THE 3 ¾ BORE CONTINENTAL ENGINE

The 3¾-bore Continental engine was introduced into production in 1954 with the start of the D series cars. It has often been written that this change was made to answer the need for more power and torque as customers specified increasingly heavy options. In fact it was due to the power robbing qualities of the recently introduced automatic gearbox. The larger bore increased displacement to 4887 c.c. and power at the flywheel was raised from 158 to 172 BHP.
The new engine featured many improved components adopted from the latest developments on the B-61 Rolls-Royce military engine. These included full-length steel cylinder liners, lightened piston design, stronger connecting rod bolts, and hardened exhaust valve seats. All these changes later became standard on the S-1 engine.
The 1/8th inch increase in bore size was physically possible because the engine block had been recast in 1950 with siamesed cylinders as part of the development of the B-60, 3 5/8-bore engine. Although it was never publicised the compression ratio was also raised on the Continental 3 ¾ bore engines to 7.8-1 by machining the cylinder head.

ENGINE EXCHANGE PROGRAMME

Some owners of the early 4.5 litre cars wanted the increased power and performance that was available from the larger 4.9 litre engine. Therefore, when the new engine was introduced an engine exchange programme was established to replace the earlier 35/8-bore engines without excessive delay. While most exchanges took place shortly after the introduction of the larger engine, the program continued for over two decades with some cars receiving exchange engines well into the 1970s.
The first exchange was made on the 13th of December 1954. BC-24-A received a new 3¾-bore engine from stock and its original engine was converted to the new production specification and placed in the exchange program. Most but not all conversions were done on an exchange basis. Several cars did have their original engines converted.

Most engine exchanges were done at the Hythe Road or at Crewe. Records were kept of the exchanges and replacement engines were meant to be re-numbered to correspond with the original engine number of the chassis.

However, because some of the exchanges were done at regional service depots, the original engine number was not always removed and the new number was not always stamped. Therefore some exchange engines still have their original number, or possibly both engine numbers. There is no evidence of any exchange engines having no numbers. Several new engines used in the exchange program, such as BCD 75 and BCD 77, had original D series engine numbers that are higher than those of any engine fitted to a D series chassis. It is believed that this was the result of overproduction and the surplus engines were placed in company stock.
A total of 37 4.5 litre cars with 35/8-bore engines were refitted with 3 ¾-bore exchanges engines. Some cars have had more than one engine replacement to date. These were spread amongst the cars as follows, 9 in the A series, 6 in the B series and 22 in the C series. In the D series, 4 cars received exchange 3 ¾-bore engines as a service replacement for their original 3 ¾-bore engines.

CYLINDER HEAD CONVERSION

A further programme to fit a specially modified, big valve, 6 port cylinder head to the R-Type Continental engine was developed in 1955. This modification was designated OPWAS. The first head was fitted at Crewe by the experimental department. Power at the flywheel was raised to 178 BHP. Further conversions were carried out at the Hythe Road. Additional cylinder head kits were sent to regional service depots. This modification has caused confusion, as the modified engines are sometimes mistakenly identified as S-1 type replacement engines.