The Z Magnette was fitted with a 1500cc version of the BMC B-series engine originally developed for the Austin 12. It has a well-deserved reputation for being reliable, resilient and tolerant of neglect. However it has also proved remarkably amenable to stages of tuning that dramatically improve its power output and was developed by the factory to provide a power unit for a range of BMC and, later, Leyland models including the MGA and MGB as well as the more mundane Austin-Morris range. The last vehicle to use it was the Sherpa van!. As a result, the modifier has a wide range of alternatives to choose from and from which to obtain standard parts that offer enhanced performance.
Experience of Magnette modifications over the years has generally suggested choosing one of the following specifications for a combination of improved performance but also ease of installation. (Note: the best way to identify an engine is to look at the number cast into the side of the block. Engine number plates are very easily interchanged by unscrupulous vendors!)
MGA 1588cc:: This engine is physically very similar to the 1489cc unit and has simply been bored out to provide the extra 100cc. In standard form the additional performance achieved by fitting this engine is marginal (80BHP vs 72BHP in the standard ZB), though like all B-series engines it will take a fair amount of tuning. All the MGA engines are fitted with a mechanical tachometer that is driven by a skew gear on the rear end of the camshaft, so you have the choice of using this to add the gauge to the Magnette or reverting to a plain camshaft. "1600" cast into the block.
MGA 1588cc Twin-Cam engine: mentioned only really for academic completeness but extremely rare, extremely expensive and if one comes up for sale you will probably trampled in the stampede of "Twinkie" enthusiasts. The block is similar to the conventional engine but the front end of the engine and the twin overhead cam head are very different.
MGA 1622cc: With this development of the engine came more significant benefits, although it will still fit very easily into the Magnette. Introduced in 1961, it is stronger than its predecessors and had bigger valves in the head, producing a useful 90+BHP with the possibility of tuning for more. "1622" cast into the block.
Farina Models 1622cc: Although having a lower compression ratio, these engines can easily be rebuilt to MGA performance standards because compression ratio was only adjusted in the various engine versions by using dished or flat pistons. 1622cc blocks are the same, regardless of application and if you don't mind having a "AMW" engine number, you will probably get a cheaper bargain than someone who insists on MG.
MGB 3 main bearing 1800cc (Type 18G & 18GA): the best of the 3- bearing options but becoming hard to find. Very similar to the 1622cc but bored out to get the extra 176cc. It offers an excellent combination of extra grunt (95 BHP) with minimal modification hassle because it still accepts the Magnette engine end plates and sump. In order to bore out the cylinders, their spacing was changed, which in turn required a stronger block, a new crankshaft design and narrower main bearings to fit it in. The cylinders were now closer together, so there is no water jacket between pots 1 & 2 and 3 & 4, leading to weak points in the head gasket. Always use the best gasket you can get for this engine. Not recommended for serious competition work as the 3-bearing crank was now reaching the limit of its ability to handle the horse-power. "1800" cast into the block.
MGB 5 main bearing 1800cc (Type 18GB et seq) A significant increase in power but at a price in terms of complication. The modification is possible but requires significantly more work than the 3-bearing engines. On the other hand, the 1800cc units are readily available as they were produced in vast numbers. "1800" cast into block. Some later versions were burdened with emissions systems that actually reduced performance and have air injector ports in the head.
Other 5 main bearing 1800cc: as fitted to the Morris Marina and the Sherpa van. Similar in most respects to MGB. Easily rebuilt to match MGB power output and may offer a cheaper alternative. "1800" cast into block.
As time goes on, some of the above are getting older and scarcer, so the more logical choice for maximum benefit would be the 5-bearing 1800cc version. It is suggested that the 18GB units of 1965 to 1967 are the easiest. If using a later type you must be prepared to obtain an 18GB type flywheel which is suitable for the inertia type starter motor although with the introduction of small strong geared after-market starters this is less of an issue and a pre-engaged starter can probably be fitted in. If you are happy to stick with a 3-bearing engine, don't feel that the 1622cc engine is a poorer option than the rare 1800cc because the difference in output is very marginal. With a bit of work on the cylinder block face, the MGB head with bigger valves can also be fitted. Any B-series head will fit any B-series block but with different combustion chamber sizes and different valve sizes you have to be careful not to ruin the engine rather than improve it.
There are more exotic possibilities that will not be described technically here until we have full details of a real-life conversion on which to base guidance. Known conversions include the ubiquitous Buick-derived Rover V8 3.5 ltr and other small-block V8s in the USA. The light construction of the Rover unit means that another four pots can be slotted in without the disadvantages of extra weight at the front end.