Pressure carburetors versus suction carburetors in the 1930-1940 boosted (supercharged) racing engines era which might be of interest to some.
From the 1937 Mercedes-Benz M125 5.7-litre eight report:- “Expectations for a dramatic power increase had been high since late September 1936, when Mercedes first tried a suction carburetor system on the 1935-type engine. Suction systems, in which the supercharger draws a fuel/air mixture from the carburetor and then pumps it to the engine, had been widely adopted by competitors like Auto Union and Alfa Romeo but were not fondly regarded by Mercedes-Benz at Unterturkheim. All its new racing engines since the mid-1920’s had superchargers that blew through the carburetors, so the company had a vast found of knowledge on the operation of pressure-carburetion system. But Scheerers findings brooked no argument. Tests on a 1936-type engine with a suction system gave power increase of 32% at 2000rpm and 11% at 5800. Peak output was 488bhp at 5500. Cautioning that these results did not reflect the changes that would be needed to get smooth throttle response in the car, the engineer estimated that the M125 would deliver 583bhp on the normal racing fuel blend and as high as 597bhp on WW fuel, a blend that was good for power but not for fuel economy. It was to happen, but not easily. A power curve taken on the first M125 in mid-February with suction carburetion, showed 580bhp@5800rpm on standard fuel blend and even better output at medium and low speeds than had been forecast. This was an excellent result. But when installed in the car the eight refused to respond to full throttle smoothly unless carburetor size was drastically reduced. Nor did it want to react quickly to a blip of the throttle for downshift. After these discouraging trials the 1937 season was started with pressure carburetion of the proven type.
The sacrifice in peak power incurred was relatively small, a matter of 5 to 10 hp, but the loss was substantially greater at lower engine speeds. Torque was strikingly reduced. A comparison made in June 1937 on a single engine with the normal test fuel and settings suitable for racing showed the difference:-
Pressure carburetor = 3000rpm, 6.2 psi boost, 316bhp, 750Nm torque.
= 5800rpm, 13.3 psi boost, 550bhp, 675Nm torque.
Suction carburetor = 3000rpm, 10.2 psi boost, 361bhp, 857Nm torque.
= 5800rpm, 11.3 psi boost, 556bhp, 683Nm torque.
To find a suction-type carburetor that would give the needed throttle response the Daimler-Benz design office reverted to the example of a 1924 2-litre racing Mercedes eight, the M218, which had an updraught instrument incorporating an Automat, a weighted sliding venturi. Much like that unit in principle, the new carburetor had 2-updraught 62mm throats, each with a 48mm Automat venturi."