In a previous post about turbo basics we discussed the basics of a turbo and how it increases engine output power, we also touched on a few of the disadvantages, turbo lag being the major one. The terms ‘turbo lag’ and ‘boost threshold’ are often confused, but they are two separate things. Turbo lag is the time delay between opening the engine’s throttle valve and when the turbo accelerates and delivers positive pressure (boost) to the engine when engine speed is above the boost threshold. Put simply, the turbo does not react immediately to a change in throttle position or engine speed, it needs time to react and accelerate. This situation occurs to some degree in all turbocharged engines but it is more prominent when a large capacity turbo with a large mass moment of inertia is fitted to a small capacity engine. Inertia is basically an objects resistance to a change in direction or speed. This means that a large, heavy turbine wheel will have more resistance to accelerate than a smaller, lighter version which is why reducing the mass and diameter of the compressor and turbine wheels is one way to reduce turbo lag. A turbo’s boost threshold is the engine speed (rpm) equivalent to the required exhaust gas flow for the turbo to produce boost, below this level the turbo simply will not produce boost and very little benefits will be achieved. A common problem with performance vehicles is that people tend to fit large capacity aftermarket turbos to their engines opting for high output power but neglect to verify if their engine can actually produce the required exhaust gas flow to have boost delivered to the engine in the desired rev range. When this is overlooked it often results in decades passing before the turbo finally spools up and provides the engine with boost. This problem is commonly described as the car being ‘laggy’ but it is actually because the engine is not producing the required exhaust gas flow and is not really compatible with the turbo’s boost threshold at low engine speed, basically the turbo isn’t matched with the engine and is oversized.
Obviously the effect of using a turbo with a boost threshold outside of the range of your engine is undesirable, as are the effects of turbo lag, so what can you do to minimise these factors? It really depends on the application as to the length you are prepared to go, but there are a number of ways that turbo lag can be reduced even in a street car. More than anything a turbo needs to breathe so two of the most important things are to have a free flowing air intake and exhaust system that will provide little impedance to air flow entering the compressor inlet or to the exhaust gas flow as it leaves the turbine. In a street car this can sometimes be difficult due to noise and emission restrictions where mufflers and catalytic converters are required which impede exhaust gas flow. A good exhaust centre can normally provide performance options whilst still conforming with these legislations. Another way to minimise turbo lag which also effects the boost threshold is to fit a turbine housing with a smaller aspect ratio (A/R). These are readily available for most aftermarket turbochargers and can be interchanged whilst retaining the original turbine. The aspect ratio has a big effect on exhaust gas acceleration as it enters the turbine due to the sudden change in volume, which affects turbo lag.