By TURBO TORQUE - www.mazdarotary.net
Turbo Basics
A turbocharger is used to force air/fuel mixture into an engine at a pressure greater then the natural atmospheric pressure of around 14.5 PSI. When a turbo produces 7 PSI of boost, this means how much extra pressure it applies on top of the natural atmospheric pressure. The way a turbo works is the exhaust coming out of the engine is pushed through a turbine. This turbine is mounted on a shaft, which in turn spins an air compressor. The compressor draws air in and blows it into the inlet manifold, and this produces BOOST. The whole point of forcing the air/fuel mixture into an engine is to allow it to burn more fuel and make more power with the same engine capacity. This can get complicated, as there are several factors that get in the way of efficency gain. For one thing when you compress air (with a turbo) it gets hotter. The problem with hotter air is that it contains less oxygen than cooler air, so there is less oxygen to help burn extra fuel that’s going into the engine. This is why many turbocharged cars use “intercooling” of various types, to cool the pressurised air back down into the engine.
Compressed Air = Hot Air
It’s important to consider problem of too much heat inside the engine. Once the hot compressed air goes inside the engine it is then compressed again by the piston in the engine. Therefore by the time the mixture of fuel/air mixture is ignited it is really hot. When it is too hot it could ignite itself before the spark plugs fires (known as pinging). When the engine pings the smooth, well-timed normal ignition mishaps, costing you power and damaging ports to the engine. Damage is caused to the engine, and can therefore lead to blowing the head gasket, this then chain-reacts to more severe cases. Another problem when the engine has an air/fuel mixture which is too lean, that is not enough fuel for the air coming in, which creates too much heat, or having ignition which is not suited to the engine. With private turbo installations these problems are often ignored. The ignition timing required for optimum power and smoothness from a turbocharged engine is in fact totally different to that required for a non-turbo engine. This is because the more efficient the engine is, the less advanced the ignition timing needs to be to get power combustion of the fuel/air mixture. When the engine gets more and more boost, the ignition should happen later and later in each cylinders or rotors combustion cycle. To get best out of the turbocharged engine it needs a balance of boost, mixture richness, charge air temp and ignition timing that allows it to run smoothly and efficiently with the type of fuel you're using. If you use low-octane fuel, it is more susceptible to pinging as it ignites more suddenly and erratically. The higher the octane of your fuel, the more smoothly and progressively it will burn, which helps prevent pinging. This means that with better fuel you can run more boost or more advanced ignition without any engine difficulties, which indeed means more POWER.
Turbo Engine Setups
Carburetor
There are two types of choices in a carburetor turbo setup: “Suck-through” or “Blow through”. The Suck-through (or draw through) setup involves mounting the carburetor before the turbo inlet (usually in front of the impeller mouth). This means that both fuel and air are drawn into the turbo already mixed and then blown into the inlet manifold. This is by far the simplest way to set up a turbo, as the carburetor doesn’t need to be especially modified and tuning is quite easy. The main disadvantages are that you can’t use any intercooling with such a setup, as it is dangerous to run air/fuel mixture through an intercooler core. The reason for this is that fuel can condense inside the intercooler core and stay there – if you then have an engine backfire the intercooler can explode. As a result water injection is about the only option for cooling the charge air with this setup. This also corresponds to a blow-off valve because instead of just venting pressurised air, it would be releasing a fuel/air mixture which is very dangerous. The Blow-through arrangement, logically enough, means the carburetor is mounted after the turbo compressor, so the turbo only draws in air and then blows it through the carburettor, which adds the fuel. To use a carburetor this way it has to be specially modified so that the jets will still add the right amount of fuel. This means specially sealing the carburetor and pressurizing the fuel bowls to match the turbo boost. The good thing is than an intercooler and also a blow-off valve can be used with such a setup.
Fuel Injection
Fuel Injection is the best setup for a turbo engine. As the injectors are controlled by a computer you have full control over the fuel delivery and can tune the engine’s fuel/air ratios much more accurately rather than with jets. A turbo charged EFI engine would have an air filter before the turbo, which then blows through a pipe to the throttle body. The throttle body controls how much air goes into the inlet manifold, which is where the injectors add the fuel. The intercooler (if fitted) is mounted after the turbo, but before the throttle body. When adding a turbo to a naturally aspirated EFI engine, more often then not the injectors themselves will have to be upsized to cope with the fuel demands of a turbo charged engine. The smaller injectors will fail to keep up with the engine and cause it to lean out. Likewise the fuel pumps will often need to be upgraded, because its no good having big injectors if the fuel pump can't keep the fuel pressure up to them at high RPM. Rather than designing a new fuel map and changing the factory injectors, an independent injector driver can be used to drive one or more extra injectors. This is cheaper and easier to do, so it is quite popular for cheaper turbo kits and upgrades. However, it does not offer the same degree of fine-tuning as complete fuel system upgrade. For the best drivability and reliability a full after market programmable computer system is definitely the way to go. Using a programmable computer also gives you much more flexibility for making future modifications.
Twin Turbo
Factory twin turbo systems are invariably designed to make the engine more tractable, rather than more powerful. To understand why this is so, you have to realise that big turbos and small turbos behave quite differently. A small turbo has minimal inertia, so it takes more gas flow and boost. On the other hand a small turbo may not be able to flow boost up to a maximum level when a large engine is revving to its greatest. A larger turbo will have greater further capacity, so although it takes longer to speed up, it wont run out of puff at the top end of an engines rev range. To give an example say we have a 2.0 litre engine with a small turbo like a Garret T2. Such an engine would come “on boost “ at very low engine speeds possibly before below 2000RPM, but due to the limited flow capacity of the turbo, by 4000RPM boost would start to trail off and by 6000RPM you might have less then half your normal boost level. If you change this turbo for something very large like a Garret T4, the engine wouldn’t come on boost until much later – probably around 4000RPM, which may make the car a bit difficult to drive, but the good thing is that by 6000RPM the turbo would still happily provide full boost. Normally you would look for a compromise somewhere in between, like a T28, so you would get boost just before 3000RPM and only trail of very slightly at the top end. Ready to get the best of both turbos, however, some manufacturers have used a set up called “sequential turbo charging”. Put simply, this system uses a small turbo to give boost at low RPM, with an additional larger turbo kicking in at higher RPM. These systems take a lot of development to get working smoothly and are also expensive to do in an aftermarket engine, but factory sequential turbo engines like the 13B-REW rotary in the series 6 RX7 are certainly very impressive. A slightly simpler approach is to use two small turbos instead of one big one. This way both turbos come on boost at relatively low RPM, but because you have twice the flow of one small turbo, boost doesn’t tend to drop off at higher RPM quite as much. For ultimate power potential, however, one big turbo still has more potential due to superior to its flow efficiency. This is why drag cars generally use one huge turbo even if the engine came with a twin turbo setup originally.
High Flow Turbos
Turbo sizes need to be properly calculated to get optimum performance and drivability for a given engine. Generally a hi-flow turbo will use the existing exhaust turbine from a factory turbo, with either a larger compressor wheel in the existing housing, or a whole new compressor assembly grafted on. It is generally the case that the existing turbines on factory turbochargers can support a larger compressor, so this is convenient way to improve the flow capability of your turbocharger while still keeping the original manifolding, oil feeds and etc. This process is relatively cheaper than going for a new turbo and associated parts required for that turbo.
