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The downsizing of spark ignition engines should be an issue to decrease the consumption and to fulfil the ACEA commitment, i.e. 140 g CO2/km in 2008 and maybe 120 g/km in 2012. To achieve very low specific fuel consumption, the use of very downsized engines should be a solution. However, it is well known that one problem with such engines, that means very small turbocharged engines with high specific power (up to 100 kW/l), will be the turbo lag [5-6]. Different ways are possible to avoid it: some changes in intake layout, exhaust manifolds, turbo inertia, valve timings can be considered, or more sophisticated systems (such as electrically assisted compressor [3], volumetric compressor …) can be envisaged. To classify the interest of such solutions, it is very useful to compute their transient behaviour and, thus, to have accurate models to predict their impact under transient conditions like tip-in at constant speed but also tip-in on a vehicle (varying speed conditions).