Have you ever wondered exactly what those articulating chunks of metal atop your intake manifold actually do? As a Gen Xer who was weaned on GM EFI systems, for years the inner workings of a carburetor were as mysterious to me as an auto trans or the opposite sex. Many years on, women still stump me—but thankfully, the ingenious inner workings of a carburetor are no longer a mystery.
For this installment of our running tech/buyer’s guide, we’ll be covering some basic carburetor tech info, then diving into a heap of today’s best Corvette-compatible aftermarket carbs. This is by no means a deep dive into carb complexities—that techy goodness can be found here, here, and here. Instead, it’s a basic introduction to carbs, followed by beauty shots and info about the hottest carbs for your Corvette. Let’s floor it.
Carburetors were invented in the 1800s, and George and Earl Holley began screwing together carbs in 1904. Years before four-barrels came along, hot rodders bolted up Stromberg 97s and Holley 94s to their flatheads. And when the revolutionary and efficient four-barrel debuted in the early 1950s, it only took the explosion of pushrod V-8s to create a match made in car-guy heaven.
Four-barrel builders started cranking out carbs: Will Carter fourbarrels (WCFB, now Edelbrock) came out in the early ’50s, Rochester built 4-Jets in the mid-’50s and Quadrajets in the mid-’60s, and Holley introduced the world-famous 4150 model in 1957 and the hardcore 4500 Dominator in the late ’60s. As the power wars raged, carbs increased in size, performance, quality, and efficiency.
In the 1970s, OEM carbs had to deal with new emissions laws, then merged with computer controls in their mid-’80s swan song. But even though carbs haven’t accompanied a new car down the production line since the late ’80s, today’s aftermarket options continue to push the performance, drivability, and economy envelope. And even the most hardcore racers can now simply plop down the dough for a damn good out-of-the-box piece, or pay a little more for specialty carburetors tuned by dedicated experts. The cars change, the market changes, the goals change. But performance carburetors are still here, and better than ever.
Before we jump into an abridged version of how a carb works, let’s quickly discuss some of its components. Carb designs vary, but you can typically see a carb’s body, fuel bowls and sandwiched-in metering blocks, fuel inlet fittings, linkages, vacuum/mechanical secondaries, vacuum ports, adjustment screws, and manual or electric choke assemblies. Looking into one from the top can reveal the air horn, circular venturi inlets and boosters, air bleeds, choke plate, and the straw-like fuel bowl vents. And flipping one over can show the base plate and throttle plates.
While it’s easy to imagine a steady stream of liquid fuel pouring into your intake, garden-hose like, the reality is much different. In truth, carburetors are complex devices that do a complex job: directing fuel and air through a maze of passages, to turn liquid fuel into atomized droplets. Why? Well, liquid fuel doesn’t burn, so fuel must be atomized and vaporized before it combusts—completely vaporized fuel leads to better combustion, bigger power, and lower emissions.
Low-Pressure Fuel “Injection”
Here’s a big-picture look at how that happens: first, the vehicle’s fuel tank, lines, and fuel pump supply fuel to the carburetor’s float bowls. Float bowls are reservoirs that use a float, an arm, and a needle-and-seat assembly to regulate the float height—and the fuel level. When the float goes up, the needle and seat prevent more fuel from pouring in. When the float goes down, the needle and seat allow more fuel in. In doing so, the carb gets a steady diet of petrol. But unlike with fuel injectors, fuel in a carb’d engine isn’t forced in; it’s pulled into the engine by a pressure differential, caused by a phenomenon called the venturi effect. This scientific wizardry involves fluid pressure, flow restriction, and Bernoulli—the scientist, not some Mustang driver from Englishtown.
Fuel travels from the bowls through emulsion tubes—small cylinders with tiny holes. Combined with air from air bleeds, these tubes pre-atomize the fuel before it gets to the venturis. Now, let’s talk venturi effect. A venturi uses physics to push fuel through the carb and into the intake manifold. How? It’s shaped like an hourglass: wide on both ends, and necked down in the middle. When fuel hits the first wide end, it has low speed and high pressure. But as it passes through the narrow middle section, the fuel’s speed increases but its pressure decreases. And it’s that pressure difference—about 1 psi lower than the normal, 14.7-psi atmospheric pressure seen at the fuel bowls—that pushes fuel into your engine.
A Circuitous Route
Carburetors contain numerous air and fuel circuits, which mirror the engine’s varying rpm and load situations. Here’s what they do:
The idle circuit provides fuel when the engine is at idle.
The primary circuit provides fuel for the primary side of the carb during light-throttle operation.
The secondary circuit provides additional fuel when the secondaries open during hard acceleration.
The accelerator pump circuit speeds up fuel flow by adding gas during transition from light to heavy throttle—like when the secondaries open and the engine goes lean from the extra airflow. FYI, a carb’s accelerator pump circuit is the only one not affected by airflow.
The fuel enrichment circuit (aka the power valve) is normally located on the primary side, only opens during WOT, and can add up to 10 more jet sizes of fuel on demand. And because it’s held closed by manifold vacuum under non-WOT conditions, smaller primary jets can be used for crisp part-throttle operation, with the power valve providing enough fuel for full-throttle performance, too.
And there’s a bunch of idle-related circuits that are both air-only and air/fuel mixtures. Those include idle air bleeds, idle transfer air bleeds, and idle discharge holes/slots.
A carburetor’s jets are simply restrictors in varying sizes that determine how much fuel passes through—and what an engine’s air/fuel ratio is. Jets are numbered based on what they flow; jet sizes can sometimes correspond to the jet’s diameter, and sometimes not. But the bigger the jet number, the greater the fuel flow so larger jets richen the mixture, and smaller jets lean it out.
Edelbrock/Carter AFB carbs also use metering rods—Allen-key-shaped devices with stepped ends that extend into the carb’s main jets. Similar to power valves, metering rods are available in various sizes, are actuated by specially tuned springs, and restrict or increase fuel flow through the jets depending on engine load.
Different carburetors are available in different flange types: there’s the Holley 4150-style square-bore, or the QuadraJet spread-bore. Be sure that whatever carb you buy will mate with whatever intake manifold that you have.
Vacuum vs. Mechanical Secondaries
Carburetors use two methods to open up a four-barrel’s secondaries: Vacuum-secondary carbs use engine vacuum in the primary venturis to open the secondaries. As the rpm increases the vacuum increases, and the secondaries open gradually. This type of carb is more street-friendly and gets better gas mileage—though heavy vehicles with tall gears can benefit as well.
Conversely, mechanical-secondary carbs are “hard wired” in; that is, directly actuated by your right foot. They typically open right around half-throttle, hit very hard compared to a vacuum-actuated carb, and trade drivability for a more high-performance feel.
While it’s easy to say “vacuum setups are for street and mechanicals are for race,” the truth is that today’s highly refined carburetors can be set up to work fine in both worlds. You gotta love progress.
Manual Vs. Electric Choke
Your choke options are either manual, which are controlled by a cable, or electric, which are controlled automatically. Manual is more work, what with the pedal pumping and rod-choking and cold-weather frustration and what not, but race-intended mills can use ‘em (or not if you like). And electric is hands-off and easy, and great for street-driven rides. Combined with vacuum secondaries, electric choke makes for a killer daily-driven combo.
Tuning, Tweaking, and Buying Tips
“Leave It Alone” And Other Useful Carb Wisdom
The gearhead mentality means not leaving well enough alone. Seventy years ago, being a hot rodder meant building a performance part literally from scratch, so fiddling with stuff came in handy. But in today’s climate of insane R&D time and precise quality control, that mentality can come back and bite us.
Case in point: buying a new carburetor. Here’s where we mess up:
Don’t overestimate how much carb is needed. Instead of sifting through 100 suggestions from well-meaning gearheads, check out our “Carb Sizing” section below—preferably before you throw a Dominator on your stock Vette.
Don’t think that an out-of-the-box carb running rich is a problem. This is not a design flaw that requires instant jet changes, it’s a wise move from the aftermarket that keeps your engine from running lean.
Don’t assume that your new carb isn’t tuned properly. If you’re not building a dedicated race engine and you’re installing a brand-new, properly sized carburetor, just leave it alone. Aftermarket companies like Edelbrock and Holley put a lot of engineering into these suckers, and most of the time they’re dead-on right out of the box. It’s when you start changing things that the tune suffers.
And if you must start to change things, always increase jet sizes first. Decreasing jet sizes is a good way to lean out and possibly damage that pricey engine of yours. So if you’re gonna fiddle with the jets, go up one size first—once you know it’s running rich and losing power, then you can drop down two jet sizes.
How To Size and Buy A Carb
Choosing a carburetor is a fun yet confusing parts purchase. Here’s how to do it right.
Step 1: Use a general carb sizing formula to get into the size ballpark. This formula looks like so:
(engine cubic inches) x (max engine rpm) / 3,456 carb cfm.
So if you have a 350 that sees a 6,000-rpm redline, simply input those numbers and calculate:
(350) x (6,000) = 2,100,000 / 3,456 = 607 cfm
Step 2: Adjust the above flow rating based on the specific engine that the carb will go on:
If your engine is stock, go one cfm rating lower.
If your engine has a larger cam or high-flowing heads, go one cfm rating higher.
Step 3: Choose your options. If it will be going onto a mild driver, consider electric choke and vacuum secondaries. If it’s destined for the track, look into a double-pumper with mechanical secondaries and larger float bowls. Consider the type of ignition advance needed, and the type of intake manifold design that you plan on using as this affects the flange type—4150-style square-bore or spread-bore Quadrajet—as well as height and vacuum line routing/obstructions. Finally, consider details like coated vs. polished, etc.
Step 4: Pull the trigger. If you’ve done your homework, your new street carb should reward you with a good idle and smooth transitional and WOT performance. And your new track carb should snap your neck back when the secondaries open! But of course, you can make tweaks as needed–see below.
Step 5: Tune it up.
If you’re up for spending the coin and going to an experienced dyno tuner, by all means do it. However, another, possibly cheaper way to get your tune on is to pick up a wideband air/fuel meter, like this one from FAST. It only takes a welded-in bung and wiring routing to install, and it’ll give you visual and logged air/fuel info about every move your engine makes.
After the A/F meter install, the first thing you should do is check that the fuel pressure and base/advance timing is correct. Next, take a drive with the meter recording, and be sure to idle, transition from low to high rpm through the gears and back down again, cruise, and make a wide-open throttle hit.
Then you can park it and start making changes. Start in the idle to 3000-rpm range—14.4-13.5:1 is okay for idle. Typically, a naturally aspirated engine cruising between 2300-3000 rpm likes a 14.3-13.9 air/fuel ratio, so first change your primary jets/rods, then set the idle mixtures, to hit that target. If the off-idle to 1800-rpm band is still off, non-Holleys may need the idle channel restriction enlarged to fix it. Next, go for a drive to ensure that your idle and off-idle performance is smooth, and your cruising air/fuel ratio is good. Finally, aim for 12.3-12.9 to 1 at wide-open throttle—depending on your carb type, it may require a combo of jets/rods, power valve, and accelerator pump changes to zero this in. And engines are all different, so you’ll need to track mph at the track or get dyno time to find your max power potential.
If you’d rather take a low-tech approach, you can tune idle/off-idle/cruising by sound and feel, then check your carb’s WOT vacuum readings with a gauge to determine your carb’s peak flow potential. Granted, unless you can borrow carbs from a buddy, you’ll have to do this post-purchase. But it’s still a good procedure that’ll teach you a lot about your new squirter. So, starting with the smallest carb option from the above formula, get a vacuum gauge and connect it to your engine’s intake manifold.
Seeing 1.5 inches of vacuum at WOT means you’ve nailed your carb sizing perfectly.
Seeing around 2.5 inches of vacuum shows restriction—and the need for a slightly larger carburetor.
Seeing 1 inch of vacuum or lower means the carb is too big for your engine, and it needs to be smaller.
And as always, you can check your spark plugs to see how a certain carb is running. Golden brown is perfect, while white (lean) and black (rich) means a change is needed.
Corvette Carburetors By Year
1953: 3 side-draft, 1-barrel Carter
1954: 3 side-draft, 1-barrel Carter
1955: 3 side-draft, 1-barrel Carter / 2x Carter WCFB 4-barrel
1956: 4-barrel / 2x 4-barrels
1957-61: 4-barrel / 2x 4-barrels / 2x 4-barrels (hi-po)
1962: 4-barrel / large 4-barrel / large 4-barrel (hi-po)
1963: 4-barrel / 4-barrel AFB / 4-barrel AFB (hi-po)
1964: 4-barrel Carter WCFB / 4-barrel Carter AFB / 4-barrel Carter AFB (hi-po)
1965: 4-barrel / 4-barrel HC large / 4-barrel Holley
1966: 4-barrel 300 hp / 4-barrel 350 hp / 4-barrel 390hp/425 hp
1967: 4-barrel / 4-barrel / 3x 2-barrels
1968: 4-barrel Rochester / 4-barrel Holley (350-hp/390-hp) / 4-barrel 390/400 hp / 3 2-barrel Holleys (435 hp) / 4-barrel Holley 850 (“430”-hp L88)
1969: 4-barrel Rochester (300-hp/350-hp/390-hp) / 3 2 barrel Holley (400-hp/435-hp) / 4-barrel Holley (“435”-hp L88) / 3 2 barrel Holley (“435”-hp L89)
1970: 4-barrel (300-hp/350-hp/350-hp (ZR1)/370-hp (LT1)/390-hp)
1971: 4-barrel (270-hp) / 4-barrel Holley (330-hp) / (365-hp/4-barrel Holley (425-hp)
1972: 4-barrel (200-hp/255-hp/270-hp)
1973: 4-barrel (190-hp/250-hp/275-hp)
1974: 4-barrel (195-hp/250-hp/270-hp)
1975: 4-barrel (165-hp/205-hp)
1976: 4-barrel (180-hp/210-hp)
1977: 4-barrel (180-hp/210-hp)
1978: 4-barrel (220-hp/185-hp/175-hp)
1979: 4-barrel (225-hp/195-hp)
1980: 4-barrel (230-hp/190-hp/180-hp)
1981: 4-barrel (190-hp)
Carburetor Buyer’s Guide
Now that you know what to look for, take a look at some of today’s best street, street/strip, and race carburetors. FYI, we’re more about performance here—but if perfectly restored is your passion, “factory” reproduction carburetors can be had at some Corvette vendors. That said, all of the below carbs are four-barrel models unless otherwise noted, and prices are either from the manufacturer or from an online store.
Demon Carburetors (Holley): www.holley.com
Speed Demon 750 cfm
Speed Demon carbs are a potent mix of street manners and race technology, and are available in vacuum secondaries/electric choke, or mechanical secondaries/twin–squirter versions. Features include dual feed fuel inlets for constant high volume fuel delivery, large-capacity float bowls to eliminate fuel starvation, billet metering blocks for precise metering capacity, four corner idle and a billet baseplate with Idle-Eze for precise idle control, a quick-change vacuum secondary cover and non-stick gaskets for easy tuning, and large glass float-level sight windows for easy and safe fuel level adjustments.
Those goodies add up to balanced, high-velocity airflow, and exceptional fuel metering capacity for most street engines. In fact, these carbs are calibrated for 393- to 477-cid engines, including mild 427- to 454-inch Corvette big-blocks. Part number 1402010VE shown.
Notes: Speed Demons available in sizes from 575 to 850 cfm. Designed for engines with cam profiles between 220º to 240º duration at 0.050.
Road Demon 625 cfm
The Road Demon is calibrated for mildly modified street machines, and offers a remarkably smooth choke tower design, vacuum secondaries, and patented air entries for smooth operation and strong throttle response. Along with the features previously mentioned on the 750-cfm Speed Demon, the Road Demon also offers Electric Choke Management for easy cold starts. Perfect for mild performance engines from 302 to 400 cubes, and 250 to 350 horsepower. Part number 282010VE shown.
Notes: Road Demons available in 525-cfm and 725-cfm sizes.
Edelbrock Carburetors: www.edelbrock.com
Edelbrock Thunder Series AVS 500 cfm
Designed and calibrated for small cubic-inch engines, the 500-cfm AVS is perfect for 265- and 283-cube Chevy engines. It includes both timed and full vacuum ports for ignition advance, and is factory equipped with .086 primary and .095 secondary metering jets, .065x.052 primary rod, and a 5-inch Hg orange spring. Also available in dual-quad calibrations, with .086 primary and .077 secondary metering jets, .065x.057 (or .052 for #1804) metering rods, and a 5-inch Hg orange spring.
Available in electric-choke Satin (1801) and EnduraShine (18014, shown), manual-choke Satin (1802) and EnduraShine (18024). Dual-quad calibrations available in electric-choke Satin (1803) and EnduraShine (18034), and manual-choke Satin (1804) and EnduraShine (18044).
Notes: Non-EGR. Use Carb Studs #8008 or #8024 if needed.
Edelbrock Thunder Series AVS 650 cfm
This square-flange carb is designed and calibrated for optimum street performance in small-block and some big-block engines. Match with a variety of manifolds that include Performer, Performer EPS, Performer RPM, RPM Air-Gap, Torker II, or other brands of similar design. Includes .095 primary and .098 secondary metering jets, .068x.047 metering rods, and a 5-inch Hg orange spring.
Available in electric-choke Satin (1806, shown) and EnduraShine (18064), and manual-choke Satin (1805) and EnduraShine (18054).
Notes: Non-EGR. Manual-choke carbs can’t be converted to electric choke. Use Carb Studs #8008 or #8024 if needed.
Edelbrock Performer Series 750 cfm
This electric-choke, 750-cfm Performer is designed for 402 cubes and up, and provides enough juice to quench the biggest Vette big-block. And yet, its slightly smaller primary jet and slightly larger metering rods make the calibration 2 percent leaner, for decent mileage as well. Includes both timed and full vacuum ports for advance, as well as .110 primary and .107 secondary metering jets, .075x.047 metering rods, and a 5-inch Hg orange spring.
Available in electric-choke Satin (1411, shown); a manual-choke 750-cfm with a performance tune is available in Satin (1407) or Black (14073).
Notes: Non-EGR. Use Carb Studs #8008 or #8024 if needed.
Edelbrock Performer Series 800 cfm
Designed and calibrated for maximum street performance, this 800-cfm EPS carb features a high-capacity accelerator pump and an improved primary and secondary cluster design. Plus, the EPS 800 has the same out-of-the-box performance and two-piece design as the rest of the Performer Series line. Includes .113 primary and .101 secondary metering jets, .071x.047 metering rods, and a 5-inch Hg orange spring.
Available in electric-choke (1413, shown) and manual-choke (1412).
Notes: Non-EGR. Use Carb Studs #8008 or #8024 if needed.
Edelbrock Vintage 94s & Intake Manifold Kit
If you want something different for your 283 to 327 small-block Chevy, check out this vintage 94 six-pack and manifold combo. Vintage 94 two-barrel carbs use a die-cast bowl/air horn and an aluminum, three-bolt base. Thanks to extensive dyno and road testing, their power valve and jetting combination delivers a wide calibration band for smooth and strong performance in various applications.
They’re bolted to an X1 Ram Log intake manifold, and include Edelbrock’s exclusive progressive throttle linkage #1033, which was originally designed by Vic Edelbrock Sr. back in the late 1950s for small-block Chevy. All kits also include high-quality Edelbrock gaskets, carb studs, and all of the necessary hardware.
Available in six-pack/manifold kit (2018, shown), triple-carb kit with C-357-B intake manifold (2015), triple-carb kit for E-Tec or Vortec heads and C-357-B intake manifold (2016), or as individual carbs in primary (1151) or secondary (1152) configuration.
Notes: Use carb stud, nut, and washer kit #8006 if needed.
Holley Carburetors: www.holley.com
Holley Classic 600 cfm
This Classic series Holley 4160 has a new aluminum trick up its sleeve, which makes it up to 40 percent lighter compared to its zinc competitors. Its 600 cfm of airflow, vacuum secondaries, and single fuel inlet makes it amazingly universal. Add to that an electric choke, straight booster, and 1.563-inch throttle bore, and this puppy can be used effectively on dang near anything. Part number 0-80457SA shown.
Notes: applicable to certain models of 1963-’82 Corvettes
Holley Classic 650 cfm
This zinc 4150 is the perfect performance upgrade for stock to mildly modified vehicles that need the classic Holley look. Its dual feed fuel inlets provide double-pumper flow, and a 1.688-inch throttle bore, vacuum secondaries, and an electric choke round out the features. Finally, it’s coated in classic Gold Dichromate for a classic look. Part number 0-80783C shown.
Holley Classic 750 cfm
This classic 750 features dual fuel inlets for mega flow, along with a 1.688-inch throttle bore. Its vacuum secondaries can compensate for different vehicle weight, transmissions, and gears, and a preset electric choke results in easy startups. All in all, it equals one of the most widely used Holley carbs of all time. Vibratory polished (heh). Part number 0-80508S shown.
Holley Tri-Power 350-cfm 2-barrels
Holley’s Tri-Power system is legendary in Chevrolet circles. They were the ultimate power statement when factory-mounted on cars like the 1967 427 Corvette—and you can still buy ’em brand new. This model 2300 is the non-choke “outer” version used in the front and back; naturally, they’re finished in Gold Dichromate for a stunning look. Part number 0-80684 shown.
Notes: Holley also offers the 325-cfm center carb with electric choke to complete your Tri-Power setup. Part number 0-80683 is $375.95.
Price: $339.95 each (outer version)
Holley Tri-Power Kit
Nothing says “cool” like a tri-power carb setup, and Holley’s Tri-Power Kit turns every green light into a battle for self-control, every car show gawker into a Holley acolyte. This system combines the previously mentioned inner and outer two-barrels, with a medium rise, dual-plane Weiand 3x2 intake manifold. The result is economical cruising with the 325-cfm, electric-choke center carb, and ungodly acceleration when the two 350-cfm outers kick in.
This system includes the Weiand intake, three two-barrels, a progressive throttle linkage, high-quality steel fuel lines, and reusable air filters with polished housings. Part Number 300-522 shown.
Notes: Front height from the front sealing surface to the top of the air cleaner nut is 11.5 inches; rear height from the front sealing surface to the top of the air cleaner nut is 12.5 inches. This kit is also available in a Dichromate finish (+$100).
Holley HP Supercharger 950 cfm
When its time to get blown, supercharger HP carburetors have you covered. They feature a contoured venturi inlet for balanced airflow, 1.375-inch venturis and 1.75-inch throttle bore, screw-in air bleeds for precision tuning capabilities, high-flow metering blocks, down-leg booster, and Dominator-style fuel bowls that allow plumbing from either side for ease of installation.
These 4150-based Supercharger HPs also have a unique capability that allows their power valves to reference the intake manifold vacuum below the supercharger. This allows the power valve to operate as it should, based off intake manifold vacuum. A power valve provides further enrichment to the main metering system under load (low vacuum) conditions. Part number 0-80577S shown.
Notes: No choke, mechanical secondaries. Supercharger HPs also available in 600- and 750-cfm sizes.
Holley Gen 3 Ultra Dominator 1,050 cfm
Holley’s Dominator is a true legend, and the all-aluminum Gen 3 Ultra Dominator builds upon that fearsome reputation. It features a patent-pending main body that’s 5/16-inch taller, which allows for larger radius air entries. The air entries are optimized using Computational Fluid Dynamics software to deliver smoother flow into new, fully machined, 1.69-inch venturi (and 2-inch throttle bores). The main body also features an integrated idle bypass valve that allows for idle adjustments, while maintaining the correct throttle plate-to-transfer slot relationship.
The Gen 3 utilizes high-capacity, starvation-eliminating fuel bowls with 20 percent more fuel capacity, an integrated fuel shelf to minimize fuel aeration, and internal baffling to control fuel slosh. Other features include 12-hole billet booster inserts, fully tunable billet metering blocks, adjustable external linkage, throttle position sensor mounting points, and knurled (hand adjustable) curb idle screws. And to top it off, each calibration has been dyno and track tested to deliver proven performance! Part Number 0-80903HB shown.
Notes: Sizes up to 1,475 cfm
Pro Systems Carburetors: www.prosystemsracing.com
Pro Systems 4150 Series
Pro Systems carburetors are built in the USA and to customer supplied specifications. Each unit is handmade and serial numbered. Each design offers incredible throttle response, improved response to tuning changes, clean idle and driving characteristics, as well as improved power output over corporate/generic style carburetor designs.
Pro Systems 4150 series carburetors feature Pro Systems-designed CNC Billet VENOM II metering blocks, deep-notch drag float to eliminate fuel starvation on launch, adjustable air bleeds (complete with wet-flow jetting portfolio), high-flow brass screw-in jet extensions, glass window sight bowls, high-flow, low foaming needle and seats, and “unbreakable” billet baseplates. And, they’re electronically wet-flow calibrated and live engine tested!
Price: Starting at $640.00
What to look for in a race carb
Track thrashing a street car is fun, but rarely results in optimal e.t.’s. The same goes for using a street carburetor at the track: a carb set up for cold starts, drivability, and good mileage is fine for spirited driving, but hardly perfect for racing. We wanted to know more about the differences between street and racing carbs, so we contacted Patrick James of Pro Systems. This carb company counts some of the most hardcore racers as clients, and sells modified units from the other guys as well as some revolutionary in-house designs. In short? They know their way around a carburetor.
Vette: Thanks for speaking with us Patrick. So, why are off-the-shelf street carburetors inferior for drag racing?
Patrick James: This biggest thing that you will find in a street carb versus a drag race carb is that the drag carb is flowed with the blades in a wide open throttle position. It is at that point that the fuel curve is calibrated to as close to perfect as possible, throughout the intended rpm range of the design. Now, the design is operated at part throttle and changes are made to make it drivable, BUT only changes that will not affect the fuel curve at WOT. That’s why some of the best carburetors in drag racing have some of the worst part throttle operation.
However, many designs for all-out drag racing are actually quite clean in operation, but there are exceptions to every rule. In a street and road course racing carb, priority is given to drivability: the fuel curve is built and laid out, and then part throttle operation is dialed in and compromises are made to maintain streetability versus WOT operation. That’s why we designed our Venom II billet metering blocks—we build them in house on our own CNC machines to eliminate the compromises needed when we are forced to employ other manufacturers’ metering block designs. The improved layout of the VENOM II block we build offers us the capability to get great drivability, and still achieve maximum horsepower output. Plus, since they are CNCd right here in our own shop, they come out of the machine with your calibration already in them ... we just simply dial them in after that and off your carburetor goes, ready to be bolted onto your vehicle.
Vette: Do street functions like electric choke and vacuum secondaries affect a carb’s power potential or response time? If so, why?
P. J.: Yes, the choke blade is a restrictor plate on the combination that hampers airflow and even alters the fuel curve for that half of the carburetor. They make things pretty messy, and it’s difficult to correct for them as the air does odd things as it travels around the choke blade at various engine rpms. As a result, we don’t build carburetors that require chokes. Most engine combinations above 400 hp, and at or above 10:1 compression, will fire up on a 50-degree day with just a few pumps of the throttle. When it fires up, you hold the engine at about 2,000 rpm to get some heat in the oil for 90 seconds, and then you should be able to drive off.
As far as vacuum secondaries go, we only build them for race series that require them. If you have properly selected your converter stall rpm and/or know how to drive a manual transmission in a performance vehicle, if the carb is properly sized, there is no sense in taking off at half throttle—which is essentially what a vacuum-secondary carb is doing to your engine off the line.
Vette: In your opinion, which aftermarket carb brands and models are the best designs to modify for dragstrip performance?
P. J.: Anything that Holley builds is good stuff to work on. They are a reputable company, and the components they use are easily modified to get great results to maximize a program.
Vette: When Pro Systems modifies an off-the-shelf carb, what is done to turn it into a killer drag-race carb?
P. J.: Our biggest tool is our wet flow bench—there aren’t too many of them out there. But it’s the only way to verify that the work you did to the customer’s carb was actually done properly. We simply bolt the carb down on the wet-bench, draw air through the carburetor in the cfm range of the engine’s needs, and use a fuel simulant that allows us to not only measure the fuel curve that we put in the carburetor, but actually allows us to measure how well the fuel atomized by measuring what percentage fell out of suspension before it reached the return tank. It’s a great system, and one that I guard with my life. About 20 years ago when I first came out with the design, I was doing the Pro Stock Program for the Jegs racing team. I had to give their people a personal tour of the system in order to obtain the contract for that project, and since then only one other person from Dodge’s Pro Stock Program required that they were given the tour. But now the tours are over, and I keep the system in a locked room in the shop and that part of the shop is off-limits to shop tours. We’ve proven it works!
Vette: How do different engine mods like nitrous affect custom carb design and tuning?
P.J.: If you watch the show Street Outlaws you’ve seen Monza’s car. When he won the whole deal a couple of years ago, he was running our carburetor. When I built that design for him, many would’ve deemed it too small for the application. But he had a big cam, a very tight nitrous converter, and lots of nitrous. These combinations struggle to get up on the converter when you hit the throttle. Then when you dump all that nitrous in there, the air speed from the engine is so low—and made even lower with the introduction of nitrous—that the carb tends to “fall off” the booster. Meaning, its air speed has gotten low enough that the carb can no longer draw fuel from the bowl area and with enough venturi speed to atomize it into a proper fog. As a result, the engine can’t get up on the converter, or worse yet, when you hit the nitrous the intake manifold is suddenly full of lean air-to-fuel mixture as a result of the carb falling off the booster, or as a result of the fuel not being able to be properly atomized, the engine blows flames out the carb or blows the hood into the air. Monza was successful when he was using our carburetor design, and that’s a direct result of properly designing and sizing the carburetor for the application.
Vette: At what power levels, and for which racing types and classes, do you recommend that people invest in a custom racing carb?
P. J.: We design carburetors for anything above 400 horsepower. At levels below that, those engines just aren’t finicky enough to notice or take advantage of the mods we do, so most out-of-the-box carburetors do the job just fine.
Vette: How did the idea for Pro Systems’ innovative SV1 series racing carb come up?
P. J.: We were sitting in a restaurant one day, and a guy said, “If you were going to design a carb with a clean sheet of paper, how would you do it?” I told him, “Well, you don’t move oil through a pipeline with four small pipes, you use one big one—it offers less resistance, and in an engine that means more potential horsepower.” And then I grabbed a napkin at the table and started drawing out the concept!
Vette: That’s wild. So what were your goals when the SV1 was being developed?
P. J.: It needed to employ as much booster surface area as possible to atomize the fuel better than current designs, and it needed to be able to respond to throttle movement faster than the current booster layout of four-barrel carbs. The SV1 senses any changes in air speed the moment the blade starts to open, as the booster has openings in it at the very edges of the venturi and they travel to the middle. Current four-barrel carburetors only sense the changes in air speed that go through the central area of the venturi. As a result, the SV1 jumps up onto the converter faster, and because it atomizes fuel better the car launches harder off the line.
Racers that can take advantage of these features have gathered lots of records and championships for the design. In only its first year, our SV1 gathered over a dozen serious records and championships. And now that it’s been out there for five years now, I have no idea how high that number has become.
Vette: What are the main physical differences between a normal aftermarket carb and an SV1 carb? And how do those differences help performance?
P. J.: The single venturi is the biggest single difference visually, and it allows the cfm to be easily changed. For example, by simply choosing our Kamm, Centerline, or Power boosters. When they are employed, the same carburetor that saw 1,120 cfm can flow as much as 1,380 cfm with only a booster change. And it only takes five minutes to change while it’s on the vehicle!
Vette: Good stuff. Last question: what’s the best way for a racing novice to learn more about race-prepped or dedicated racing carbs? What should they look for in a race-prepped carb? And how do they get started?
P. J.: The real stuff goes on in the inside of the carb, there are lots of special tapers we use to modify main wells, as well as reversion steps to assist in shift recovery and even relocation of emulsion jets and booster locations. Some of this stuff is tough to see, but even a small .005 emulsion change in just the right spot can add 10 pounds of torque on a 400-horse engine.
As far as getting started, the best education you can get about building racing stuff is to go racing. You will learn more in a year of actual racing than you will in five years of talking to people about racing. Get out there and live it, it’s great!
Vette: Thanks a bunch for your time, Patrick.
Special thanks to Patrick James and Pro Systems for their help with this article. You can learn more about Pro Systems and their killer race carbs at www.prosystemsracing.com, or by calling 727-490-5717.