[Thor]

The manifolds on my 409 (with PAS) are the same, LHS has rear exit and RHS has central exit, and I recall seeing the same on other Bristols of the similar period. The RHS manifold has the central exit and the valve controlled by a bi-metallic strip which forces exhaust gas back across the inlet manifold to warm it up quickly from cold. All evidence, cars, and reports I have ever seen would say that all the Bristol 'Poly' engines had these features and the 4-barrel Carter carburettors, when they were built. In a comprehensive history of car engine design, I think the Poly engines would deserve an honourable mention for the good port and combustion chamber shapes allowed by the valve layout, without incurring the complication of the 'Hemi' double rocker shaft arrangement. It really is remarkable for a single cam 'V' engine.