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What can I optimize on my engine? Is there anything I should pay special attention to? The costs and complexity of engine tuning is dependent on how much more power is needed and what you want to achieve. Often it is not enough just to switch certain parts for better ones, because they must work together and have to be tuned to reach their full potential, and it can be easy to get yourself into trouble. However, with turbo engines you get a significant increase of power with a few simple steps and the right combination of parts. We have collected the most important technical basics about engine and turbo tuning and explain their function to make things a bit clearer. If you are planning a modification and have further questions, please contact us.
The turbocharger contains the central bearing housing, and at the ends the turbine and compressor housings. In these housings there are paddle wheels fixed rigidly to a shaft, which is supported in the middle by the bearing.
The turbine housing is mounted directly on the exhaust manifold, and the turbine wheel is driven by the exhaust gasses of the engine. Since both wheels are rigidly connected, the compressor wheel rotates with the turbine, which inducts fresh air. At high enough speeds, a pressure build-up occurs, and the engine can be supplied with considerably more air than it could suck in itself. Assuming enough fuel can be supplied to match the increased airflow, this results in a higher engine power.
The bearing housing should be water-cooled, as this drastically reduces the risk of coking of oil after engine shutdown.
The task of the turbine side is to utilise the exhaust gas energy to turn the compressor wheel, so that it delivers the required airflow and pressure fast enough. With the same pressure ratio, a small turbine responds faster than a large one, but has higher back pressure at high speeds, which is the difficulty in selecting the turbine size.
The compressor wheel increases the pressure and density of the inducted air. The efficiency of the compressor is determined by the pressure ratio and the volume flow. At the optimum size, the optimum efficiency (about 75%) must be positioned in an RPM range that is often used. The lower the efficiency of the turbine, the higher the temperature of the compressed air, so it is important that the efficiency should be kept as high as possible over the entire speed range.
The performance maps of the compressor and the turbine are diagrams of their efficiency at different pressures and speeds. This allows conclusions to be drawn about their efficiency and behaviour under different conditions, helping you choose the best turbine and compressor wheel sizes for your application.
The compressor performance map compares the pressure ratio with the mass air flow. The left-side limitation of the efficiency islands shows the surge limit. This line represents the maximum amount of pressure the turbocharger can produce while flowing the least amount of air. Beyond this, no more flow takes place, since the airflow breaks off at the compressor blades. This can happen when the throttle is closed when the intake pressure is high but the volume flow is low. Backward curved blade ends and a recirculating air valve can prevent backflow of the gases and positively shift this limit.
The right side of the efficiency islands is the choke limit, where the compressor is at the limit of its flow rate. This happens when speed of sound is reached at the compressor wheel. By displacing every second impeller blade back, manufacturers achieve a delay in the choke limit.
The turbine map compares the turbine pressure ratio with the turbine mass flow. The behaviour of the turbine is determined by the temperature and pressure gradient before and after the blade wheel.
The charge pressure control by means of a wastegate is the most common. The wastegate is a valve that diverts exhaust gas past the turbine, controlling its speed.
The wastegate is held closed by a spring and opened by the wastegate actuator. At a certain boost pressure level, pressure from the compressor side acts on the diaphragm in the actuator, which moves to open the wastegate.
The control by means of a wastegate is the best possible, but it wastes valuable exhaust gas energy. This is because the valve begins to open before the turbo reaches the desired pressure, and this energy could still be used to accelerate the turbine wheel before it has reached the target speed.
The wastegate can be internal or external. The internal wastegate is installed within the turbo itself. A disadvantage of this system is that the diverted exhaust gas is usually directly behind the turbine wheel, and immediately joins the exhaust gas from the turbine wheel, resulting in high turbulence.
With the external wastegate, these two exhaust gas streams are brought together much later to a freely selectable location where a cross-sectional expansion can also take place. The minimum distance from the turbine outlet should be approximately 50 cm. This external wastegate has the disadvantage that, if not optimally arranged, it can form vortices in front of the turbine which disturb the airflow. The wastegate output in the exhaust manifold should ideally be located in the volume flow of all cylinders, at a flat angle from the main mass flow (not at right angle) and symmetrically flowed to the turbine housing.
Bleed valves allow you to adjust the boost pressure without having to change the basic setting of the wastegate. It is installed on the pressure line between the compressor and wastegate actuator. The valve bleeds off the pressure before it acts on the wastegate diaphragm, so that the wastegate remains closed for longer, resulting in higher boost pressure.
A blow-off/recirculation valve is installed on the intake upstream of the throttle. When the throttle is closed abruptly under high boost pressure (during a gear change or sudden lift-off), the high intake pressure flows backwards out of the compressor. This can stall the compressor and cause pressure oscillations. This can damage the compressor wheel and shorten the life of the turbocharger.
A recirculation valve and releases the excess pressure back into the intake before the compressor side, so that the compressor wheel remains at high speed for longer. A blow-off valve works the same way, except that it simply releases the excess air into the open air, giving the characteristic sound.
The charge air pipes should be as short as possible to minimise throttle lag, and without sharp bends to minimise pressure loss.
One consequence of increasing the charge air pressure is also increasing the air temperature. This makes the engine more prone to knocking and pre-ignition, and because of the reduced air density, the power is reduced.
The goal of the intercooler to cool the air after it has been compressed by the turbocharger. This gives more power, torque, air density, reduces the chanced of pre-ignition. The same power can also be achieved with less pressure. The size of the intercooler depends on the mass air flow and the air temperature. There are two types of intercooler: air-to-air and air-to-water:
Air-Air Intercooler - This is the most common type of intercooler. The charge air is cooled by the ambient air flowing through the intercooler. It is advantageous to place the intercooler neither in front or behind another radiator in order to ensure that the airflow is as free as possible.
If this is not possible due to the packaging at the front of the car, it should at least be the first cooler in the airstream. Cooling air ducts can also improve efficiency. The best design in terms of cooling technology is a large surface area, low cooling network depth (airflow through Intercooler is better) and a high charge air back pressure. However, if there is too much turbulence, blocking flows occur. Here there is a balance between low charge air pressure loss (high cooling network depth) and high cooling efficiency (sufficient turbulence). The efficiency of an air-air intercooler decreases as the ambient air temperature increases. An efficiency advantage can be achieved by spraying water against the intercooler.
Air-Water Intercooler - The charge air is cooled by water, which in turn is cooled by a small radiator at the front of the car. The water is circulated by an electric pump. The air temperature can be controlled more accurately, and the overall intake length is reduced. An air-water intercooler can be more efficient than an air-air intercooler, but is more complex and more expensive.
For short full load runs (1/4 mile), there are also related dry ice air heat pumps, which have an extremely high efficiency in racing operation and, like the air-water intercoolers, also contribute to a shorter intake path.
For a turbo petrol engine at full load, the EGT is around 850-950°C. It can be higher for a short time, depending upon tuning, but should remain below 1050°C. Excessively high EGT can damage the turbo and catalytic converter. For turbo engines, the temperature should be measured in or near the turbo, if possible before.
The exhaust manifold is subjected higher thermal load in a turbo engine compared to a naturally aspirated engine It is also subjected to higher back pressure up to the turbine and it has to support the weight of the turbo and the wastegate. As a result, the manifold works more and needs to be made from high-quality materials (including the gaskets, screws, and nuts).
For a high performance turbocharged engine, a metal catalyst should be used instead of a ceramic catalyst, as this has larger cross-sections in the honeycomb structure, reducing back pressure in the exhaust system.
To minimise back pressure, the exhaust piping should be as large as possible and without tight bends. The silencers should not have any chambers or constrictions, especially at high charging pressures.
To withstand to the increased temperatures and pressures, the following options are available: high-quality bearings for connecting rods and crankshafts, connecting rods, pistons, stronger valve springs, pins/screws for connecting rods, pistons, cylinder heads, a nitride crankshaft and a stable cylinder head gasket.
Occasionally even diesel blocks and crankshafts can be used, since these are designed for higher combustion pressures. The increased temperatures are counteracted by additional oil and water coolers, oil spray cooling of the piston heads, sodium-cooled outlet valves and higher speed fans.
The compression ratio determines economy and performance at certain boost pressures, turbo lag, required fuel octane rating and intercooler efficiency. However, it must be reduced in the turbo engine due to the higher temperatures and pressures compared to a naturally aspirated engine. The usual ratio is 7 to 8.5:1.
Compression reduction by means of an intermediate plate is the simplest method of reducing the compression ratio, whereby the other engine components are usually retained. On the VR6, for example, a spacer sleeve for the chain tensioner must be used. The control times are also shifted in the early direction. When increasing the performance of turbo engines with low compression, a further reduction is achieved by thicker cylinder head gaskets.
Any machining shouldn?t be too aggressive. Only a smoothing or very slight channel expansion should be carried out in the outlet channels. The combustion chambers and inlet ducts can be treated more generously. However, due to the higher temperatures and pressures during turbo operation, a little less machining should be done here as well. Great attention should be paid to the transitions from intake manifold cylinder head, cylinder head exhaust manifold and exhaust manifold loader. In professional machining, seat rings for valves and their shafts are also enlarged, requiring modified valves.
These are present in almost all standard turbocharged engines. While the intake valves (300 to 500°C) are still relatively well cooled by the incoming gases, the exhaust valves (up to 700°C) are much more heated by the exhaust gases. The temperatures can be reduced by about 100°C due to the sodium filling.
Compared to a naturally aspirated engine, extremely "sharp" camshafts with long opening times, overlap and high lift are not necessary for a turbo engine, as they can cause a high proportion of the exhaust gas to return to the combustion chamber due to the high pressure in the manifold.