JAGUAR - Gearbox Swap

Q
The question I have concerns a letter in the magazine, where it appears that someone with a V12 was thinking of improving the performance of it through the use of a Supra five-speed gearbox and by fitting twin-turbos.
I would like to know if it's actually possible (or worthwhile) to fit this gearbox?

A
It is certainly possible, but not something we could recommend wholeheartedly as the standard Supra gearbox suffers from a weak third gear, which can quickly fail if the car is driven hard. The Supra turbo gearbox is much stronger, but also much harder to find.
The best, if expensive, gearbox for the job will be the Tremec T5, but an affordable and reliable alternative is the Getrag five-speed unit as fitted to the six-cylinder XJ-S, but be
sure to use a pre-1987 gearbox in a pre-1987 car otherwise there will be no drive for the mechanical speedometer. Try The Driven Man (www.thedrivenman.com) for the Tremec and Simply Performance (www.simplyperformance.com) for the Getrag kit.