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Originally Posted by dellguyz
1. Why the boost will build up to so high level when I step down the throttle fully? Though I step down the throttle fully at the mean time, the engine speed haven't build up quickly, the exhausted gas should not have a big change in flow to make the turbine spin fast which generate much more boost.
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When you are at cruising speed, your at 15-20% throttle. When you go to 100% throttle, you are adding 5 or 6 times more air and fuel. That will have a huge effect on turbo speed even though the RPMs don't go up.
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In steady speed driving on flat road, the boost value mostly between -40 to -50 kg/cm2, why the pressure will be in negative value?
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A car without a turbo operates in negative pressures all the time. vacuum is greatest at idle and deceleration and least at full throttle. With a turbo, the same is true, except you get positive pressure when the turbo makes more pressure than the engine is taking in. On a flat road you are using very little throttle, so the turbo isn't making much pressure. The engine is sucking more air in than the the turbo makes, therefore you have negative pressure.
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4. From some of the information from internet, it said the turbine is start spinning when the engine is started, is it true? In my knowledge, if the compressor make the boost reaching limit, pressure will trigger the wastegate to let exhaust gas bypass the turbine and balance the pressure. So if I drive the car in low RPM (cruise on highway at 2000rpm, 80km), the compressor will keep charging and build up pressure, doesn't it will damage the comprees under long time?
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The turbine will start spinning when the engine starts, but not fast at all. It won't be making pressure. Its not about building up pressure and then letting it go, it makes pressure constantly. Think of it like this example: Put a small drinking straw in your mouth and blow through it. The blowing is like your exhaust. Now blow harder. You increase the pressure in your mouth because of the restriction in the straw. Now blow harder. Once you reach a certain amount of pressure in your mouth, it will start "leaking" past your lips. You won't be able to keep your lips closed against the pressure. That's how a wastegate works. After you reach a certain pressure, it starts letting it leak past. It doesn't "build up" pressure, its a constant balance of how much exhaust goes past the turbo and how much is let out the other side.
Have you ever blown on a fan and made it spin? That's how a turbo works. Blow harder and the fan spins faster. Its just that the fan is attached to a compressor which pushes intake air.