![]() ![]() Pontiac's objectives were to match T/A 400 acceleration, to reap all the fuel-economy benefits offered by trimming piston displacement 25 percent, and to keep drivability up to GM's usual high standards. So far, this is standard turbo-engine practice, but all these conventional components were screwed together and dialed in to produce some very un-turbolike behavior. The compression ratio has been lowered to 7.5:1. Pistons are cast-aluminum, but they've been redesigned (along with the wrist pins) for greater strength. Crankshaft fillets receive a pressure-rolling treatment to make this part more fatigue-resistant. Extra material was added to the block's bearing webs and top deck, and main-bearing-cap bolts have been increased from seven-sixteenths to one-half inch in diameter. Several internal modifications have also helped the 301 stand the gaff of nine pounds of boost. This happens continuously every few milliseconds, so the engine runs with optimum spark advance all the time. Upon hearing the first few rattles of detonation, the transducer tells an electronic module first to back off spark advance, and then immediately thereafter to restore just enough to keep ignition timing at the threshold of detonation. 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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