Testing, Testing, Testing
Is this formula correct for all vehicles? We have our doubts. If you have a modified Corvette with a tiny carburetor (which was added, presumably, to increase torque), there's a chance you could be hurting the output rather than improving it. Of course, the question of a useful power band also arises. Given this thought, we used a computer model in an effort to prove a point. The computer program is Racing Systems Analysis' Engine Pro. We've found this computer engine-simulation program to be reliable, and perhaps a bit on the conservative side when compared to dynamometer test results. The basic model was a hypothetical 355ci small-block Chevy (pretty common Corvette fare). It incorporated a 1.94-inch intake, 1.500-inch exhaust valves (mild port-matching for a flow of 230 cfm at 28-inches of water at 0.460-inch valve lift), a mild hydraulic camshaft (short duration of 260 degrees at 0.050-inch tappet lift, and moderately high lift of 0.460 inch for maximum torque production), an 8.6:1 compression ratio, and a common, average-flowing, dual-plane intake manifold. In our computer simulation, we tried several different cfm carburetor combinations. When the torque peaks were monitored, the results were rather surprising.
390-cfm carburetor: At this cfm rating, the 355 small-block produced a maximum horsepower of 279 at 5,300 rpm. On the torque side of the equation, the computer predicted a maximum of 308 lb-ft of torque at 4,150 rpm.
500-cfm carburetor: We noticed a substantial gain at this cfm level. The computer predicted 307 hp at 5,450 rpm and a maximum torque output of 328 lb-ft at 4,300 rpm. Improvement over baseline: 20 lb-ft.
600-cfm carburetor: At this point, torque and horsepower still increased. Keep an eye on the rpm level at which the torque peak occurred. It's rising, but not as quickly as you might have first guessed: Peak horsepower was 324 at 5,550 rpm. More importantly, peak torque improved to 340 lb-ft at 4,350 rpm. Improvement over baseline: 32 lb-ft.
700-cfm carburetor: The engine is still producing more torque. The torque peak rose to 347 lb-ft at 4,400 rpm while the horsepower increased to 333 at 5,600 rpm. Improvement over baseline: 39 lb-ft.
750-cfm carburetor: Once again, we're seeing an improvement in torque. The computer predicted a maximum torque peak of 350 lb-ft at 4,400 rpm (identical to the rpm level of the 700-cfm carb), while the horsepower increased to 337 at 5,650 rpm. Improvement over baseline: 42 lb-ft.
800-cfm carburetor: We're starting to see diminishing returns, but the maximum torque is still increasing. At this point, the computer "sees" 352 lb-ft of torque, again at 4,400 rpm. The horsepower peak is now 341 at 5,650 rpm. Improvement over baseline: 44 lb-ft.
850-cfm carburetor: At this cfm rating, the torque peak is improving, but the rpm is increasing. The engine produced 354 lb-ft of torque at 4,450 rpm and made 344 hp at 5,650 rpm. Improvement over baseline: 46 lb-ft.
What's The Point?
As you can see, our computer engine tests show the traditional carb-sizing formula may be too conservative. In fact, if you adhere to the common formula, you could be costing yourself plenty in terms of torque output. While our simulated tests do not show the complete torque curve, they do point out that carburetor size has a definite effect upon the maximum torque output of an engine-even one that's destined for low-rpm, street-car use.
Bottom line? There's a good chance General Motors wasn't so dumb after all when it came to carburetor size. Just remember that carburetor tuning plays just as important a role as cfm when it comes to driveability.