cinders

I know nothing!

I admit the only thing I know about the turbo is it pushes exhuast gasses back in to the engine (I think) and when my impreza hit's around 2,500rpm it makes a nice whistling noise and goes like hell!....

Can someone give me a quick description of how it all works?
What is boost? How much boost does my car run? Why do people run different boost?
What does a boost gauage show? Is there any point me having one?...
What is a wastegate? What is a dump valve?...

If someone could spare some time to explain how this all works, it would be appreciated, or point me towards some reading matter.

Thanks,
Matt.

greasemonkey

Will try.

"Normal" (aka naturally/normally aspirated) engines receive their air at or near normal atmospheric pressure. In simple terms the air flows freely into the induction system, passes through the throttle, is mixed with fuel and flows into the cylinders.

The simplest way to generate more power from an engine is to cram more explosive air and fuel mixture into each cylinder, so the corresponding bang is bigger and lasts longer. This is where turbocharging and supercharging (so-called forced induction) come in.

Again, as simply as possible, the air goes through a fan (or compressor) on its way into the engine, forcing more in. When mixed with a correspondingly larger amount of fuel, you get the bigger, longer bang, and more power/torque.

The boost pressure is simply the measure of the air pressure in the inlet manifold.

A turbocharger is simply two fans connected together by a shaft. One fan is in the exhaust gas stream, and is driven by the gas passing through it. The shaft connects to the other fan, which rotates in the inlet airstream, thus compressing the air on the way into the engine. An efficient design, seeing as the energy in the exhaust gas (which would otherwise be wasted) drives the turbo.

The downsides are that below a certain engine speed, there's not enough gas to drive it, which is why boost is only available above a certain engine speed. The other problem is that the turbo components themselves have mass, and thus they can take time to accelerate (so-called turbo lag).

Superchargers are directly driven from the engine via shafts or belts, so are usable at a wider range of engine speeds, and don't suffer from turbo lag.

You'd need to use a boost gauge to find out for certain, although if you tell us what model car you have we'll be able to give you a rough idea, provided it's standard.

The short answer is that more boost equals more power and torque, but the long answer is slightly more complex, as you can't simply run as much as you want.

[quote] What does a boost gauage show? Is there any point me having one?...
If your car's bog standard, and you don't intend to modify it, a boost gauge is not essential. However, if you modify it in any way, or indeed are seeking to raise the boost, you obviously need an accurate way of reading it.

It's a valve that allows the exhaust gas to bypass the turbine. The ECU controls the opening and closing of this valve, and so is able to control the rotation speed and thus the amount of boost generated by the turbo.

A dump valve is located between the compressor and the throttle body. It is designed to open when the throttle is closed, allowing unwanted boost pressure to vent off, either to atmosphere, or back into the induction system, preventing the air being forced back into the turbo and causing stall or damage.

Leviathan

Very acurately explained as usual GM
Just to finish off and i know you didnt ask but theres sod all on the telly!
The other feature which your engine has is the intercooler which is the air to air type,ie, it is cooled by the outside air flow.
This cools the air charge after it has passed through the turbo. Turbo's by their opperation,ie exhaust gases flowing through them get very hot. This heat is transfered through the turbo and hence heat up the air being compresed by the compressor wheel of the turbo. This is a bad thing as it becomes less dense and also leads to other problems like pre-ignition/dettonation. (another can of worms).
The intercooler then cools it down and makes for a more dense charge (more power) before it enters the inlet manifold where it is mixed with the fuel and hence enters the cylinders.

MJW

How Stuff Works.com is pretty good for this sort of thing.

STi-Frenchie

Greasemonkey, Leviathan, you guys should get together and do a glossary of terms for SN - those explanations were very clear and easy to understand. Thanks to you both.

Brit_in_Japan

Started to write some extra explanation, then checked the website given by MJW. That really is an excellent site which explains it all, and pictures too