequently flow more air. As the valves open fully and the valve area becomes the limiting factor, this effect diminishes and ultimately disappears.
The effect of this improved low lift flow is to give the two small inlet valves a power advantage over a single valve of the same area. The effect depends on the cam profile used on each engine but is in the region of 10% to 15%.
