FIAT 124 Spider
Engine Maintenance + Modification
7
2.3 Carburetor
A carburetor is a device that mixes fuel and air together for use in an internal combustion
engine. A throttle regulates the amount of airflow into an engine and the subsequent
increase in air speed and drop in pressure. A restriction in the carburetor barrel, known
as a venturi, forces the air stream to increase in speed as it passes by a number of fuel-
delivering orifices. The orifices enrich the stream of air with fuel. The resulting mixture
is a near lambda combination of air and fuel.
2.4 Fuel Injection (FI or EFI)
A fuel injection system delivers a metered amount of fuel under high pressure into a
stream of air. Unlike carburetion, fuel injection does not rely on the pumping of the
engine to draw fuel into the air stream. Instead the fuel is forced through a small orifice
(a fuel injector) at very high pressure. The flexibility of fuel injection systems and
accuracy of fuel delivery leads to increased horsepower, better atomization of fuel,
reduction in emissions, and improved fuel economy.
2.5 Compression Ratio (CR)
The compression ratio is used to measure the performance of an internal combustion
engine. The compression ratio is calculated by taking the cylinder bore (diameter), piston
stroke, and volume of the combustion chamber prior to ignition. The formula looks like
this:
Where b is cylinder bore diameter, s is the piston stroke length, and Vc is the volume of
the combustion chamber. It is important to note that your author inserted this only to
demonstrate use of the pi (π) character and because formulas make guides like this one
look more scientific and authoritative.
FIAT Spiders had compression ratios that ranged from a low of 7.5:1 (Spider Volumex) to
a high of 9.8:1 (124 Sport Spider 1608). The Volumex relied on a supercharger to
increase the horsepower of the engine; therefore, it is notable that compression ratios
alone are not the sole indication of an engine’s capabilities. Generally speaking, and
since most of us would not install (and could not find) a Volumex supercharger on our
FIAT, mild increases in CR towards the 9.8:1 range result in good performance
increases. CR is typically increased with a piston swap, although other methods
(described later in this guide) will bump it up a point or two.
Pistons are available that will drive the CR of the engines upwards, all the way to 11:1. A
general rule about compression ratios is that the higher the CR the higher octane fuel
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