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Discussion Starter #1
This article aims to answer most common questions about turbochargers and how they work.



What Is a Turbocharger?
As I explained in my article All About Intakes, internal combustion engines "breathe". Performance air intakes allow more air to get into the engine, and faster.

Turbochargers take this to the next level. They act effectively as an air pump. Rather than letting air flow in more easily as with a performance air filter, turbochargers physically force air in.

How Is a Turbocharger Powered?
Often people mistake turbochargers for "superchargers". A turbocharger is powered from the cars exhaust, where as a supercharger is powered by the engine itself.

The exhaust inlet of a turbo charger receives exhaust air from the engine, which spins a fan-like wheel. This in turn sucks air in at a high compression, and passes it into the engine.

What About Intercoolers?



Often the words "turbocharger" and "intercooler" come together. An intercooler is basically a device which cools down the air that has come out of the turbocharger, before it is put in the engine. This allows for a higher density of air. To get technical, decrease in air intake temperature provides a denser intake charge to the engine and allows more air and fuel to be combusted per engine cycle, increasing the output of the engine.

As Simple As That?
The basic principal behind turbocharging is fairly simple, but a turbocharger is a very complex piece of machinery. Not only must the components within the turbocharger itself be precisely coordinated, but the turbocharger and the engine it services must also be exactly matched. If they're not, engine inefficiency and even damage can be the results.

I hope this cleared up any questions!
 

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The exhaust inlet of a turbo charger receives exhaust air from the engine, which spins a fan-like wheel. This in turn sucks air in at a high compression, and passes it into the engine.

edit :
nearly the exhaust side of the turbo is spun up via spent exhaust gases ,this in turns spins the pressure side of the turbo which blows a high volume of air at variable pressure depending on engine speed into the engine via the intercooler if fitted,

the pressure is controled at max boost via the waste gate which then dumps the exhaust gases slowing the turbo down to stop over boosting and turbo/engine failure.

sorry alec couldnt help myself .
 

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high pressure = denser air =more oxygen to go bang :D:D
an intercooler is used to cool the air after it is heated by the fast spinning turbo, this increases the efficiency of the turbo.

paul mentioned the wastegate^

a dump valve (real not a stick on speaker one) stops the boost stalling the turbo when revs drop (change gear etc). when the revs drop the engine doesn't use all the boost so some is left and needs to be released before it heads back to the turbo and stops it from turning as it is travelling in the opposite direction. The dump valve usually is only used after a lot of boost is created through revving the engine high. so its not all for show ;)
 

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dump valves and wastegates are 2 different things thou ,
wastegate controls the turbo pressure , and as rightly said the dumpvalve stops the turbo stalling,
because when you take your foot off the throttle the throttle flap closes thus resulting in turbo stall as the compressed air has no where to go hence the dump valve opening to release the built up air , this also allows the turbo to spool back up quicker , resulting in less lag.
i used a hks dumpvalve on my last car which dumped air at low boost to as it had a high and low pressure valve system.
 

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Cams also differ :

A turbo is driven by exhaust gas velocity. Turbochargers are an exhaust restriction (which raises the exhaust gas pressure), but since they use energy that would otherwise be wasted, they are much more efficient than a belt driven supercharger. Normally when the exhaust valve opens, there is still useable pressure in the cylinder that needs to be dumped so it will not resist the piston trying to go back up the bore. That pressure makes high exhaust gas velocity. With a turbocharged engine, this is the energy that is used to spin the turbine.

With a well matched turbo / engine combo, boost pressure should be higher than exhaust gas pressure at the low side of the power band (near peak torque). As the engine nears peak hp, the pressure differential will get nearer 1:1. At some point the pressures in the intake and exhaust will be equal then crossover making the exhaust a higher pressure than the intake. At peak hp there will usually be more exhaust gas pressure than boost pressure. The ultimate goal is to have as little exhaust backpressure possible for the desired boost.

If the turbocharger is matched well to the engine combination, the camshaft selection will not need to be much different than that of a supercharged engine. The problem is that most factory turbo engines have turbo's that are sized too small and will usually have more back pressure than boost pressure over much of the useable power band. Car manufactures do this in an attempt to reduce turbo lag. When a turbocharger is too small, it will be a bigger restriction in the exhaust, causing more back pressure. A big mistake of turbo owners is to crank the boost up as high as they can thinking they are going faster, but in reality, chances are that they are just killing the efficiency of the turbo and most gains are lost. If you want to run higher boost levels and back pressure is a problem, cam timing can be altered to give respectable power increases for much cheaper than a new turbocharger. Before you go increasing boost and changing cams, remember that the oxygen content into the engine will increase power, not boost pressure. A good flowing head with a good intercooler can make a lot of power without high boost. You may not need more boost to get the power you want.

Valve Overlap

If you're one of many factory turbo car owners with a turbo sized too small, there will be higher exhaust pressure than intake. You should see that if both valves are open at the same time, the flow would reverse. Any valve overlap is a no-no if you're looking for higher boost with a restrictive turbine housing. The exhaust valve will usually close very close to TDC, but there is will still be more pressure on the cylinder than in the intake. You must allow the piston to travel down the bore until the pressure is equalized. If the cylinder pressure is lower than the intake manifold pressure, no reverse flow will take place. This means that the intake valve needs to open 20-35° ATDC, depending on the amount of boost you're using. Most street turbo's will work well when the valve opens close to 20° ATDC, only when boost gets near 30 psi will you need to delay it as much as 35° ATDC. In low boost applications (under 15 psi or so), opening the valve closer to TDC and maybe keeping the exhaust valve open a little after TDC is a compromise for better throttle response before the boost comes on. As you increase boost, you will need to delay the opening of the intake valve to avoid reversion. You want the intake valve to open as soon as possible, in an ideal situation, the intake valve should open when the pressure in the cylinder is equal to boost pressure. This can cause a little confusion with cam overlap. If the exhaust valve closes before the intake opens, the overlap will be considered negative. If the exhaust valve closed at TDC and the intake opened at 20° ATDC there would be -20° of overlap. In this type situation, pumping losses are quite large, although the turbo will still use less power than a crank driven supercharger.

If you have a well matched turbo for the engine and application, it is a different deal altogether. A well matched turbine housing on the turbo will usually work well with cams with a lobe separation in the 112-114° area. If there is more pressure in the intake than in the exhaust, a camshaft suited for superchargers or nitrous will usually works well. When the exhaust backpressure is lower than the intake, reversion is not a problem, actually just the opposite is a problem. More pressure in the intake can blow fresh intake charge right out the exhaust valve. This can be a serious problem with a turbo motor since the charge will burn in the exhaust raising temperatures of the exhaust valves and turbo. This is also a problem with superchargers, which is why supercharger cam profiles usually work well with turbo's. In this type situation, the power required to turn the turbine is nearly 100% recovered energy that would have normally been dumped out the tailpipe, basically free power. Many will argue that nothing is free and you need pressure to spin the turbine and this must make pumping losses. They are wrong because a turbo is not getting anything for free at all, it is just making the engine more efficient. It is true that there are pumping losses, but on the other hand there are pumping gains as well. If the exhaust back pressure is lower than the intake, the intake pressure makes more force on the intake stroke to help push the piston down. At the same time another piston is on it's exhaust stroke. So the intake pressure is more than canceling out the exhaust pressure. Not free, just more efficient.

Valve Lift

By delaying the opening of the intake, the duration of the cam will be much shorter. A short duration intake works well with a turbo, but the problem is that sufficient lift is hard to get from such a short duration. This is where high ratio rockers can really pay off. A cam for a turbo engine can delay the intake opening by over 40° compared to an cam for a normally aspirated engine. This makes for much less valve lift when the piston is at peak velocity (somewhere near 75° ATDC), any help to get the valve open faster will make large improvements.

Roller Camshafts

Turbo motors place a large flow demand at low valve lifts, and roller cams cannot accelerate the valve opening as fast as a flat tappet. They do catch up and pass a flat tappet after about 20° or so, but up until that point the favor goes toward the flat tappet cam. The area where rollers really help in turbo motors (and supercharged) is cutting frictional losses. Any forced induction engine will need more spring force on the intakes. If you run a lot of boost, you'll need quite a bit more spring force to control the valves. As spring forces gets higher, the life of the cam gets reduced. A roller tappet can withstand more than twice the spring pressure as a flat tappet with no problems. On the exhaust side, it's not the springs that put the loads on the cam lobes. The problem there is that there is still so much cylinder pressure trying to hold that valve closed. This puts tremendous pressure on the exhaust lobes. So when high boost levels are used, consider a roller cam. I would definitely consider a roller cam on engines making more than 20 lbs. of boost.
 
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