This article first appeared in the June, 2016 edition of SportsCar Magazine. SCCA members can read the current and past editions of SportCar digitally here after logging into their account; To become an SCCA member and get SportsCar mailed to your home address monthly in addition to the digital editions, click here.

A little technology takes the guesswork out of tuning carburetors

Eking the most power from a carburetor-powered racecar has always been a challenge. It often seems only those truly talented in the black art of carb tuning can nail the setup – and even then, it requires the assistance of lady luck. Modern technology, however, is changing that, removing much of the carb fuel jetting guesswork that has baffled racers for years. The best part: these modern tricks can be applied to any tried and true carburetor that already sits atop your SCCA racecar’s motor.

Along those lines, it’s hard to imagine Holley’s four-barrel carburetor isn’t the most prevalent type of carb in the SCCA. Likewise, the double-pumper might be the most common model, considering Holley’s p/n 4776, 600cfm, four-barrel carb is the spec part number for full prep American Sedans. While there are many tricks to making the Holley 4776 run at its peak (and as American Sedan racer and 2009 National Champion Tom Sloe is quick to point out, the part number is spec in the GCR, but the carb is not a spec piece) tuning for optimum fuel delivery is key to racing at the front.

Sloe knows how to make a carb sing. Case in point, at the 2015 SCCA National Championship Runoffs presented by Garmin VIRB, going into the AS race, Sloe sat atop the grid with a blistering 2:01.746 lap time, clicking through the traps at 170.962mph – his Mustang powered, like all other full-prep AS cars, by the 4776 Holley double-pumper. While his race didn’t go as planned, Sloe’s qualifying lap time and top speed have yet to be bested. So, what’s his secret?

“Your first [air/fuel] reading is going to be from the dyno, which is a controlled environment,” says Sloe, pointing out that you need to have your carb dialed in by someone who really knows what they’re doing long before reaching the racetrack – that’s the baseline everything gets compared to. Before heading to the track, Sloe recommends installing a wideband oxygen sensor with data logger, and also purchasing an air density meter. “The technology is out there,” he says. “You don’t have to try to read the plugs anymore.”

Now you’re at the track and the race weekend is ramping up. “You’ve got your data running, so your first session’s going to give you that data,” he explains. “You’re going to look at the air temperature and the air density, and you may make an adjustment, or you may go back out and take another swipe at it.”

Sloe’s specific data-gathering setup includes Daytona Sensors WEGO sensors on his car and a Longacre Air Density Gauge in the paddock. These two items, working in tandem, give him the information he needs to make educated fuel jet adjustments on his carburetor during a competition weekend.

“You need to record the data and then keep notes on what carb jet changes work, so when things change you can see where you came from and where you went,” he says.

According to Longacre’s Air Density Gauge instructions, the gauge combines the factors of air temperature and barometric pressure to give a direct reading in percent air density. By paying attention to that number, as well as using the calculation that Longacre provides on its website, you can work out which direction you need to go on your carb’s fuel jets. But, in a nutshell, it comes down to this: “If the temperature goes way high [less dense air], you’re probably going to pull some jet from the setup,” says Sloe, “if it gets cooler [higher air density], you’re going to add some jet.”

Holley recommends making adjustments in increments of two when it comes to their jets. Through experimentation, you’ll quickly learn what works for your motor.

Along those lines, Sloe is insistent on keeping notes through the weekend, logging your engine’s air/fuel ratio along with the air density, temperature, and humidity, because when race time comes, you only have one chance to get your carb’s jetting right.

“According to [my engine tuner] Ron Hutter, with the right air – and you adjust for it – it’s a four- to five-percent increase in horsepower,” says Sloe. It’s that kind of an advantage every racer wants when the green flag waves.

But what if your racing budget doesn’t include sensors, data loggers, and air density meters? “It’s possible to do if you have no equipment,” says Sloe. Old school methods still work. Exhaust temperatures can be used, where increased temperatures may indicate lean conditions while cooler temperatures might show an abundance of fuel. Sloe also says you can listen to your motor. “If your fuel jetting is off by enough to have the carb make a sound, it’ll talk to you a little,” he says. “If you’re lean, you’ll hear little sounds, but when it’s rich, it’s blubbery and just won’t rev up.”

Holley’s YouTube channel has extensive how-tos on correct carburetion jetting, including how to read spark plugs (the short story: white plugs indicate lean conditions, black means rich). But the reality is that reading spark plugs is tricky, if not impossible, once a race weekend has begun, as it requires shutting the motor off while on track, and then pulling the spark plugs before firing the motor again.

Through the use of wideband oxygen sensors, data loggers, and air density meters, however, knowing when to change fuel jets in a carburetor is easier than ever. Knowing how much to change the jets, well, that takes trial and error – and if you’re running something like a Weber carburetor where you can change air jets and emulsion tubes, too, things just got really complicated. But, as Sloe says, make adjustments and take plenty of notes. “If you know where you’ve been, it’s easy to make changes, or go back if you got it wrong. But one thing’s for sure, if you never touch your carb, you’re probably never getting to the front.”

Words and Image by Philip Royle