DUAL carbureters, as equipment for eight-cylinder passenger-car engines, have recently come into special prominence and, compared with a single carbureter, give a gain in power in the middle-speed range, between 1400 and 2800 r.p.m. This is an adaptation from airplane-engine practice, in which greater power-output and better distribution have been realized by multiplying carbureter units as the number of cylinders is increased. An absence of overlapping and interfering suction-strokes and the use of larger manifold-passages are apparently responsible for this gain.
Tests made on a number of eight-cylinder engines of both the in-line and the V-types confirmed this gain, which was, however, unaccompanied by any particular gain in fuel economy. With dual manifolding, the weight and heat-capacity of the manifold mass are high in proportion to the piston displacement, with a resultant lag in mixture temperature, as compared with changes in the manifold temperatures, the extremes of the former being greater than in other engines.
While the dual carbureter has been blamed for in-ability of the engine to idle smoothly and for poor fuel-economy at light loads, the substitution of a single carbureter on the selfsame engine failed, in virtually every case, to improve either condition.
Carburetion problems are relatively simple with the dual system. Low velocities can be used, and a nearly uniform mixture gives good results except at low speeds. Some minor variations between the suction and the airflow of the two carbureter barrels result from the difference in the length and volume of the two manifold members. In service, some difficulty was experienced at first in securing a satisfactory setting of the two idling adjustments.
Dual carbureters have not successfully replaced the single system on six-cylinder engines because of “blowback of fuel spray and air charge from the mouth of the carbureter.” This condition probably can be remedied by a slight change in the valve timing.
The discussion covers the necessity for overcoming spiralling of the airstream, the increase in aircraft-engine power-output following the change from a six-cylinder manifold to a three-cylinder grouping and the “ramming” and blowback effects in six and eight-cylinder engines.