Tech Talk: McLaren's unique top-mounted radiator
McLaren have continually referred to the MP4-30 as the "first size zero" Formula 1 car because it has such a compact rear, allowing for slimmer sidepods (see Fig.1) which allows greater airflow to reach the diffuser.
But how exactly have they achieved this? Well the answer - at least some of it - might have been uncovered in the above photo which shows the MP4-30 with its engine cover removed.
If you take a look at our anotated version (below), then you'll see that the airbox (see Fig.2) is split in two. Most cars have one or more airboxes to feed certain areas of the power unit, so this isn't uncommon.
McLaren's version is however unique in that located on top of the airflow ducts is multiple small radiators. Traditionally a car would have a radiator located in each sidepod, one to cool the engine/oil and one to deal with the ERS and turbo.
By placing a radiator above the airflow duct, it allows cool air to enter the airbox  and be channelled directly through the radiators  where it then travels down the spine of the engine cover to the large exit (see Fig.3) at the rear of the car .
A second airbox and duct  provides cool airflow for the remaining systems which require it, such as the power unit's compressor .
By relocating part or all of one radiator unit, it allows McLaren to split the second radiator into two, with half in each sidepod, allowing for them to be far slimmer.
(Above image © Octane Photographic and Grand Prix Times)