Triumph's rugged, easily tuned wet-liner four had done yeoman sports-car service since 1953 but was literally out of breath by the mid-Sixties, when it became clear that the larger new TR4/4A would need more power to stay competitive. Enter the Triumph TR5 PI/TR250.
Back in the Fifties, Standard had introduced a small inline four that was later developed for Triumph's compact Herald sedan and the "junior TR," the Spitfire. There was also a derivative six, first seen in Standard's four-door Vanguard Six and later used in the Triumph 2000 sedan. With modifications, this 2.5-liter engine was duly installed in the TR4 to create a more potent successor.
Despite its greater displacement, the six was no heavier than the old four, though it was significantly longer. Luck was on Triumph's side, though, as the six just fit the TR4 engine bay and proved relatively easy to clean up for the new U.S. emissions standards that were being enacted during its development.
Nevertheless, Triumph ended up producing two versions of its latest TR. America got the TR250 with a detoxed 104-horsepower six fed by twin Zenith-Stromberg carburetors. Europe and the rest the world enjoyed the 150-bhp TR5 PI, the suffix denoting petrol (fuel) injection by Lucas, the same basic system used by Maserati on its contemporary 3500GTI and Sebring. Needless to say, the performance of these cars was utterly different.
Visually, they differed little from the TR4A, with changes confined to the grille (now minus its vertical support bar), nameplates, and minor trim. The TR5 even retained the original hood bulge needed for carburetor clearance on the TR4. Both rolled on standard steel wheels, adorned (if that's the word) with "mag-look" covers, but many examples were treated to the traditional center-lock wires. TR250s wore rather silly transverse racing stripes on their noses, and their soft tops had reflective tape above the doors and rear window for nighttime safety.
Chassis specs remained much as before, but radial tires were standard and the rigid rear axle option for American models was scratched. Final-drive ratios were numerically lowered to suit the torquier six.
Returning to performance, the TR250 was no faster than the TR4A from standstill or flat out -- no surprise, since it had no more horsepower. But, of course, the six was a lot smoother than the old four, and its extra torque greatly improved drivability at low speeds in the intermediate gears. By contrast, the TR5 would wind to an easy 6,000 rpm and near 120 mph, while cutting no less than 20 seconds from the 4A's 0-100 mph time. Unfortunately, it idled like a race car and used a lot more gas than the 250 (perhaps as much as 16 mpg U.S.), and the fuel injection proved far more finicky than the 250's good old-fashioned carburetors.
Despite their six-cylinder smoothness and greater flexibility, the TR5 and TR250 were seen by many as new wine in old bottles. What was needed, some said, was fresh styling to match the new engine and its more "manly" character. Triumph, in fact, was already working on it, which explains why TR5/250 production lasted just a year and a half. The TR6 was on the way.