Engine Maintenance and Modification Bradley Artigue

FIAT 124 Spider
Engine Maintenance + Modification
8. Exhaust System
The exhaust system conveys spent gasses from the cylinder head to the exterior of the
car. Exhausts perform a number of valuable functions and exhaust design is crucial to
good engine performance. Emissions, noise, and backpressure are key concerns in
exhaust design. Emissions must be safely conveyed away from the passenger
compartment; in the case of the FIAT Spider they are reduced by a catalyst and expelled
behind the vehicle. Noise produced by an engine must be muffled in order for the
vehicle to function in society; therefore a muffler is installed at the rear of the vehicle to
reduce exhaust noise. Spiders also have a center resonator, an additional muffling
device located above the rear axle. Backpressure exists if gasses escaping the engine
meet resistance and, taking into account the safety and environmental features noted
above, is the focus of good exhaust system design.
A poorly-designed exhaust manifold may create pressures that inhibit the engine’s ability
to breathe. Many vehicles have intricate exhaust systems, designed specifically to
equalize the length of each cylinder’s exhaust pipe. FIAT Spiders used two types,
known as 4-2-1 and 4-1; the former is considered to be a superior design. Aftermarket
headers are also produced, each touting improvements to the original designs (with
dubious pedigree; refer to the section titled “improvements” later in this book.)
There are two types of the 4-2-1 design. The first was fitted to 1756cc Spiders
manufactured in 1974. There are four pipes running from the exhaust side of the engine,
tapering down to where a flanged two-pipe fitting mates and extends under the car.
Here a single pipe leads through a catalytic converter (or not, depending on year),
through mufflers and out of the car. The second type of 4-2-1 was installed on the fuel
injected Spiders. It is similar in design except the four primary pipes bend into two and
into a collection “box” where the oxygen sensor is installed. The 4-2-1 was designed for
Spider engines in the 100+ HP range and is at least as good (arguably better) as most
“performance” aftermarket systems.
The 4-1 design was installed on most Spiders and in every engine displacement. Four
short pipes extend from the exhaust manifold into a single pipe that runs under the car
to the catalyst and onwards. The design is simple and the exhausts are very well built.
They suffer only from the rather abrupt turns from the exhaust ports to the down pipe
and, unless the engine is going to produce more power than stock, may not be worth
replacing.
8.1 Exhaust System Identification
Look in your engine compartment at the exhaust side of the engine. There may be a
metal heat shield on top of the exhaust - it will have three 13mm nuts securing it down.
The exhaust system parts have the part numbers stamped into them. It is very unlikely
that a Spider will have the original parts from the down pipe back.
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