EngineLabs: What are the basics of a typical fuel system on a Pro Stock car?
Vickous: There’s a front mounted, 2-gallon fuel cell that takes away the gravity issues they had with rear-mounted cells. It’s also safer inside the cage, and there are fewer fuel lines. If there’s an accident, you won’t get fuel inside the car. Most use four single pressure regulators, one for each bowl, or a four-port regulator. A lot still use the old small-regulator technology because it still offers the best flow, fuel control and delivery when used one per bowl. Most use AN-6 lines. The needle and seat is a restriction at .130-inch, so a 3/8-inch line will feed that just fine. Teams generally run around 5 or 5.5 psi with a single pump rated at 350 to 500 gallons per hour, depending on how it’s rated by the manufacturer.
Vickous: We have to start with an approved cast carburetor, then we can do just about anything within that footprint. The holes are generally 2.300- to 2.350-inch diameter on the bottom with a venturi in the 1.950- to 2-inch range. If you were to actually rate them, a typical Pro Stock carb would flow at least 1,650 to 1,700 cfm, but that all depends on the engine. The vacuum will determine how much is pulled through the carburetors. In the old days where they didn’t have the vacuum pumps on the engine, they wouldn’t pull as much fuel through. Now that they’re controlling vacuum on the engine and in the crankcase, the engines are a lot more efficient and they pull more cfm.
EngineLabs: What’s the difference between a dyno carb and a track carb, and what’s the biggest challenge in setting up a track carb?
Vickous: The dyno carb is a known piece that they can use over and over, because the team knows how it reacts. The ones on the race track are refined versions of that. Controlling the fuel in the fuel bowl is the biggest challenge at the track. When a car launches that hard, the fuel wants to go towards the rear of the bowls, because they are mounted sideways. If you do not have the fuel pressure set up right and fuel level high enough, it’ll uncover the front jet of each metering block and give you a lean spot. That hurts the front- and third-row cylinders. And then you got all the fuel going back, it’s going to make the adjacent cylinders go rich. So you got four trying to go lean and four going rich. You have to try and tune the barrels of the carb individually with air bleeds, emulsions and booster pin size to bring more fuel back to the lean cylinders and take away fuel from the rich.
EngineLabs: And then when the car levels off down the track and the G-forces are lower?
Vickous: You do something in-between so it makes it happy at launch and doesn’t kill it down track. The crew chief can also play with the timing on the individual cylinders to help the fuel curve as flat as possible. When you first hit it, you’re going to get spikes, one way or the other. But you want to get [the curve] leveled out. Carbureted engines inherently try to go richer at the end of the run and high rpm. The one thing you can do is take care of it with timing. Let’s say peak torque on Pro Stock comes in at 9,200 or 9,400 rpm, and they’re now turning up past 10,500 rpm. Once they reach peak torque, you can add timing to help burn off that extra fuel.
EngineLabs: What tricks learned from Pro Stock carb development has Quick Fuel trickled down to sportsman and street carb setups?
Vickous: Most of the technology and things learned with Pro Stock really are limited to big-bracket engines and definitely Top Sportsman and Top Dragster. The biggest things are matching the venturi and throttle bores with a booster that still is responsive to the engine signal.