We've all seen it happen (or worse yet, it might have happened to us). A fellow Saturday-night racer shows up at the track with a new four-barrel carburetor and-over a friendly soda in the pits-promises to leave the field in his dust. From the $500 setup he's bought, he should get an extra 20-40 hp over a lesser carb. But during the race, he repeatedly gets passed coming out of the turns. His carb runs great along the backstretch but bogs down when the rpm falls off in the corner. He finishes in his usual middle-of-the-pack position and wonders, "How the heck could I spend all that money and run no faster than before?"
The solution to this racer's dilemma is one that we're sure could help a lot of you weekend warriors. The short answer is that no box-stocker carb is made for his specific application, and if he really wanted to go faster, he should have turned to a custom carb tuner. For the $500 he spent on a big four-barrel monster carb, he could have spent the same money on a custom-tuned two-barrel and gotten better results.
Now we know what many of you experienced wrenches are saying: "Carb tuners? Why the heck do I need them? I know how to change jets and turn an idle screw." Well, hold on a minute. Combined, the three companies we spoke to for this article-Competition Carburetion, Chuck Nuytten Carburetor Specialty Service, and JET Performance Products-have more than 70 years of professional experience building and tuning carbs.
Sure, you could try to perform the same sort of carburetor service these companies do in your garage. All you would need is a flow bench, a special booster install tool, an engine dyno, several special mills, a test engine, a lathe, numerous gauges, and more. Even if you had all these tools, it still wouldn't be enough. "The experience alone to understand what the flow bench is telling you simply is not attainable by do-it-yourselfers," Chuck Nuytten says.
Sean Murphy from JET agrees it could get pretty tricky for the guy at home to do his own carb building. "For one thing, you have to have a basic understanding of how the carburetor functions," he says. "Then you have to know what areas of the carburetor affect what areas of the fuel curve-and how to change those to get the carburetor to perform properly for the different specs of that particular engine."
Sound confusing yet? That's what we thought, so that's why we turned to these carb builders for insight and explanation on how a custom-tuned carburetor can help produce better results on the track.
Stock vs. ModifiedSo why is a custom modified carb better than a mass-production, box-stock unit, even one that is designed for racing applications? The answer is quite obvious: A factory carb is built to mate with its factory specs. Once you begin to change your car for a race setup by adding a different gear ratio, upgrading the tranny, or any other modification, that carb may no longer match the rest of the setup. To get the car running at its maximum performance level, all the aftermarket parts have to be tuned together, including the carburetor. As Nuytten put it, "Production-line carburetors are just that-a very good starting point to customize and modify for a particular use."
OK, so an aftermarket carb is needed to help win races-we already knew that. Holley, Edelbrock, Demon, and Rochester build some remarkable racing carburetors, and many are designed for specific engines and particular racing applications. However, there are many critical factors that contribute to peak carburetor performance that these manufacturers can't take into consideration when building mass-produced units. Bob Oliver of Competition Carburetion explains, "A production carburetor, even a racing carburetor, has to be built to be a general carburetor. Basically, the customer should be able to pull it out of the box, put it on, and not drive it into the ground. They have to build them that way-there's just no way they can put those things on the edge.
"A prime example: I live at 4,200 feet up here, and if I buy an Edelbrock carburetor off the shelf and put it in my car, right out of the box, it's dead rich and will run terribly. And people might ask, 'Why doesn't Edelbrock build one for this elevation?' Well, they can't, because number one, they'd be building too many different units. But more importantly, if a carburetor manufactured for Denver accidentally got shipped to Los Angeles, and the guy burns four pistons, then boom! Who gets in trouble? Edelbrock. They just can't do it."
Want further evidence of why major carb manufacturers could never tailor a unit perfectly to your race car? Here's just a small list of the elements custom carb tuners take into consideration before building your unit: weight of car, type of transmission, engine cubic inches, camshaft design, compression ratio, cylinder head specs, exhaust header, intake manifold, rpm range, gear ratio, track conditions, track length and style, track altitude, average humidity, air density, and much more. If you think manufacturers can factor in that many variables, think again. As Murphy states, "When a carburetor comes out of the box, it doesn't know what size motor it's going on, it doesn't know what type of rpm range it's going to running at, it doesn't know what size camshaft the engine uses, what design the intake manifold uses, and that type of thing."
Track Conditions and DriveabilityOf all the variables that affect carburetion, perhaps none is more important to the Saturday-night racer than track conditions. These are also the most difficult for major manufacturers to account for, and the reason custom carb tuners are your best bet for getting the most out of your race car's fuel system.
Track conditions are important because they dictate at what rpm ranges your race car can operate, and carb tuners take that into consideration. "If a racer knows that he only runs between 5,000 and 7,500 rpm, we can set up the carburetor so that it operates more efficiently in that range," says Murphy. "For a guy that says, 'I have to go from 3,000 out of the corner to 7,000 at the end of the straight,' that's more of a limiting factor, but we'll take that into consideration as well. If you can't come off the corner quickly, it doesn't matter how fast you are at the end of the straight-you're still chasing."
On a large banked track, however, a racer could conceivably run at about 6,000 rpm all the way around. In that scenario, a custom carb tuner will build the fuel curve to peak at 6,000 rpm.Unfortunately, most of us don't race on a WOT (wide-open throttle) track. Instead, we swap paint on tight-cornered ovals, tiny bullrings, and other tracks that require peak carburetor performance in a sweeping rpm range. Therefore, the driveability of a carburetor, or its ability to get you in and out of different conditions efficiently, is of utmost importance. Again, it's a tough aspect for major carb manufacturers to perfect.
"There's nothing worse than when you hit the throttle off the corner, and two guys get by you because it doesn't go-you may never get those two guys back. So driveability is as important as horsepower, torque, and all that sort of stuff," Oliver says. "Also important is the way the carburetor and motor build torque and horsepower. You want it to be a smooth curve; you don't want this big, peaky, straight up and straight down curve. That's very hard to drive."
Murphy says, "A carb that can keep a linear flow throughout the rpm range will atomize the fuel more efficiently, so it burns cleaner."
All the companies we spoke with mentioned driveability as a major concern. Nuytten agrees, "A good 350ci motor with a 750-cfm carburetor can see gains of 20hp, 30hp, and even more with a properly modified carburetor. It will also have driveability and quicker lap times." Aha, driveability, a five-syllable word for quicker lap times. That's all they had to say to convince us.
Attention to DetailPassing a tech inspection is always a major concern for any racer. We were wondering what tech inspectors look for when checking a carburetor, and we found yet another area where a custom-tuned carb far surpasses its box-stock original.
First of all, without ripping your carburetor apart and flow testing each orifice, tech inspectors can only measure the main components, such as venturi size, throttle-bore diameter, throttle shaft diameter, and booster (fuel-discharge nozzle) size to see if you're within the specs.
"The inspectors know that if everything measures out, you're equal to the next guy. That's basically what they're trying to accomplish, to keep everything as equal as possible," Murphy says.
As far as airflow goes, those components are the way by which custom carb tuners building class legal units are restricted. But there are other tunable components that can help your carb become a track winner.
"A lot of times, there are things we can do to the metering blocks and metering circuits that will make the carburetor run a lot better," Murphy says. "And that's where you can get horsepower gains from classes that don't allow you to tamper with the airflow a lot."
In that respect, we discovered what custom carb tuners are really all about. They pay careful attention to every detail of the carburetor, going over every millimeter with a fine-toothed comb. As Oliver states, "There is a difference between assembled and built-in everything." While major manufacturers can assemble a race carb, it's altogether different to build a custom-tuned carb.
"We machine all the body surfaces to make sure they're all perfectly flat, which prevents leakage," Murphy says.
All the custom builders we spoke to machine, true, mill, port, and grind surfaces, then measure and check each venturi to ensure proper flow. That's important, because most of the materials in the carburetor are cast, which means they are bound to have slight imperfections. And when we're talking about fractions of seconds in throttle response making the difference between winning and just banging fenders in a crowd, every imperfection counts. It's this attention to detail that really distinguishes custom-carb builders.
"On all my carburetors that get flow checked, each individual venturi gets checked. It's not just pull, 750-cfm, and ship it," Oliver says. "I'll actually, with my flow bench, draw air through each individual, and get them to balance side to side. This is because balance, meaning the draw on the jet, is as important as cfm."
Oliver continues, "You can have one hole that flows more cfm than the other hole-for example, side by side. But if one has a much weaker signal than the other, you better get that signal right-even if you have to give up a little cfm. You want that signal; it's really important, because that's what drawing on the booster, and that's what's atomizing the fuel."
But even though they are carb builders and not just assemblers, custom tuners pay special attention when putting a carb together as well. For instance, believe it or not, installing a butterfly is critical.
Oliver explains his approach to butterfly assembly, "I get those things to where they are dead on the money. Sometimes I even have to go in the primary shaft because they're opening too far right out of the box. The manufacturers actually have the positions incorrect, and they go past center. I have to weld that thing up and grind it so that it goes vertical again, and then get the secondary to match."
