The system relies on an accelerometer-type transducer bolted to the intake manifold to listen for vibrations characteristic of unscheduled combustion. This is why Pontiac has adopted a very sophisticated spark retarder, pioneered by Buick in its 1978 turbo Regal. A side effect of using a later spark to limit detonation is the fact that it also sacrifices power. It can be combatted by at least three means: by adding water injection, which Pontiac has avoided for a number of good reasons by enriching the fuel-air mixture, which is unfortunately contrary to efficiency goals and therefore used sparingly and by retarding ignition timing. Nine psi worth of overpressure is a rather ambitious undertaking for an engine certified to run on 91-octane fuel, and realizing this, Pontiac has taken several precautions to avoid the mechanical destruction all too common when a turbocharger blows an engine the wrong way. It heats the charge to maintain cold drivability, but once the cooling system reaches 217 degrees Fahrenheit, the thermostat shuts off flow to keep fuel-air mixtures as cool as possible. Instead, there's a water jacket surrounding the plenum chamber. Heat is also detrimental to a high-mass flow, so the normally aspirated 301 engine's exhaust crossover has been omitted from the turbo engine's intake manifold. All passages are as smooth in shape and as generous in area as possible to minimize power-limiting restrictions. Air is picked up by a four-inch-diameter duct just above the front air dam, flows unimpeded to the carburetor, where fuel is added, and then turns through 90 degrees in the plenum to enter the turbo compressor in a horizontal stream.
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