Ah, the Triumph Stag V8, the stuff of classic car legends.
It´s all there for a long chat at the pub: dashed hopes, shoddy Midlands workmanship, the dark days of British Leyland´s decline. There´s even a bit of Italian in there, as Giovanni Michelotti styled the car. The bit we´re interested in is the V8 though. This unit was conceived in the middle of the 60s in response to the growing demands of the UK market for more powerful engines as the motorway system expanded. Triumph had some sixes and jolly nice they were too but they were not enough. They also needed more swept volume for a planned push into the US market and, no doubt, to compete with the Germans who were about to offer V8s in their Mercedes-Benz R107 (SL) and C107 (SLC) vehicles.
What was the Triumph V8? It was 3.0 litre unit designed by Triumph themselves, being more or less two Triumph slant-4 engines placed together. It had one OHC per cylinder bank and had a cast iron block with – crucially – aluminium heads. It had a single crankshaft which is partly responsible for the engine´s rather pleasant noise (if it runs).
In order to meet expectations at the time, the original plan for a nice 2.5 litre unit was changed, rather late in the day it would seem. Spen King, the chief engineer, insisted the Stag´s ought to be a 3.0 litre unit, to be run with carburettors. Increasing the diameter of the cylinders to increase the volume meant the walls were thinner and less able to conduct heat. The aluminium cylinder heads were designed to close tolerances as well. As a result of poor management and a lack of experience on the part of the workforce, the heads were often not produced to a high-enough standard.
The other problem was that the losses Triumph were running lead to cost cutting. The purchasing department found a cheaper supplier for the head-gasket which often failed. There was not enough time spent on engineering the problem of water-circulation either. The results of this meant the engine was liable to overheating when used hard.
If all had gone well, the Triumph Stag V8 would have made a convincing alternative to similar GTs from Mercedes and BMW but was much less costly (about half the price). It got good reviews and rather had the field to itself. There were not very many alternatives to its formula of V8 power and open-top style. But the problem of over-heating engines ruined its chances. And had all gone well, Triumph might just have avoided the ghastly decline that followed from 1970 to 1980, ending in the bathos of the Triumph Acclaim, a four-cylinder family saloon, a re-badged Honda.
Post-script: there is some debate as to whether the Rover V8 might have been a better engine for this car. Allegedly the engineers lied when asked if the Rover V8 might fit. When this option was mooted a lot of time had already been spent on the Triumph V8 so understandably the engineers did not want to see their work discarded. Since engines matter, it would have been preferable to have V8 expertise in BL rather than to have to buy-in the know-how by using the GM-derived Rover unit. Additionally, even if the Rover V8 did fit, it may not have had the required characteristics of the Triumph design.