The Triumph 6 is a cast iron block, cast iron head pushrod engine of 2 or 2.5 liter displacement. The 2 liter (1998cc) engine was found in the GT6, as well as the Triumph 2000 sedan. An earlier version of 1600 cc was fitted to the Vitesse saloons. This 2 liter 6 has a bore of 2.94 inches and a stroke of 2.992 inches. Basically square. The 2.5 liter engine, actually 2498cc, uses the same bore but increases the stroke three quarters of inch to 3.74 inches. The early TR-250 and TR5 cars even used the same block as the GT6, but later the larger displacement cars got a slightly different block with a bit more clearence at the bottom for the crank throws. Later TR6s have still a third block with no significant changes. The blocks are interchangeable for all practical purposes.
The early GT6 Mark I engine (66-68) had a head with pushrod guides being pressed in tubes, while later cars of both displacements used the same head casting with integral guides. In the long stroke motor the head was used as cast, but the 2 liter got it trimmed down to maintain good compression. [Actually some early cars had different port spacing than later cars.] This will be covered in more detail in the section on heads and intake systems. Rather than write a lot of confusing numbers, here's a chart in which all the confusing numbers are well organized. I will use the US of A market and model designations and specs unless noted.
| Model | Years | Disp. | Stroke | CR | bhp | torque |
| GT6 Mk. I | 66-68 | 1998 | 2.992 | 9.5 | 95@5000 | 117@3000 |
| GT6 Mk. II (+) | 69-70 | 1998 | 2.992 | 9.25 | 95@4700 | 117@3400 |
| GT6 Mk. IIIa | 71-72 | 1998 | 2.992 | 8.5 | 79@5000 | 117@3000 |
| GT6 Mk. IIIb | 73 | 1998 | 2.992 | 7.5 | 79@4000 | 110@3000 |
| TR250 | 68 | 2498 | 3.74 | 8.5 | 111@4500 | 152@3000 |
| TR6a | 69-71 | 2498 | 3.74 | 8.5 | 104@4500 | 142@3000 |
| TR6b | 72-73 | 2498 | 3.74 | 7.75 | 106@4900 | 133@3000 |
| TR6c | 74-76 | 2498 | 3.74 | 7.5 | 101@4900 | 128@3000 |
There is some discrepency as to when the low-compression TR6 came about, and
what the actual power output is for that motor. Maybe I should have thrown in
the specs for 9.5:1 injected TR6 just for fun. Anyway, later cars (75-76?) had
106 bhp but same torque and compression. Look at page 43 of the Bentleys for
some further info about which year did what. Also, the GT6 Mk. III was
manufactured up until december of 73, so you may actually come across a car
labeled as a '74 model year car.
All the information about the Triumph Six naturally also applies to those TVR models fitted with Triumph six running gear. Being several hundred pounds lighter (1800 for the TVR versus 2300 for the TR6) a few judicious changes to the TVR's motor will have more dramatic effects on the car than those same changes on the heavier Triumph. Since a GT6 weighs the same as the TVR, perhaps the thing to do would be install a 2.5 liter motor in one of those little guys...
Add in some power to weight figures for GT, TR6 and 250.