Frequently Asked Questions
Q: Can I add a turbo to my car?
A: Possibly. There are many factors which have to be taken into account before you can consider an engine ready to have a turbo attached to it. On modern engines the biggest factor is usually the fuel injection system – an injection system for a non-turbo engine is very unlikely to behave in a manner that would allow a turbo to operate without it damaging the engine. Adding a turbo to an engine is not a quick fix for more power: it must be fitted as part of a carefully planned and tested modification.
Q: Can I change/upgrade my existing turbo?
A: Maybe. All turbos operate on the same basic principles, however very few turbos are directly interchangable due to their different physical characteristics. Changing a turbo will usually require replacement of more than just the turbo, and may require fabrication of new parts. A radically different turbo may also impact on other factors such as fuel/ignition maps, suitability of the intercooler, and so on.
Q: Can I modify my existing turbo?
A: Turbos can often be upgraded to different specifications of the same basic turbo (for example by fitting a larger compressor wheel). This is very much dependant on which turbo your vehicle has to start with, and whether there are different options available for it. In some circumstances parts from different types of turbo can be made to fit – again there may be compromises to nearly any upgrade.
Q: Can I raise the boost on my turbo?
A: Possibly. Raising the boost will always place higher stresses on both the turbo and the engine, and you must carefully consider the possible consequences of doing so. For example, in extreme cases the turbine shaft may completely snap, or you may melt a piston in the engine – in any case the results may be very costly. If you are planning to raise the boost on a turbo engine it must be done in a carefully controlled manner. We recommend avoiding using single ‘bleed valve’ systems to raise boost – the level of control provided by this method is not adequate, and can result in boost spikes and some times an actual drop in performance.
Q: What is a boost spike?
A: The level of boost produced by a turbo is dictated by the speed at which the turbine rotates. If the wastegate cannot react to the changing speed of the turbine rapidly enough it may briefly run to an excessively high speed (called overboost) before dropping back to its normal maximum speed. This brief excessive speed is called a boost spike, and can occasionally cause damage to the turbo or engine or cause problems with the fuel management system.
Q: What is a blow off valve?
A: Please see the Turbo Basics section
Q: What is the function of the wastegate?
A: Please see the Turbo Basics section
Q: What is ‘lag’?
A: Turbo lag is the delay between the throttle being opened and the engine producing more power due to the output of the turbo. This lag exists due to several reasons:
- The rotating assembly of the turbo has to be accelerated to many tens of thousands of revolutions per minute before it can compress the air that is passing through it.
- The air in the intake system takes some time to compress to its ultimate pressure.
- There is insufficient exhaust gas being produced by the engine to fully accelerate the turbine, meaning that the turbo must begin to produce some boost (and therefore more exhaust gas) before it has enough energy to reach full speed. This phenomenon is more obvious on larger turbos where the first few pounds of boost takes far longer to build than the rest of the pressure does.
Lag has always been the Archilles heal of turbo systems. Engines with high lag can become difficult or at least unpleasant to drive. In recent years much development has happened to help reduce the problem of lag. Ceramic shafts, ball bearing cores and variable vane turbines are three examples of methods for making turbos more responsive.
Q: What is a ceramic shaft?
A: A normal turbo uses a steel shaft with a steel turbine wheel. In all but a few cases the turbine and shaft is a single inseparable piece. A ‘ceramic shaft’ is a combination of a steel shaft bonded to a moulded ceramic turbine wheel. The ceramic wheel is lighter than an equivalent steel wheel, producing lower rotational inertia therefore lower lag. The downside to this is that the ceramic wheel is brittle and likely to shatter should it impact against the exhaust housing (in the case of failing bearings).
Q: What is a ball bearing turbo?
A: Conventional turbos use oil filled floating journal bearings to allow the shaft to rotate, and thrust bearings to prevent the shaft from moving backwards and forwards in the turbo core (please see the diagram in the Turbo Basics section). Ball bearing turbos use a ball bearing pack in the turbo core to both allow rotation and prevent shaft movement. The advantage of this system is a marked reduction in rotational friction, producing lower lag.
Q: What is a variable vane turbo?
A: Because turbos have to deal with a very wide variation in air flow through a fixed size and shaped housing with fixed shaped turbine blades this represents a compromise rather than optimal performance. A variable vane turbo uses a method to dynamically alter the shape of the exhaust housing depending on how much boost it is producing. This allows for a more efficient gas flow across the turbine and therfore lower lag. The shape of the housing is altered by using a concentric ring of blades in the exhaust which can be forced to fan out in the air stream. The second benefit of this set up is eliminate the need for a wastegate. Once the turbo reaches its maximum speed the change in the angle of the blades reduces the efficiency of the turbine, preventing it from spinning any faster.
Q: What is EGT and why is it important on turbo cars?
A: EGT stands for exhaust gas temperature. With every internal combustion engine the gas exiting the engine is at several hundreds degrees, however with turbo engines this temperature typically gets forced higher. With restrictive exhaust manifolds and extreme combustion conditions caused by high performance turbos, the EGT can often exceed 900 degree celcius. At these temperatures the materials in the engine and turbo are liable to suffer damage or exhibit outright failure. When designing or changing a turbo set up it is very important to ensure that EGT is kept within safe bounds.
Q: I’ve been told my turbo is water cooled – what does this mean?
A: Every turbo is lubricated with oil that runs through its bearings which also serves to extract surplus heat from the turbo. Some turbos, more often ones found on production passenger vehicles than aftermarket units, are also cooled by way of a water jacket that surrounds the centre housing. Water is drawn from the engine cooling system and passed through the jacket to draw off extra heat.
Q: Why is keeping a turbo cool so important?
A: Because turbos operate under extreme stress and extreme heat, it is vital that they maintain very high tolerances in order to prevent them self destructing. Very high heat in a turbo can directly damage some parts, however the major heat damage usually occurs after the engine is shut down. Due to turbos being constructed largely of cast iron, they very effectively absorb heat from the engine. Oil left soaking around the bearings will get baked by this heat, causing small but abrasive pieces of carbon to form. This carbon will wear away at the bearings, eventually leading to turbo failure. Please see the Turbo Damage Guide for more details.
Q: So how do I prevent this failure?
A: A turbo will survive for many tens of thousands of kilometers driving if it is cared for. The simple rule is to ensure that before an engine is shut down that it is left to idle for a short time, particularly if it has been pushed hard or raced. Leaving it to idle will ensure that the heat is dissapated away from the turbo. Water cooled turbos are usually better in this regard, as the water jacket is very effective in soaking up the excess heat. However they are still limited in their operation after engine shut down as the water is no longer circulating around the cooling system.
Q: Do I need to use a turbo timer then?
A: Not necessarily. Careful maintainence habits such as ensuring the engine is idled before shutdown will be sufficient to ensure that a turbo is not ruined through heat soak. A turbo timer is however a much more convenient way of ensuring that this happens without having to wait around each time you turn your engine off.
Q: Ah, but I’ve got an intercooler, so I don’t need to cool my turbo down!
A: Please go sit at the back of the class. An intercooler does not in any way shape or form assist in cooling the turbo. The function of an intercooler is to lower the temperature of the compressed air coming out of the turbo before the air enters the engine. The typical net effect of an intercooler will be to slightly raise the load on a turbo rather than lowering it.
Q: So what’s the point in an intercooler then?
A: Because turbos by their very nature will heat as well as compress any air that passes through them, intercoolers must be used to bring the temperature of the air back down again. Cooler intake air makes for more efficient combustion and in high performance engines it assists to prevent detonation (uncontrolled explosive ignition of the fuel mixture). An intercooler is simply a large radiator used for cooling air that passes through it.
Q: Why is using high octane petrol on a turbo engine important?
A: This is not a problem with the turbo itself, but rather with the engine attempting to burn a mixture of petrol and highly compressed intake air. Lower octane fuel has poorer resistance to detonation, and therefore should be avoided on turbo engines. The rule of thumb is to use the higest octane fuel you can – 96RON (Super/Premium), or 98RON (BP Ultimate/Mobil 8000) if it is available.
Q: How do I know what condition my turbo is in?
A: There are a number of easy ways to get a basic idea what state it is in:
- There should be no visible blue exhaust smoke produced.
- There should be minimal whine or whistle produced by the turbo (very large truck turbos tend to be an exception to this rule).
- If you remove the intake pipe off the front of the turbo and wiggle the front of the shaft there should be less than about 1mm play from side-to-side (lateral play) and no detectable front-to-back play (axial play).
- There will be minimal oil in the air intake pipe (turbos will always leak a little oil).
Please see the Turbo Damage Guide for more details.
Q: Is there a separate oil system for the turbo?
A: Except for very unusual cases, No. The turbo will use the engine oil for its lubrication and cooling needs.
Q: Do turbos have specific oil requirements?
A: Yes, good quality oil is essential. You do not need to use specific ‘turbo grade’ oil, however good quality is a must.
Q: How often do I need to change the oil?
A: Every 5000kms on a petrol engine. Leaving oil changes for too much longer can spell death to a turbo.
Q: Are there any problems with fitting a high performance air filter?
A: Yes. To ensure a good lifespan of your turbo it is very important that your air filter properly removes small particles from the intake air. With compressor blades spinning at over 100000RPM small debris passing through the turbo will make a very effective job of sand blasting the compressor blades. While cone filters and mesh inserts (such as K&N) are fine to use on a turbo vehicle, it is very important that they are regularly cleaned and oiled – without oil their filtering abilities are severly reduced. Please see the Turbo Damage Guide for more details.
Q: I’ve seen “electric turbochargers” for sale. Do these work?
A: A typical 12V fan as used in electric turbo kits pumps in the region of 15 cubic feet of air per minute. A typical 2 litre turbocharged engine will require approximately 500 cubic feet of air per minute. You do the maths.