Actually, the reason that the Magnette was released with the B Series engine and the Wolseley with the XPAW engine was nothing to do with giving the Magnette more power, although this was a desirable side effect of that decision. Both cars were designed prior to the Nuffield/Austin merger, but the Wolseley was ready ahead of the Magnette and it was decided to release it with the Nuffield designed engine and change over to the B Series at a later date (which resulted in the introduction of the 15/50 a few years later - basically a FourFortyFour with a B Series and a handful of minor cosmetic changes). The Magnette was still under development at the time of the merger and it was relatively straightforward to change the design to accomodate the B Series engine.
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