Background and Details of the MDS Motorsport Cylinder Head developed for best Gas Exchange and Highest Performance
Maximum power and torque is reached with the maximum amount of air in the cylinder. It allows the input of the largest quantity of fuel with the correct fuel/air mixture. For a given displacement, it means that the engine will take in and retain as much air mass as is possible in the cylinder in each cycle and across all speeds. In theory, for a naturally aspirated engine, the maximum air volume would correspond just to the cylinder displacement. It is 100% volumetric efficiency of each intake stroke. But in a real engine this can reach more than 120%, provided the use of an effective cylinder head, intake port and cam system and with a favorable external gas exchange system which results in a certain "low pressure supercharging effect".
For High Performance Engine this means:
- Largest possible displacement (often set by class/category regulation)
- Use of high revolution (regulation deciding)
- Highest combustion efficiency (compression ratio as per regulation)
- Low mechanical losses (base engine geometries in regulation
- Best possible gas exchange (cyl.head often unrestricted in regulation)
- High air utilization/ air efficiency (particularly in case of restrictor regulation)
Best gas exchange and combustion requires an optimized cylinder head design. This is independent of 2 valve or 4 valve cyl. heads. This goes together with best possible detailed valve/ springs and cam system and transmission. The cylinder head must also work efficiently with the external gas exchange system, on both intake and exhaust side.
Highest performance is linked to cam profiles with high lift and time area inside a given duration. High lifting cams must have enough clearance in the cam housing; also have the provision for corresponding and matching strong valve springs. In the direct acting cam systems, the tappet diameter must be of largest possible diameter; this allows the fastest lifting cam profile with resulting large valve opening area at the important phases of low valve lift. See figures below.
Advantages of a unique Motor Sport Cylinder Head compared to a Series Based modified Cylinder Head
Std. Based Cylinder Head modified from series casting. Converting a series based cylinder head to high level of gas flow and tune frequently is limited by:
- Possible valve sizes & max dia. and valve center location are decided of series base design
- Valve inclination and included angle is set by the series base design
- Strongly compromised flow at near cylinder walls when large valves are used
- Machining of ports is restricted and limited due to series casting constraints
- Maximum port area, area run and overall shape/ geometry is decided by the series casting
- Casting puts limits on machining of ports, comb. chamber, min. wall thickness must be retained
- Water jacket cores, cam box area as well as oil handling all defined by casting with limitations
- Machining for very high lift cams and largest tappets can reduce structural strength
Design of a series casting is a compromise for effective low cost high volume production. The casting, process and use of material are often to a lesser specification than in a unique dedicated motor sport casting. This brings limitations in structural integrity, strength and reliability and forces compromises of the main engine parameters. It can lead to a limited CR, a reduced max. cyl. pressure level; alternatively force the use of non-optimum ignition settings or richer mixture to reduce pressure or thermal loads.
This can degrade potential engine performance and response. It can limit the applications of the cylinder head, as well as its operating life and result in shortened rebuilding intervals. It is easily understood a series based cyl. head with its limitations must be compromised in almost all areas to retain a sufficiently strong structure and design. Special methods like offset machining, can be forced, most often compromises in wall thickness must be accepted, for instance in port walls. Added material by welding must be used to create required geometries for highest flow and performance, typically in ports. But also for functional design in high performance cam systems, like tappets and valve spring sizing and areas.
This takes out the only remaining advantage, the potential lower cost of a modified cylinder head from a series based casting. When all modifications and time consuming machining is carried out this moderate potential in cost gain usually cannot be realized and is effectively wiped out.
Two cylinder heads, left Volvo B23 std., right MDS EVO 2V
Motor Sport Cylinder Head based on an unique casting
The solution to above problems is simple and straightforward, that is to start from a dedicated and unique casting for the motor sport cylinder head. This is from the start designed to minimize all mentioned limitations and to provide best possible conditions for good gas exchange and overall performance.
Specifics MDS 2V EVO- cylinder head
This cylinder head is a logical product of above reasoning. Compared to a series based cylinder head casting the design changes are substantial.
- Design uses an optimised valve inclination of 8 degrees to vertical
- Effective and straightened ports with favorable geometry for both intake and exhaust
- Combustion chamber of compact wedge type and with a central positioned spark plug
- Very large valves can be used & as well as large tappets capable for high lift aggressive cams
- Structurally stiff and stable cyl. Head with optimised wall thicknesses
- The casting is high grade heat treated aluminium for maximum strength.
The short and effective exhaust port with a large inner radius.
The excellent geometry of the intake port as well as the compact wedge combustion chamber is clearly seen in this cross section of the MDS EVO 2V cylinder head
Details of exhaust ports, std. B23 left and EVO2V right (red port, vs. std port yellow line)
The combustion chamber must have space for the largest valves, the spark plug as central as possible and be compact for high compression ratio and fast burn. Effective combustion, high knock tolerance is also required. Unhindered gas flow at low lift is a must for a broad torque curve. Std. based cyl.heads allow only for small machining and modifications. A machining of the bottom deck is used for reduced chamber volume and higher CR, with risk for a reduced bottom deck strength and stiffness. The series B23 head has a deep bath tub chamber and vertical valves. EVO 2V with inclined valves gets a natural wedge combustion chamber with very little masking around the valve outflow. The basic shape can manage a high CR and with full CNC machining the combustion volumes are equal across the head, for repeatable, stable combustion characteristics.
Left: EVO 2V wedge combustion chamber Right: B23 std. bath tub combustion chamber
Some technical definitions
- Flow is measured in CFM; units are English Imperial and equals ”cubic foot/ minute”. It is recorded at a chosen pressure drop, often in inch H2O water pressure. Flow data are recorded by developers at different pressure drop, which should be noted when doing comparisons. Adjusted comparable numbers can be created with equalization calculation methods.
- Port size is measured in millimeter (mm) diameter if round shape, sometimes an equivalent diameter is used in case of more square ports.
- Gas Speed in the port can be measured directly from the flow test, but can also be calculated as a theoretical mean speed with a known flow and known area of the port. This number is theoretical since it corresponds to an ideal flow across the port without turbulence, which seldom is found in reality.
- Valve Area is calculated in following formula: Valve Radius x Valve radius x Pi (3, 1416). For the 4V it is of course multiplied by 2 due to the double number of valves.
- Valve Opening Area is calculated with following formula: Valve Diameter x 3, 1416 x Valve Lift. For 4V-engines it is multiplied by 2 to for the double number of valves.
- Flow Balance between the intake and exhaust side can easily be calculated. The exhaust valve flow is divided by the intake valve flow.
- The basic geometry of the port decides at high degree the maximum port flow at very high valve lifts. This basic geometry is very difficult to alter through only the porting work; it is inherent in the basic head design. A motorsport cylinder head has often ports with a raised entry to create as straight port shape as possible.
A given engine will require a specific minimum intake gas flow to achieve a set power output. It is not possible however, to only use the porting work alone to create a very large port area, to reach this required gas flow. An overly large port area results in a very low gas flow speed which reduces among other things the inflow effect/ or ram charging effect. Not least, to achieve the optimum flow conditions, the complete external gas exchange system, including the intake runners, the throttle bodies, and the plenum/ air filter as well as exhaust headers and silencing system has to be correctly designed and dimensioned.
Key comparison numbers: A four-stroke engine only flows the intake when the valve is open at every second revolution. This means at say 8000 rpm, the intake valve opens and closes 4000 times a minute, which corresponds to 66 times per second. A very intermittent gas flow indeed! This explains why the measurements of stationary gas flow performance in test rig is mainly a base for the later so important detailed gas exchange optimization and development in the engine tuning work.
The main principle and criteria for the intake port development is very simple, always search for a port with smallest cross section area giving the sufficiently high flow for the set power and performance target. This port will then provide the best conditions to achieve broadest torque curve with lots of useable performance as well as a tractable and overall good engine.
4 Valve vs. 2 Valve Heads?
Obviously there are differences in area for intake valves when a 2 valve engine is compared to a 4 valve ditto. One large intake valve is then compared to the two smaller intake valves for a given cylinder. The 4 valve head has most often a valve area about 20 % larger than the area for a standard 2 valve head. This difference can be reduced somewhat when the 2 valve engine is designed for the largest possible intake valve diameter.
Of more interest is the comparison of the valve opening areas when calculated at very low valve lifts. Here, the 4 valve head can show a valve opening window and area up to 40 % larger than for a standard 2 valve head, at valve lift typical for said engines. This means the 4 valve head can have a larger valve area flow window than can be seen for a race tuned 2 valve cylinder head. Still, this doe not results in an overall max flow for the std. 4 valve head than for the race tuned 2 valve head. The reason for this is the cam duration plays a great role. And even more it is linked to that at higher valve lift the flow is more decided by the port, its geometry, inclination, angle of attack which decides the maximum flow. But at low valve lift, the large flow window, together with the valve seat shape are decisive factors for the gas flow. The 4 valve engine does not need so long cam duration. It can be described as the 4 valve flow diagram is more of an “on/ off” character.
This benefit of best port geometry is very important and underlined in the EVO 2V cylinder head with it’s from race principles upraised intake and exhaust ports. Large and smooth with inner radius and with a small angle to valve center line (on the intake side further benefiting from the angled valve 8 degree towards intake port) results in high flowing ports with high max. flow up to he very highest valve lifts (up to 19, 5 mm can be used with good effect).
Scavenging of exhaust gas should take place as quickly as possible after the power stroke. The outflow is under pressure from the cylinder and the piston moving upwards. Burnt gases are effectively pushed out (opposite to the conditions at the intake process with its limited suction pressure in the cylinder as the intake flow driving force). The first phase of the exhaust blow down at initial opening of valve is at critical pressure ratio, flow then in direct proportion to the opening area. Then, the pressure differential is reduced and the piston takes over as the pushing out force. At the end a suction pressure can assist. Careful tuning of the exhaust system helps the complete scavenging and pulls through of the intake gas flow in the overlap phase. Outflow under pressure means the exhaust valve and port are smaller compared to the intake side. Important is the port shape and its area run is smooth with no sharp corners inducing turbulence, to avoid pressure losses and unwanted high heat transfer.
Balance of Intake and Exhaust, Valves and Ports
Balance of intake and exhaust flow is decided from the engine application and performance level, the engine displacement and not least of the max engine speed and the wanted useful speed range. A naturally aspirated engine puts normally a high priority to a high flow at the intake side. Deciding factor is the maximum flow capability of the port at high valve lifts. In case of turbo engine the exhaust flow get higher priority, turbo charging efficiency is linked to the most effective gas flow to the turbine. The unique developed motorsport cylinder head makes it easier to optimise fully to the particular requirements of each application. In particular the matching of cylinder head to power, performance and displacement has proven very advantageous with the EVO 2V cylinder head.
Port Geometry for high gas speed and high maximum flow at very high lift
From above, it is understood the port performance is decided strongly already at the design phase. Porting work later normally results only in modest or moderate improvements. Very often series based heads have too small radius at critical corners (or to too sharply bent ports) limiting absolute max flow and inducing turbulence. Ports with too small inner radius are hard to port effectively, due to the compromise to open up the port for high flow at the same time reducing unwanted turbulence. The exhaust port of the Volvo B23 cyl. head have these characteristics with almost no inner radius in standard shape, well shown in the figures above.
Volvo B23 also has some of this problem in the intake port. It is seen cyl. heads of type 530, also on the types 405, 531. A solution can be found with added material by welding, if regulation allows such methods. One further difficulty is how easy they can be fully shaped and procured in the CNC-machining. For accurate repeatable and exacting flow in all heads this should be fully CNC-machined with a minimum finishing off. A high flow port shape in a standard based casting gets too pushed to its limits, only a fully manual process can be used. Resulting forced very thin local wall thickness can create risks in mechanical structure and stability of the head.
The motor sport cylinder head EVO 2V is from the beginning designed for CNC machining, which guarantees a repeatable high flow product. For 4 valve engines, the situation is usually better. In case of Volvo 16V cyl. heads, the basic geometry is correct with good form of ports, well sized and positioned valves and a good combustion chamber. MDS has used this to develop own CNC machined EVO version 4V heads, versions are available for both naturally aspirated and turbo applications.
Influence of surface treatment and finish of port on maximum gas flow?
A common question, fact is precise CNC machining is a real guarantee for high flowing heads. A costly surface super finish brings a very small benefit. It is always extremely important to use cylinder heads and ports not only good in gas flow tests but even more proved to be successful at race conditions.