Everyone knows smaller carburetors are the better choice when attempting to generate the most efficient power figures in your Corvette. Or are they? Is this the right plan for a modified Corvette? Poll some experienced, knowledgeable engine builders and you might find it isn't. As an example, a Winston Cup car with a restricted carburetor (for the high banks of Daytona and other Super Speedways) will always be down on torque. To offset the small carburetor, the engines are required to spin higher to generate more torque. In years gone by, it was also common practice to build super-high compression ratios to offset the loss in torque, but this is now frowned upon by the NASCAR rulebook. It doesn't take a rocket scientist to figure out the relationship between higher engine speed and higher engine carnage and link that principle to your Corvette street or race engine. On another front, a number of accomplished race-engine builders have done considerable research on the topic, and the common consensus is: As the carburetor cfm increases, so does the maximum torque output of the engine.
Carburetors come in all sorts...
Carburetors come in all sorts of different shapes, configurations, and, of course, sizes. Typically, Detroit has used carburetors with cfm ratings much higher than what might be considered correct. For example, Chevrolet used carbs as large as 800 (780) cfm on engines such as the Z/28 302. In other cases, GM regularly used large-cfm Quadrajets on any number of engine combinations, large and small. To be fair, the QJ incorporates small primary bores; but, nonetheless, it was still a "large" carburetor.
There's more here, too: Think back to pre-fuel-injection times. General Motors used Quadrajet carburetors for decades on its engines (Corvettes and otherwise). These stock, production QJs typically had flow ratings well in excess of 700 cfm. How many of these Quadrajets were junked in favor of the latest 600-cfm or smaller, flavor-of-the-week carburetor? Plenty, we'd bet.
Of course, there are formulas to determine the correct carburetor size for a given application. The following is a common formula we've relied on for years.
| Carburetor Size In CFM |
| | Carb. size | = | (cubic-inch displacement x maximum RPM) x VE |
| 3456 |
Note: VE = Volumetric Efficiency
The above formula calls for several different numbers to be plugged in before completion. The displacement of 355 ci is relatively easy to determine in the case of a hypothetical engine we'll soon build, but what about the maximum rpm and volumetric efficiency (VE)? Let's assume the Corvette will redline (maximum engine rpm advised) at 5,800, a relatively safe figure for a mild powerplant. The VE figure is much more difficult. Typical race engines will feature a VE of anywhere between 95 and 110 percent, but since this hypothetical street-driven Corvette is no ultimate racecar combination (and for the purposes of this article, we don't have access to a dynamometer to determine VE), we'll have to make an assumption. Assume this small-block is efficient as a street piece, but also keep in mind this is simply a multipurpose car that can be driven to work during the week. A conservative VE figure of 80 percent would work well in our application. Continuing with the formula, this is how it works with the numbers in place
| Carburetor size | = | (350 x 5800) x .80 |
| | 3456 | | | | | | |
| = | ( 2030000 ) x .80 |
| | 3456 | | | | | |
| = | 587.3842592593 x .80 |
| = | 469.9074074074 cfm |
While we didn't have instantaneous...
While we didn't have instantaneous access to a dyno, such a device is the perfect way to pick the right carb for an application. But here's the catch: Many of you are in the same boat as we. Dyno access isn't always available or affordable. So what do you do? Simple. Test carburetors on your engine in your Corvette. The bottom line is, the best carburetor is the one that produces the best e.t. and the best driveability.
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.
Test 1
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.
Test 2
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.
Test 3
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.
Test 4
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.
Another way to get close to...
Another way to get close to the right carb size from the beginning is to use a computer. Engine-simulation software is readily available from a number of sources, and it can be put to good use when selecting a carb size for your application. When using any simulation software, keep in mind it's easy to get lost, particularly when using optimistic numbers for things such as head flow, compression ratio, and so on.
Test 5
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.
Test 6
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.
Test 7
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.