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Old 07-21-2004, 03:12 PM's Avatar is offline Moderator
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Terje; your observations are correct. Regardless of the carb size, the engine is a positive displacement air compressor and will demand a precise amount of air at any given speed. The problem for carb designers is in the large range of speed that an auto engine runs. A Dominator carb on a 500cuin engine at top speed is perfectly sized for the application. Friction losses are relatively low and throttle vacuum signals are high enough to induce proper and responsive gasoline flow. The problem with this combo come is when you want to idle the engine. The huge carb that was so good on the top end is WAY too big for the low air velocity at low speed and the pressure drop through the venturi is too small to signal gas flow. That is why manufacturers developed vacuum (or in the case of the QuadraJet, baffle) operated secondaries. With this modification, you have your cake and eat it too. At low speed only small primaries are in service so there is plenty of pressure drop in the venturis and responsive gasoline flow. However when the engine speeds up, the secondaries kick in and the engine still gets it's gas and air through the larger venturi area. The spread-bore Q-Jet is a great idea and wonderful ranging carb. Unfortunately, the maker never refined it to appeal to the performance market. Don't understand why.

Huge carbs with mechanical secondaries were developed for and work best on drag racing engines that are designed for wide-open-throttle (high engine speed) operation. Those don't need all the low speed jewelery associated w/ vacuum secondary carbs. They can be made to run on the street by increasing accelerator pump size and other such tricks at the expense of fuel economy and other operational niceties of the vacuum secondary units.
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