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Overview of Turbos

Turbos are a very simple device. In comparison with some of the other engine systems it is easy to grasp the basics of turbo operation. However, a lot of factors must be considered when choosing a suitable turbo for a particular engine. Controlling a turboed car can be a nightmare if the wrong setup is used.
The use of suitable turbos, electronic engine control, and compatible inlet and exhaust systems have virtually eliminated turbo lag and let to excellent turbo setups available on modern cars.

How Turbos Work

 - Exhaust gases turn a turbine, which turns a compressor, which pumps air.
 - More air means more fuel can be burned, which leads to more power.
 - The turbo size needs to be matched to the application and engine.


A turbo greatly increases the power of an engine. Its hard to find any other device which provides such excellent and usable power increases. A turbo produces this extra power by forcing extra mixture into the combustion chamber, above what can be pulled in under atmospheric pressure. Compressing air adds heat which reduces the overall power increase, however, this can be combated by using effective charge cooling (intercooling). The turbo itself is made up of a centrifugal compressor, connected by a shaft to the turbine which is joined to the exhaust system. The engine produces exhaust gases, which drive the compressor, which in turn drive the turbine. The impeller turns and forces air/fuel mixture into the engine. Turbos are usually highly engineered items which are manufactures to very fine tolerances. They must withstand extremes of temperature while at the same time spinning at very fast revolutions( typically 1000deg and 100000rpm). The turbine and impeller and the housings must be tailored to provide the required throttle response and power.



At lower rpm boost is non existent and at higer rpm boost is excessive. This is due to the nature of centrifugal compressors. Much of the skill involved in designing and fitting turbos concerns addressing these characteristics and working around them. Mods to a naturally aspirated engine which reduce low down power will have an equally or greater effect on turboed engines. Because of the lower compression ratio of turbo engines(1.5 points lower on road cars/upto 6 points lower on race cars), all efforts should be made to keep low down torque to a maximum.

This means retaining the stock ports and cams. In a twin-cam cars increasing the valve overlap period of the stock cams by using adjustable sprockets will normally yield good results, if the exhaust has been improved to reduce back pressure. 3-4 deg advance on the inlet cam and 4-6 deg retard on the exhaust cam usually does the trick.



The other factor influencing low speed performance is the low down boost available. The size of the exhaust turbine and the housing, play big parts in this regard. A large diameter turbine will flow a lot of exhaust has at high rpm but is very heavy. Therefore, it requires a lot of exhaust gas just to get moving. It takes time to accelerate the turbine to usable rpms which means poor low down boost and heavy turbo lag. A smaller turbo has less inertia so it spools up much faster. However, if it’s too small it restricts exhaust gas at higher rpms and is therefore, limited in its maximum boost capabilities. This restriction in exhaust gas also leads to increased fuel consumption. A compromise must be reached between low down boost and top end boost.

Most manufacturers prefer to choose small, responsive turbos which give minimal turbo lag and lots of low down torque and power, however, most people interested in power would prefer to go for a bigger turbine and increase the medium to high end power.

A quick way to estimate the characteristics of any turbo is to look at the extruder bore. A large bore means good top end power at the expense of low down performance. A smaller bore means good low down power and minimal turbo lag.



One or More Turbos

 - Tne turbo for 2.0 4-cyl is best
 - For straight six engines it’s 50/50
 - For v8s two smaller turbos are better and cheaper than one big turbo.
 - For v10 and v12s two or four turbos are best.


It is often hard to justify going for multiple turbos. The advantages of using multiple turbos are often outstripped by the disadvantages. Multiple turbos mean more cost in both initial outlay and in ongoing maintenance and servicing. It is harder to find space in the engine bay for two or more turbos and spool-up time on full boost can be just as good with a single turbo as with multiple smaller turbos.
On 4cyl engines it simply doesn’t make sense to go for twin or quads when single turbo setups yield equal levels of response and power with less weight and complexity. For 6cyl straight engines (like the RB26) it’s a 50/50 call with most people preferring twin turbos(Supra and GTR). When we goto v8s, v10s and v12s the choice should always be twin or quad turbos. In this instance it can actually be a lot cheaper to go for multiple turbos rather than a single large unit. Also, because of the exhaust path, it is actually less complex to go for a twin setup rather than a single setup.

Turbos respond best to pulsed exhaust flow. When many cylinders dump into a common exhaust manifold, these pulses get smoothed out. On a twin turbo setup, the pulses are kept separate and have a more pronounced and higher amplitude nature. When the turbos are spooling upto speed, they receive a series of vigorous shoves instead of a steady stream of lower pulses. It is for this reason that some twin turbo setups have reduced lag, and it has less to do with the fact that they have smaller geometry.

Wastegates


 - Two basic types, internal and external.
 - External is always better, especially with big power applications.
 - Almost all production cars use internal – cheaper & more compact.
 - Almost all competition cars use external – more free flowing & less back pressure.




Because of the reduced price, many turbos use internal, integrated wastegates. These should be avoided at all costs. An external wastegate is essential for any performance car. With internal wastegates, more flow is forced through the turbine, when off boost, so boost and inlet temps are higher. Also the flow out of the turbine is disturbed. Anything that can be done to improve flow through the turbine will yield good performance results. With two turbos, exhaust flow through each turbo is halved so theres less flow to be dealt with by the wastegate and turbine. Splitting the exhaust also means less problems with exhaust maintenance. The manifold is shorter so there is less expansion and contraction which reduces metal fatigue. With flow halved the turbos will run cooler, which improves turbo life.

Hybrid Turbos

 - A hybrid turbo is any turbo derived from two or more other turbos.
 - Usually means replacing ceramic internals with steel internals to allow more boost.
 - But any mod to a production turbo can be called a hybrid.


Any modified turbo can be called a hybrid turbo. Just replacing the thrust washers can change a turbo to a ‘hybrid’. Any turbo which is made up of two or more different model of turbo can be called a hybrid. If you want increases is power well beyond the standard power, you have to go for something that will increase the strength and air flow through the turbo.

If the compressor on a standard turbo has reached its flow limit then increasing the boost won’t increase the power, it will just cause your boost gauge to read a higher value !!! this is because the boost is higher but because the compressor has hit its max USABLE air flow, it rapidly heats the incoming air and reduces air density leading to higher boost but with little or no gain in power.
Hybrid turbos can improve this situation and allow additional boost with the expected increase in power. The larger wheel isn’t bigger in diameter, but has greater blade area. Normally the compressor housing must be machined to accept the new size. This normally leads to a 15% increase in performance potential.

On the turbine side, things can be improved by back cutting the turbine wheel. This mod increases top end power with a slight increase in lag.





Below. Ceramic Turbines



At a glance…


 - Less inertial than steel so less turbo lag and faster spool-up.
 - Same effect as a lightened flywheel.
 - Anything over 1 bar can cause ceramic destruction (turbo specific).

Modern ceramic turbines do wonders to reduce turbo lag. The reduced weighr means less intertia which leads to less low down exhaust gas being required to turn the wheel. However, if you raise boost beyond 14.5 to 15psi the ceramic wheel will shatter. The turbine wheel has to exert great pressure on the common shaft to turn the compressor.

This places stress on the ceramic wheel where the blades are atteched and where the hub bonds to the shaft. Both areas have a low load limit.

Another way the turbo can be destroyed is by splash damage. Before removing or modifying any splash guards make sure the turbo is sufficiently protected from water.

Turbo Lubrication

 - Even a small loss in lubrication will destroy the turbo.
 - Major consideration when upgrading a NA car with a turbo.

Turbo lubrication is a major problem for turbo installations on a non-turbo car. Turbos cant be operated above 65psi oil pressure. A restrictor must be used to reduce the oil pressure without restricting the flow. At idle a flow of 0.5L per minute is required and at load you need 2L per minute. Ball bearing turbos require a bit less, around 1.5L maximum. Obviously each turbo has slightly different lube requirements. It is usually possible to tap into the engines oil circuit to get the requires oil supply. The oil return line is more critical. You are relying on gravity to move the oil from the turbo to the sump.

After passing through the bearings, the oil is whipped up into a foam. Therefore, you need a large bore return with minor or no bends all the way to the sump. If the turbo is high enough it can enter the sump via a redundant oil dip stick opening or a tapped opening.

If the turbo is mounted too low then an aerodyne turbo can be used which has its own sealed oil supply which is not connected to the engine oil supply. Otherwise an electric or belt driven pump can be used to return the oil to the sump.

Turbo Care

 - Fully synthetic oil is a must.
 - Service every 5000mls or more frequently.
 - Use a turbo timer or take precautions.
 - Don’t rev just before shutdown or just after startup.


Always use fully synthetic ester based oil and you should keep your service intervals at 5000mls. However, this is just the beginning of turbo care. Ideally the turbo should always be allowed to idle for upto 3 minutes after a hard run. This also applies to water cooled turbos. A better option is to run the engine at 1500-2000rpm for a while before engine shutdown rather than idling. Idling sometimes provides insufficient oil flow for the turbo. People who live down country lanes or who have to go through housing estates to get to their home have the advantage of having time to let the turbo cool down before shutdown. If you are in a situation where you can idle or shutdown immediately after caning your engine then you should invest in a turbo timer because you will eventually forget to allow cool down time and will destroy your turbo.

Also, never rev your engine prior to shutting down Giving the throttle a quick blip before shutdown will spin the turbo to 30000rpm and shutting down at this point will starve the turbo of oil at just the wrong time and cause premature ware. A similar problem occurs at engine startup. It can take 30 sec for the oil to reach the turbo. You should wait this long before applying throttle.

Similarly, if the turbo is just fitted or a reconditioned or new unit is being fitted, then it must be primed with oil before the first startup. Also, if the car has been parked up for a few months, the turbo should be primed with oil if it can easily be accessed.

Turbo Reconditioning

A turbo should only need servicing every 30000 to 50000mls. If the vehicle spends a lot of time at idle or at high speed, then this interval will be reduced.