The performance world is packed with thousands of ways to improve and reshape the power curve of your engine. Engine size, camshaft profile, compression, carburetion, and intake manifold design are only the beginning. The goals for better power are simple, yet the criteria complex. How fast do you want to go, what is your budget, and how will you be using your car? To engine builders these decisions ultimately define how well a street-performance engine will perform throughout the entire rpm range.
Selecting a carburetor that balances good performance and drivability for a street-driven car is a significant step. Don't make the mistake here of trying to copy what works well on your buddy's drag-race car. On the strip, a dedicated race car is set up to spend almost no time at low rpm (below 3,000). On the street, a typical performance car will spend most of its time operating from off-idle to 3,500 rpm, and on occasion past 6,500 rpm. Therefore, selecting a carburetor for a street-performance car is often more involved than choosing a carburetor for a dedicated race car. This is because the street engine must be built to operate under a range of performance conditions, varying altitudes, traffic, cold climates, and sometimes different drivers.
If the carburetor installed on a street engine is too large, the engine will generally be lazy until rpm exceeds 3,500 rpm. Contrary to popular belief, a larger carburetor allows too much air to enter the engine quickly without enough fuel, which creates a bog. Accordingly, the flexibility of engine operation over a broad rpm range will suffer. This is because as the comparative venturi size increases, air velocity decreases through the carburetor, forcing the carburetor's main system to postpone fuel flow until a higher rpm is reached. If the carburetor is too small, overall power may suffer because the engine can't draw in enough air and fuel to develop sufficient power.
For a closer examination of proper carburetor selection and what guidelines should be considered, we visited with the tech guys at Barry Grant, Demon Carburetion. The folks at Demon spend hundreds of hours a year taking phone calls and solving carburetion dilemmas. And while no two calls or applications are exactly alike, they have found many patterns and developed guidelines to help select and size your carburetor to match your application, maximizing the vehicle's performance.
In addition to exploring the selection process, we decided to look past the typical street-performance application and summon one killer street/strip combination to test on the dyno wheels at Westech. Granted, this combination is not the norm, but we wanted to see how hardcore street/strip stuff would test. The car we used for this belongs to our buddy Bobby Mahoney. His engine is a wicked 468-inch Rat that motivates his Marina Blue Chevelle. This street-driven ride runs race fuel, 12.8:1 compression, AFR reworked 315cc aluminum cylinder heads, a five-speed trans, and a huge solid-roller camshaft. Although Bobby's Chevelle makes regular road trips of more than 250 miles to car shows, it is not your typical street setup, and certainly not a car built for docile street manners.
Which Carb Is Right for Your Engine?
Don't be tempted to buy the biggest carburetor you can find. As with all other engine modifications, parts should be selected to match your engine and vehicle's performance level. When deciding which carburetor is best for your engine and driving style, realistically set your goals.
Typically, smaller carburetors will offer better performance than larger ones. If you're not sure which carburetor of similar cfm ratings and styles to choose, you might consider swapping carburetors with some of your friends to experiment.
As the majority of hot rodders understand, most street-style four-barrel carburetors open the primaries first, and as driver demand dictates (as the throttle is depressed) the secondaries open next. At cruising/freeway speeds, only the primaries are barely open and engine vacuum is high. The opening of the secondaries occurs either by air speed through the primary venturis (in conjunction with the primary throttle angle) or by mechanical action when the throttle angle is opened significantly, to or approaching wide-open.
For most street applications, good low-speed power with excellent midrange and improved top speed power is achieved with a carburetor system that provides a minimum pressure drop in manifold vacuum at WOT. For most applications, a four-barrel carburetor with vacuum-operated secondaries supplies added airflow capacity only as the engine needs it. Therefore, the vacuum operated secondary feature makes it very difficult to overcarburete an engine. Good candidates for vacuum secondary carburetors are performance vehicles with moderately modified engines (camshaft duration less than 240 degrees at 0.050-inch lift), dual-plane manifolds, and automatic transmissions with less than 3,000 stall speed.
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.
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!