According to David Edgington's very much worth having book,
The Lister CS Story http://www.stationaryenginebooks.co.uk/listercs.htm, (pages 35 - 36) the 8/1 first appeared in 1958 as the CS 6/1 was showing its age: too heavy, too expensive, old technology. According to Edgington's book, Lister was scrambling to compete with more modern engines in a post-war economy where construction was booming. Lister's management decided they had to both increase power and reduce manufacturing costs in order to remain competitive. The 8/1 had an aluminum piston to lower reciprocating weight, and as a cost cutting measure, did away with the "Listard" chrome plated cylinder bore in favor of cast iron, with a chrome plated top compression ring. The dual chamber head was modified to include the now ubiquitous plug,which reduced the compression ratio from 19:1 to 17.5:1 for both start and run conditions. And though there appears to be more metal in the 8/1 flywheel disk, Edgington says the flywheels were lighter than those of the 6/1s, although the air cooled VA variant that was manufactured mainly for export had flywheels that in some cases were heavier than those of the standard 8/1 for generator service.Â
The Indian variants seem to simply use a 6/1 piston (cast iron) and flywheels. I've only seen one or two examples of Indian 'roids with the "proper" 8/1 flywheels.
As to what the draw of the 6/1 is over the 8/1 or 10/1, I think it varies. Seems the 6/1 and 10/1 are popular, but not the 8/1 so much. Edgington's book says the 8/1 was capable of running at both 650 rpm (6 hp) and 850 rpm (8 hp). For my part, I wanted low fuel consumption. I didn't foresee any need for more power.
Quinn
