Most of this information is what I have learnt, and some knowledge that I have gleaned from other sources.
There are many different things to modify on a turbo car to improve performance. The methods can be relatively cheap, or more expensive. As always, there is a trade-off between PERFORMANCE and RELIABILITY.
This I can tell you from my own knowledge. As they say, 'experience is knowledge' !
Topics covered so far:
Turbocharging is essentially a simple theory, but can get tricky
to put into practice. Modifications to an engine have to be made to run with
A turbocharger is placed in the exhaust from the engine. It is made up of two sections, joined by a spindle. The exhaust housing contains an exducer, which is like a fan on one end of the spindle. These blades sit in the exhaust from the engine.
When the engine is running, exhaust is spinning the blades, causing the spindle to rotate at high revolutions (up to about 120,000rpm).The other side of the turbo contains the compressor wheel. The operation can be likened to a hair drier. As the spindle is turned by the exhaust gas, the compressor blades on the other end of the spindle take INLET air, and compress it, forcing it into the engine manifold. This compression of air into the manifold is called boost, and can be set to a maximum pressure using a 'wastegate'. The wastegate is basically a diaphragm that opens at a specific pressure. Boost is measured in Pounds per Square Inch (p.s.i), or BAR (barometric pressure).
In this way, the turbocharger uses 'free' from the engine's exhaust to create more power. With the extra airflow, the engine needs more fuel, to keep the mixture correct, otherwise pinging, or worse, detonation may occur. (see topic below). Other issues include the engine's compression (static and effective), as the engine must be built to handle 'boost' (see below). Another issue is heat from the turbocharger, and this can be partly resolved by using an ...
An intercooler is used to cool the intake air flow. The theory is quite simple. Cold air is more dense by volume, and burns better. If the fuel mixture in the cylinder is too lean (not enough air or too little fuel), the pinging or detonation can occur. This topic is covered below. To help reduce the likelihood of this, and intercooler is placed in airflow, usually front mounted in the car in between the bumper and radiator. The compressor from the turbo gets very hot, due to the friction of the turbine spindle, and the heat of the exhaust, which the turbocharger absorbs. Thus, then the turbo compressed intake air, it heats it (pressure = heat). The intercooler is placed in-line, between the turbo compressor, and the intake manifold. The hotter (than atmosphere) compressed air goes through the intercooler, and then into the engine. A good intercooler may drop air temperature from 70 degrees cent. to about 50 degrees cent.
There are 2 types of intercooler available.
Air to Air intercooler - This operates much like a radiator except intake air flows through it, not water. Air flow through the intercooler from the car travelling, cools the intake air travelling through it. This is the most common intercooler, as it is relatively light, and cost effective.
Water to Air Intercooler - This cooling method uses water, instead of air to cool the intake charge. A water to Air intercooler is heavier, because of the water, but more efficient than an air to air. It is usually more expensive as well.
Contrary to popular belief, a larger intercooler will not produce a lot more 'turbo lag'. The larger the intercooler you can fit, the better, as it will provide better cooled intake charge.
Turbo cars need a good free flowing exhaust, as under boost,
effective compression of the engine is high, and a much larger volume of fuel
and air is being forced into the engine (the idea of a turbo), therefore much
more exhaust gas needs to be expelled.
It is possible to put a very large diameter exhaust system on a turbo car, and although many people say that the LESS BACK PRESSURE the BETTER, this is not quite correct. If there was NO back pressure, then the turbo would not spool smoothly, and as soon as the throttle was released, the turbo would slow quickly. (turbine speeds reach upwards of 100,000 rpm at full load).
Some back pressure is required, however, not as much as you need from a normally aspirated engine. My car, for example has a 2.2 Litre, EFI 4 cylinder engine. (with intercooled turbo as well, of course!). The standard exhaust system has a diameter of about 1 7/8". This diameter of course will reduce in the bends of the exhaust pipe (as standard car exhausts tend NOT to be mandrel bent). This reduces further the amount of gas that can be efficiently expelled from the engine. The standard muffler and catalytic converter are usually restrictive, in order to keep the exhaust as quiet as possible, and produce acceptable exhaust emission levels. This restricts flow, and thereby restricts performance of the vehicle.
Many Performance Exhaust establishments that I have spoken to estimate a free flowing exhaust system for a turbo car to improve performance by 10 - 15%, without any other modifications at all. This means more power, most likely some better economy. Not to mention, MUCH BETTER SOUNDING!
Boost is the amount of pressure the turbo creates, to force
air into the engine, above normal.
Most older factory cars, Telstar TX5's, TX3's, Cordias, Starions etc etc all had lower boost pressures from the factory, usually around 5-7 psi.
This was better for engine life, and due to the technology being older back then, the cars were a lot smoother a nicer to drive. Even so, when TX5 engines first got a turbo, they would usually accelerate well to 3000 rpm, then explode in a fury of horsepower, resulting in torque steer, and wheel spin in lower gears. The lower boost levels in these cars would generally mean that they would not have problems with 'pinging' or detonation, unless boost levels were raised significantly higher without altering ignition timing and fuelling to the engine.
These days, with better turbo technology, and engine management
systems, turbo powered cars and other vehicles can run higher levels of boost,
resulting in more power (we all love that idea!), and better drivability.
A Subaru WRX comes standard with about 14 pounds of boost.
To put this figure into perspective, we need to know what boost, or bar pressure is.
1 Bar = 14.7 pounds of boost (psi, or pounds per square inch).
This means when a WRX is running 14 psi of boost under load (and full boost is being produced), twice the amount of normal air volume is being forced into the cylinder, because 1 Atmosphere is 14.7 psi.
You cannot get more than 1 atmosphere of pressure into the cylinders of a normally aspirated car, as nothing is forcing the air in. It is simply being drawn into the cylinders via the operation of the engine.
Increasing boost by a few pounds can result in immediate, and
noticeable performance gains. However, care must be taken, as increasing the
boost level too much (resulting in a higher ration of air to fuel), may result
in 'pinging' or 'detonation' in the engine, due to a lean mixture (not enough
fuel in the air:fuel mixture).
Pinging occurs when the mixture either has too much air, or too little fuel. Instead of a controlled burn taking place in the cylinder, the mixture starts to explode. This is called detonation and should be AVOIDED AT ALL COST.
For any turbo owner, especially with a modified car, it is imperative that they fit a boost gauge, and even better, a mixture display. This will ensure that you will KNOW when an overboost situation may occur, or when your fuel mixture is running lean.
To fix lean out, either reduce the boost, add an extra fuel injector, or get a new chip or ECU for the car (with altered ignition and fuel maps for higher boost levels). These sort of modifications should be performed by someone who knows what they are doing. It's easy to kill an engine if you aren't careful.
A Blow Off Valve, or BOV for short is a simple device that can save you a lot of money. It is used to vent pressure away from the throttle butterfly when it closes on full boost. Pressure is either vented to atmosphere to create that PSSSSHHHT sound you may have heard from some turbo cars (rally cars too), or back into the turbo inlet.
Situation - You are in third gear, under full boost, approaching 6000rpm, ready to change gear. Your foot comes of the throttle. the throttle butterfly closes. The turbo is spinning extremely fast from the exhaust, and the boost it was just producing, and is still trying to force air into the manifold. The shock of the butterfly closing can damage the turbo overtime, as it slows it down quickly, while the exhaust is still trying to make it turn!. Where then to vent this pressure? It's best to vent it back into the turbo inlet, to keep the turbo spinning, or 'spooled'.
In the case of my TX5, a valve is in place between the turbo compressor and the inlet manifold. It vents pressure back into the pipe entering the compressor. This helps to keep the turbo spooled, and protect it from damage.
Some BOVs also vent a small amount of pressure to the atmosphere to create a noise. Some like it, I personally think it's just an attention grabber...
Instead of buying a new/larger turbo like a IHI or Garret, you can simply highflow your own turbo. Works great for me, because externally, my turbo looks the same as stock, but the compressor wheel is now 42mm in diameter. The standard compressor wheel for a TX5 or MX6/626 2.2L engine is 29mm! That's a BIG difference to flow.
It also helps with insurance, because my turbo looks standard. "No, I haven't modified it... It's a standard turbo, see??"
Highflowing is simply enlarging the compressor side of the turbo, and sometimes modifying some of the exhaust vanes a little, to help it spool faster.
Having a larger compressor means the turbo can flow more to the engine. You can produce higher boost levels, and the turbo will not run out of 'puff' in higher revs, like the standard one does. My car used to feel a bit weak in top end. Boost was there , but the engine didn't feel strong. Now, it screams. The engine feels like it has tons of power. The turbo produces more power for the engine using the same boost level as it ran as standard. It takes a little longer to spool up because the compressor is a bit larger though.
An Air/Fuel meter is a very important item for a performance turbocharged vehicle. A simple device can be made or purchased in kit form, (Jaycar Electronics for example produce a kit) that may perform as well as an expensive name brand device.
Basically, the voltage signal from the oxygen sensor in the car is monitored. The exhaust gas passing through the exhaust reacts with the end of the oxygen sensor, protruding into the exhaust flow, creating a varying signal, sent to the ECU via a voltage change. From the voltage strength, the ECU can determine whether the mixture is running acceptably, or unacceptably (lean or rich). Usually, the ECU varies the fuel injector duty cycle to alter the mixture, either increasing or decreasing fuel flow.
A major problem with turbo cars running higher than standard boost amounts, is pinging or detonation, covered previously. Lean mixture is one of the primary causes of this dangerous condition, and an air/fuel mixture display can help you to identify whether or not your mixture is beginning to lean out.