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  1. #1
    HJC Trader
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    Default Some technical terms concerning turbos

    Just thought I'd pop this up as there's a few threads around discussing various aspects of turbos with one or two bits and bobs getting confused or lumped together when they're actually different things. So, all in my own words...

    Dump valve/bov - this is a device fitted between the turbo and throttle plate (normally the closer to the throttle plate the better) which is used to vent off excess boost pressure when it isn't required, ir. when the throttle plate is closed. This is normally the case during gear changes or when lifting off the throttle following accleration. These devices are generally constructed from a piston within a chamber. The piston, when closed' keeps all the boost within the system so it goes into the engine as designed. When you close the throttle off, the engine goes into vacuum. There is a small diameter pipe leading off the intake manifold (behind the throttle plate) which feeds this vacuum to the top of the piston. This vacuum overcomes the pressure of the spring and pulls the piston open, allowing the excess boost to be vented either back into the intake before the turbo (recirc) or off to atmosphere (vent to air). When you re-apply the loud pedal the boost begins to build again so the spring within the device can overpower the vacuum and close again, thus reinstating boost. It all happens very quicky of course, though the amount of boost and the pace of acceleration and the gearchange will affect the sound. Of course, different ones are designed to make different sounds but they mainly make the classic tshhhhh sound.



    Wastegate - this device is a flap which is used to bypass exhaust gas from the turbo. Without this, the boost pressure generated by the turbo would simply keep rising as rpms increase and the engine produces more exhaust gas. Once the desired boost level is reached, this flap is slowely opened diverting more and more of the exhaust gas away from the turbo so it holds stable pressure at all rpm. These can be built into the turbo itself (internal wastegate) or mounted externally on the exhaust manfold (external wastegate). As turbos get bigger and engines pass more exhaust gas, the wastegate generally needs to be bigger in order to bypass sufficient gas to keep pressure stable.



    Screamer Pipe - Most wastegates use a recirculating system whereby the bypassed exhaust gas get's diverted back into the exhaust system after the turbo, which keeps it going out the back and run through the silencer. However, the gas is sometimes diverted from the exhaust system straight to atmosphere by way of a small pipe with no silencer on it at all. This short length of pipe is the screamer pipe and is called such because once you get on boost and the wastegate needs to bypass exhaust gas it does so straight out, which is quite loud. There was a number of these on track at Japfest. Screamer pipes are recirc wastegates can be either the internal or external type, they just require different exhausts.



    Wastegate flutter - otherwise known as wastegate chatter and sometimes known as 'mincing squirrels' . A turbo setup will always have a wastegate and normally has a bov aswell. Working together these both vent off excess boost or gas to keep everything on an even keel. If however you don't run with a bov you will get the following situation. When you lift off throttle and close the throttle plate, there will be a build up of pressure back against the turbo. This causes the turbo to slow, but remember there's still exhaust gas feeding into the turbo keeping it spinning, and that needs to go somewhere! As the intake side is closed off it can't go there, so the excess is forced out through the wastegate causing it to bounce off it's seat until all the momentum of the turbo has been spent. This gives the distinctive chattering, purring or pigeon sound. Note that if a bov is fitted but isn't big enough to vent off all the excess boost, you will still get wastegate chatter as the remainder goes out there. Larger external wastegates can still chatter and make a noise aswell even if a bov is fitted, so hearing the chatter tends to coincide with a motor equiped with a larger turbo. On smaller turbo applications running less than 1bar it's not overly dangerous to run with this set-up as it's not enough boost to cause damage to the turbo itself. However, get over 1 bar and it becomes more of an issue so you should ideally run bovs in that case.

    Compressor Surge - this occurs when a turbo is too large for the engine it's fitted to, or the compressor wheel (intake side) is out of balance with the turbine wheel (exhaust side). In this case, when the turbo spins up and starts feeding boost to the engine, it can't actually consume all the air that's being fed to it. That causes backpressure which can try to stall the turbo and the wastegate to open briefly in an attempt to even out pressures both sides. This in turn causes a stuttery acceleration as it sounds like the turbo is trying to dump while accelerating and is more detremental to the turbo itself than the wastegate flutter mentioned above - mainly due to the fact that it's all under boost and a higher rpm situation. It can be cured in a number of ways, either get a smaller turbo, smaller compressor wheel with the same turbine wheel, or clip the ends of the compressor wheel so it simply can't produce as much pressure at the given turbo rpm to be a problem.

    Boost Creep - if your wastegate is not large enough, or you've fitted an exhaust that is so much more free flowing that it's actually easier for the exhaust gas to go through the turbo than the wastegate, it can lead to boost creep - you'll hit your desired boost but as rpm rises the boost will still rise a bit. This can be cured by either getting a larger wastegate, if it's an external type, or porting the original internal wastegate to allow better airflow. You could also improve the transit of air through and from the wastegate to make it freer flowing or in other circumstances (quick fix bodge mode) restrict the exhaust so it's not as free flowing.

    Spoolup Time - this is the level of rpm required to hit full boost. Generally considered that the lower the rpm, the quicker the spool up. Not to be confused with...

    Lag - the time it takes for your boost to reach the engine. Once you start building boost, you want it straight in the engine as soon as possible. This is done with the smallest volume of intake pipework/intercooler between the turbo and the inlet manifold. If you have yards of 100mm diameter pipework and the biggest intercooler in the world, it'll be so voluminous that when you hit boost it'll take ages to pressurise this whole system before it hits the engine. On a standard GT-Four CS/RC or ST205 you have the turbo feeding a compact chargecooler sat atop the engine with only 100mm of pipework between the turbo and cooler and then about 60mm of pipework between the cooler and intake manifold, very little lag at all as it's more compact a system.[/b]

    Intercooler - as air goes through the turbo to the engine it gets heated up. This is through a number of ways, initially by being compressed, and secondly by going through hot pipework and a steaming turbo. This hot air has notably less oxygen in it than cold air, so it's not much use for power. The added heat can also cause pre-ignition which over time can lead to engine failure. To cool it down you need a heat soak between the turbo and intake. An intercooler is a bit fat metal heatsoak that draws the heat out of the charge air, it then has air passing through it to draw the heat away from the intercooler itself. This keeps the charge temp down and more oxygen is more power and safer running .[/b]



    Chargecooler - this is similar to an intercooler. However, instead of using air to draw the heat away from it, it uses water (coolant). The benefit of this is water is much better and conducting heat than air so you can away with having a much smaller heatsoak. This leads to less volume in the system and reduces lag. On the minus side it adds more weight because the coolant that's used in it requires it's own pump, pipework and radiator to remove the heat from the water! This also tends to make it notably more expensive when looking at it from an after market point of view, so most people upgrade intercoolers.



    Crikey, that's taken nearly an hour and a half Will add more if required whne I remember

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  3. #3
    HJC Non Paid Member
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    spot on that man

  4. #4
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    Default

    Sort of turbo related, but handy to know anyway

    AFR - Air/Fuel Ratio


    What is it, what does it do?
    AFR is the proportion of air and fuel present during combustion. The chemically optimal point at which this happens is the stoichiometric ratio (sometimes referred to as stoich), where all the fuel and all the oxygen content in the air of the combustion chamber will perfectly balance each other out during combustion.

    For petrol the stoichiometric air/fuel mixture is approximately 14.7 times the mass of air to fuel. This is the mixture that modern engine management systems employing fuel injection attempt to achieve in light load cruise situations. Any mixture less than 14.7 to 1 is considered to be a rich mixture, any more than 14.7 to 1 is a lean mixture.

    The addition on an AFR gauge is recommended if you are modifying your engine. If you don't monitor your Air/Fuel ratio you could well end up running too rich or too lean with disasterous (read bloody expensive) consequences.


    EGT - Exhaust Gas Temperature

    What is it, what does it do?
    EGT is used for tuning and also as a warning indicator for your fuel system. The EGT suggests whether the AFR is relatively rich or lean. This information is most useful during wide open throttle (WOT) engine operation.

    At WOT, values between 850C /1562F and 900C /1652F are often considered ideal; temperatures below 800C /1472F are considered too rich; temperatures above 925C /1697F and approaching 1000C /1832F are considered dangerously lean and can result in excessive engine detonation, also known as knock, which can cause burnt or melted components. These can often be valves, spark plugs, pistons and piston rings.

    The placement of the EGT probe affects the measurement, the further the probe is from the head exhaust port the cooler the temperature can be. The amount of ignition timing advance can also affect this, more advance can mean cooler temperatures.

    To get useful temperatures, the probe must be installed in the exhaust manifold before the turbocharger. Most people install the probe near where the individual exhaust manifold branches have joined and the temperatures from each cylinder have combined.

    An EGT gauge is a great compliment to an AFR gauge.

  5. #5
    Super Moderator
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    Default

    Great thread, worthy of a sticky.

  6. #6
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    Default

    Great work mook, good reading especially since i just got my first turbo car

  7. #7
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    Default

    No worries guys

    Thanks for your bit too PJ, I think I'll have to dig something out on pre ignition and detonation aswell...

  8. #8
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    Default

    Good effort, I know absolutely f'all about engines and all the different components, (I want to know it all though!!!) in fact all I know (or knew before reading the above) is that the turbo makes the car react faster it uses exhaust gases to spool it up, and that bov's get rid of the excess so it doesnt slow down the turbo - but that post makes it so much clearer!

    Thank you!

  9. #9
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    Default

    very useful but leads me to my next question haha my dump valve releases the pressure as you can hear it however i still get the chatter. Anyone in the know if this is actually doing my turbo no good. As far as boost goes its 1.5 bar at peak. I have no clue about the engine side of things (gimme a pc and i'm fine haha).

  10. #10
    Club Administrator
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    Default


    stickied by Chris yesterday.

 

 

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