For more extreme performance cars, mechanically operated secondaries can provide excellent performance as long as they are properly matched to the application. Typically, these vehicles will feature manual transmissions or automatic transmissions with 3,000-stall speeds or higher, performance rearend gear ratios (3.73:1 or lower), and compression over 10.5:1.
Sizing
As noted, too large a venturi may decrease air velocity. When velocity falls below the point that fuel is no longer properly atomized as it leaves the discharge nozzle, poor performance will occur at that engine speed. This translates into poor throttle response, poor low-speed torque, and reduced drivability. In general terms, engine displacements up to 327 ci with mild modifications may run well with 525- to 725-cfm carburetors. Engines over 350 ci with moderate engine modifications may run optimally with a vacuum-operated four-barrel carburetor rated no higher than about 750 cfm. Some larger engines that are highly modified may run best with 850-cfm carbs. Extremely modified engines with displacements exceeding 454 ci may run better with larger carburetors (850-plus-cfm). Of course these are not hard-line requirements, but general guidelines. Performance changes that can affect carburetion include valve sizes, exhaust system, different intake manifolds, camshaft profiles, compression ratio, transmission type, and ignition timing.
The folks at Barry Grant have developed a section on their Web site to help you choose the correct carburetor. To use this chart you'll need to know your camshaft's duration and, for automatics, the transmission's stall speed. To do this, simply log on to barrygrant.com and scroll down to Demon Carburetion and the Demon Selection Guide.
The Triple Crown
We spent the day at Westech's dyno facility to chassis-dyno-test three particular carburetors on Bobby Mahoney's Rat-powered Chevelle. We tested 750-, 825-, and 1,000-cfm units. The engine specs include a 468 big-block with AFR 315cc fully CNC-ported, angle-milled aluminum heads, 2.25/1.88-inch valves, and 121cc combustion chambers. The camshaft is a Comp Cams solid-roller (custom grind), 254/260 degrees duration at 0.050-inch lift, 0.715-inch lift, 110-degree lobe separation. Engine compression is 12.5:1 with an HEI MSD ignition, Edelbrock Victor open-plenum manifold, and Hooker 2-inch Super Competition headers.
In this particular case, the larger carburetor provided the best power, but note that this is a highly modified big-block Chevy. For most other performance street cars smaller carburetors will typically provide better throttle response and a larger power curve. At the bottom of each test we've averaged the horsepower and torque numbers. This Chevelle is making some big power at the rear wheels!
Single-plane intake manifolds...
Single-plane intake manifolds are best matched to carburetor and engine combinations that are operated at high rpm (typically above 4,500). This is because at high air speed the path to each port is direct. Dual-plane manifolds generally provide better performance on engines operated from 1,500 to 6,500 rpm.
For our dyno-testing we ran...
For our dyno-testing we ran 750-, 825-, and 1,000-cfm Race Demons. The power levels stayed remarkably close between the 750 and 825-cfm carburetors until 5,700 rpm, when the 825 carb produced slightly more power. The 1,000-cfm carburetor produced more power during this dyno session, especially from 4,300 rpm up. Note that this is a very modified large engine with a manual transmission. Although power numbers are higher in this case with the 1,000-cfm carburetor, smaller ones typically provide a crisper throttle response on the street. For most street applications a 1,000-cfm would be far too large.