Top Gear presenter James May visits David Pipe's Somerset yard to take a look at the make-up of thoroughbred race horses ahead of this weekend's Grand National, finding the "elite athletes" have a secret turbocharger and are "in essence, rear-wheel drive".

The Grand National is live at 1615 BST, Saturday on BBC One, Radio 5 live and online (UK users only) with additional broadcasts on Thursday 1340-1630, Friday 1345-1630 and Saturday 1300-1710 (all BST).