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Mk1 Lotus Cortina, The classic racing saloon car

• 12/20/2016

The Mark 1 Lotus Cortina has earned it’s reputation as an iconic racing saloon car. From the

images of the legendary Jim Clark 3-wheeling his way around classic circuits such as Brands

Hatch and Crystal Palace in the ‘60s, a whole generation has been inspired to try and

emulate his feats themselves. As probably the first “homologation special” that was

specifically designed to allow Ford to dominate production based racing in period, it’s

attributes of a lightweight structure coupled with nimble handling and tuneable engine have

kept it at the forefront of racing ever since.

BTB’s first experience with the twin-cam engined Cortina in historic racing came in the late

1990’s in the relatively low key arena of classic saloon car club racing. BTB based their first

manifold lengths and diameters on some work they’d done on Toyota’s 4AGE engine, also a

1600cc twin cam, albeit with 4 valves per cylinder, but with a similar appetite for revs and a

not dissimilar exhaust porting arrangement. The rally developed 4-2- 1 exhaust package

yielded good power, and more importantly gave a good spread of torque across a relatively

wide rev range. This characteristic gave better acceleration out of slower corners than

competitors with 4-1 exhausts which only worked at higher rpms, and didn’t suit the 4

speed gear ratios of the relatively heavy saloon compared with the engine’s use in a lighter

single seater or sports car.

As historic racing has matured into the professional international scene that it is now, it is

inevitable that the authorities have sought to reign in the development of the Lotus Cortina,

and return the specification to something more closely related to that in which it raced

originally. The effect of this clarification on the exhaust design was a mandatory maximum

primary pipe size, and a stipulation that the pipe layout closely resembles that shown on the

original homologation papers. This limits the available length of the primary pipes, but still

allows for a 4-2- 1 configuration. In fact this shortening of the primary pipe length has made

the exhaust manifold less prone to fatigue failures that are related to the pipes resonating

to the frequencies that are fundamental in 4 cylinder engines in the 8200-8500rpm range.

The Lotus twin cam engine spends a lot of it’s time at that sort of rpm, and therefore the

exhaust needs to be able to withstand repeated harmonic oscillations without failure. Due

to the outside diameter being fixed at 38.1mm (1 ½”) any increase in the wall thickness of

tubing will consequently reduce the inside diameter, thereby restricting the engine’s ability

to breathe at high rpm. BTB pay particular attention to the quality of the material and use

careful welding techniques to minimise the risk of failure in this area.

As the boss of one of the UK’s leading competition engine preparation specialists races a

Lotus Cortina himself, BTB have had their exhaust benchmarked against a number of other

FIA approved products in the market. Our CAD designs were analysed using state of the art

simulation tools so that the flow characteristics could be modelled along with the cylinder

head and cam profiles. BTB are proud that their design has been chosen to be the exhaust

that this particular competitor has optimised his engines to, and therefore recommends to

his clients.

Whilst the layout of the Mk1 Lotus Cortina manifold is largely defined by the FIA appendix K

papers, the exhaust system layout is principally determined by the customer’s desired noise

level. BTB make their system in a variety of configurations depending on whether the car

has a leaf spring or A-frame rear suspension. The number and size of silencers affect the

noise level, so if you want to have the maximum choice of testing venues then a twin

silencer rear exit system is the one to go for. A single small round silencer mounted at the

rear is just sufficient for racing in most championships, but will be too loud for trackdays.

Some owners choose to run a large rear silencer to help with weight distribution. Others

may have the opportunity to race at unrestricted events such as the Silverstone Classic or

Goodwood revival, and may like to use a short side exit system with the straightest path

possible with minimum weight.

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