A tip that applies in this case to the MG Magnette ZA and ZB because it is in the spotlight. Both models were equipped with an 1489 cm3 four-cylinder from the factory. Nothing is wrong with such an engine, but is nowadays replaced by the power source from a later MG MGA (1622 cm3) or early MGB (1798 cm3).
A larger displacement and therefore (considerably) more powerrrr compared to the standard mounted engine. The acceleration increases, the top speed remains the same. In fourth gear, the thing sounds like an excited tractor. You can do something about that, namely fitting a different final reduction. Fortunately, the Magnette used differentials that can also be found in other BMC products. Among others the MG MGA and MGB. In the ZA, a differential was mounted at the factory with a ratio of 4,875 to 1; in the later ZB, that ratio was 4,55 to 1. Which means that with 1000 revolutions per minute - with rear tires in the size 165 × 15 ”- in fourth gear with a ZB with standard mounted final transmission 25,87 is driven per kilometer. With 4000 tours per minute you only drive 103,49 kilometers per hour ...
If you want to ride with the current traffic, then you will have to turn the engine on to more than 5000 revolutions per minute and the machine does not like that. Apart from the excessive noise in the trolley, a vortex enters the tank and your wallet. You must therefore look for the final reduction of another BMC product. You can find that in the MG MGA, for example. They were equipped with a differential with a ratio of 4,3 to 1 (1000 revolutions per minute means 27,38 kilometers per hour). The MG MGB was initially available with a ratio of 4,1 on 1 (28,71 kilometers per hour at 1000 revolutions per minute) and then 3,9 on 1 (30,18 kilometers per hour at 1000 revolutions per minute). In the latter case, you are driving at 4000 revolutions per minute 120,74 kilometers per hour ... Better for the engine, better for fuel consumption and ultimately all better for your 'cut'. Such a 'joke' also applies to other brands and types. Nice to puzzle you out ?!
Photo: A handy handyman can turn an MG Magnette into a car that can go along nicely in today's - rushed - traffic! To start, install a longer final reduction in the differential ...