Weekend Workshop

Turn your Herald, Vitesse, Spitfire or GT6 into the ultimate Triumph

How to install a 2.5-litre engine in your small chassis Triumph.

Triumph’s ‘small chassis’ Herald and Spitfire models were first envisioned with modest straight-four engines. Three years after the Herald’s launch, however, the Vitesse demonstrated that a 1.6-litre straight-six fitted under the Herald’s bonnet. This opened a world of possibilities to power-hungry enthusiasts. Triumph repeated its clever packaging trick in an even tighter space in 1966, when the Spitfire grew into the 2.0-litre GT6.

A year later, Triumph increased the capacity of its six-pot to 2.5-litres for the TR5 and 2.5PI saloon. The TR5 engine produced 150bhp and its longer stroke led to an ever greater increase in torque. Suddenly all the parts were on the factory shelves to more than double the capacity and quadruple the power of a Herald. And it wasn’t just about the engine: Triumph also developed brake, transmission and suspension upgrades for the 2.0-litre versions of its small chassis cars.

The factory never went as far as fitting a 2.5-litre engine to a small chassis car.

The performance of this combination could’ve put the big-earning TR sports cars in the shade, which the ‘suits’ at Triumph would not have approved of. Doing it yourself, though, is far from rocket science.

Of course there’s more to building a 2.5-litre Herald than just dropping in the engine and roaring off into the sunset. There are a few tricks to make the engine fit in the small chassis, plus the need for a stronger transmission and better brakes. Today, the most widely available big Triumph engine is the carburettor-fuelled variant from 2500TC or 2500S saloons, which conveniently sidesteps the cost and complexity of installing Lucas fuel injection.

Follow our step-by-step guide as we transform our GT6 with a 2.5-litre saloon engine, and build yourself the performance car that the factory could have, should have, but ultimately never made.