Back Aspiration Normally aspirated Forced induction Snail shaped Jargon Advantages Disadvantages Retro-fitting
We’ve all heard about turbos haven’t we? But what actually are they and how do they work ... do you know? This article helps explain the fundamental theories behind the turbo and the pros and cons.
Going right back to basics there are 3 main ingredients to a fire/explosion. These are fuel, air and energy (heat). What we’re talking about here is how the air gets into our engine and how the engine breathes the air in. In simple terms the more air we can get into an engine the more power it will deliver, this is subject to many variables including sufficient fuel being supplied in balance but for the sake of this article more air is better.
There are basically 2 methods of getting air into an engine as follows :
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This method is the most common and the simplest. Once the piston has come up
on its compression stroke and the spark ignited the mixture the piston is forced
back down the cylinder. As the piston goes back down it turns the crank
producing the drive but it also leaves behind it a vacuum where the air/fuel
mixture used to be. This vacuum causes new air to be drawn in through the now
open inlet valve(s) meaning the engine breathes in its own air. This phenomenon
is known as the venturi effect and engines using this technique of breathing are
said to be normally or naturally aspirated.
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Taking the argument of more air = more power it stands to reason that if you
can force air into the engine then you can get more power. How do we do this
though ? There are 2 main methods of doing this. Both devices use a fan/turbine
of sorts to drive the air into the cylinders but are powered differently : the
supercharger and the turbo. This article just explains the turbo.
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It’s true, turbos are snail shaped. The turbo is basically an impeller that
sits in the air induction pipe to the engine and spins to draw air in and ram it
into the engine. The impeller is driven by a turbine, the turbine is powered by
gases from the exhaust of the car making the turbo a device that actually
increases the efficiency of the engine as a system. The reason it’s snail shaped
is because the curl around the outside of the turbo housing actually tapers so
that the pressure and speed of the gas from the exhaust is increased as it
travels through to the turbines. There is a downside though. Because the turbo
is driven by exhaust gas means that the engine must be producing enough gas to
make the turbo spin which can take a while and some revs giving a delay. This
delay is known as turbo lag. Higher power applications using bigger turbos
demonstrate more lag due to the extra weight of the bigger turbo internals
meaning the turbo comes into boost higher up the rev range. There are ways
around this like fitting multiple smaller turbos and other ways but all turbo’d
cars will experience some form of lag.
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Here are some of the things associated with a turbo and what they mean :
Intercooler – this is like a radiator that the air from the turbo is passed through. The air being forced out of the turbo is hot due to the fact it’s compressed and the intercooler helps cool this air before it enters the engine providing more power. A large intercooler can give more lag due to a time required for the turbo to fill it.
Charge cooler – the idea is the same as the intercooler above but instead of the air being cooled by cold air passing through the cooler, water is used which is pumped through the chargecooler, this water is cooled by air. They have the advantage of being smaller and more efficient at cooling than intercoolers but they have mechanical parts which can break.
Dump Valve – a common device among modifiers, they sound cool when you lift off the throttle of change gear but what do they do ? They are basically a valve that sits as close to the inlet manifold as possible and release the air produced by the turbo. When you lift off the throttle or change gear and again lift off the throttle flap will close, the turbo continues to spin however due to it’s momentum which means that it continues to push air against a now closed throttle flap. This causes back pressure right up to the turbo which will cause it to stall quicker, theoretically then a dump valve should reduce lag.
Wastegate - a wastegate is a valve in the exhaust section of the turbo which is operated by the wastegate actuator. When the valve is open, exhaust gas goes to the exhaust instead of to the turbo.
Actuator - this operates the wastegate. The pressure that is required to open the wastegate depends on the spring inside the actuator. In time this spring can go soft causing a less efficient working turbo.
Bleedvalve - this is where it gets interesting ... A bleedvalve is a way to increase boost. The bleedvalve fools the actuator into thinking that there's less boost than there actually is. The engine isn't so easily fooled and could complain by stopping altogether. If a bleedvalve was fitted to your car, don't touch it !!
Hybrid Turbo – a turbo made using custom parts often from other turbos, 360 degree thrust bearings are usually fitted to increase the lifespan of the turbo, the turbo housing is often gas flowed and the turbines re-profiled to help the turbo spin up quicker. In some extreme turbo applications fuel is injected into the turbo to explode and make it spin up quicker.
Detonation – also known as pinking, or “detting”. This is where the spark within the cylinders is not happening at the optimum time, it can be cause by too much ignition advance causing a spark too soon or often in turbo cars by underfueling for the amount of boost being used. It’s not good news and leads to powerloss and engine overheating that will lead to sever failure.
Forged pistons – these pistons are much harder than standard pistons and are made from drop forged steel. They are usually fitted to engines running high boost to handle the extreme temperatures. Often standard factory pistons will not cope with much above the boost settings the car would run from the factory, some will go for a while before blue smoke gives away the game. The pistons will often crack and sometimes melt.
Boost – Measured in pounds per square inch (psi) or bar. It is the amount of
pressure the air is at coming from the turbo. 1.0 Bar = 14.7 psi.
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An engine with turbo is easy to tune and increase the power output by
increasing the boost. They give huge amounts of torque low down in the rev range
compared to normally aspirated engines that deliver the torque higher up. They
sound awesome and are real fun, max grin factor.
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Lag can be a problem. They can need rebuilding and be expensive to restore or
replace. Using too much boost can blow your engine. Traction can be a problem in
front wheel drive cars when the turbo boosts up.
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It is possible to fit turbos to engines that were not designed to run a turbo and in-fact several kits for various engines are available on the market. The amount of work needed depends on the boost that is to be used. In some cases where low boost is used the internals of the engine can be left unaltered, this does also depend on the engine being used.
In most cases the compression ratio will be lowered using a variety of means. The most common is to fit a thicker copper head gasket, other ways are to fit different pistons or have the pistons crowns machined down. Once this is done a new exhaust manifold is required to feed exhaust gas to the turbo, suitable piping for the air feed to the engine from the turbo (via the intercooler) and of course a suitable engine management system are also required.
One thing you can be sure of is fun with a turbo, the contrast of low power/torque
to all of a sudden this almighty thrust of power as the turbo boosts up is
awesome. Taking 2 identical cars both running say 200bhp, one with a turbo and
one using normally aspirated tuning methods you would feel sure riding in the
turbo’d car that it would easily beat the normally aspirated machine but it’s a
deception because they are producing the same numbers at the wheels. The reason
is simply the way the 2 engines are delivering the power.
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Back Aspiration Normally aspirated Forced induction Snail shaped Jargon Advantages Disadvantages Retro-fitting