Although carburetor specialist...
Although carburetor specialist Doug Schriefer works exclusively with Barry Grant's Demon carburetors, his tips for advanced tuning will work with just about any brand.
Doug Schriefer, Barry Grant's tech specialist for racing carburetors, would like you to know that-much as many stock car racers may think-carburetor tuning is not voodoo. It is true that many racers aren't afraid to make suspension changes during practice on race night, but if the carb needs more than a jet change, it's going back to the engine builder.
It doesn't have to be that way. It's no secret that race engines perform their best with a well-tuned carburetor, but there is more you can do than simply bolt it up and cross your fingers. Whether you are the driver, the owner, or just the guy who puts air in the tires, here are Schriefer's guidelines for making yourself a top-flight carburetor tuner.
Before we can get into the secrets of making power, Schriefer says it is important to make sure we are all on the same page when it comes to exactly what we want. As stock car racers, we expect three things from the carburetor. First, it has to properly mix the fuel in the form of tiny droplets into the airstream entering the engine. This is called emulsification. Second, it has to control the engine rpm, and finally, it has to properly control the air/fuel ratio.
All three aspects affect each other. For oval track racers, part-throttle operation is of critical importance. You demand a crisp throttle response on turn exit with a nice, smooth feel and no stumbling. And then, of course, you also want maximum power from the engine when you hold the throttle to the floor. Tuning for one is relatively easy-tuning for both situations is more challenging. Finally, add in the fact that at some point during a race you may spin and stall your car, or you may have to kill the engine during a red-flag period. In either case, you must be able to re-fire a hot engine. If your carburetor doesn't hold idle at a consistent rpm to allow you to re-fire and return to the action, your night is over.
The faster air flows through...
The faster air flows through the carburetor, the better the carb can emulsify the fuel into the air. The shape of the venturis increases the airflow by forcing it to flow through a smaller area, and the boosters positioned inside the neck of each venturi speeds the air up even more.
OK, the title of this article is "Advanced Carburetor Tuning," but it is worth taking a minute to discuss exactly how a carb works. Simply put, Schriefer says any carburetor works on pressure differential. For our example, we are obviously only talking about naturally aspirated engines. The pistons moving downward in the cylinder bore create low pressure in the manifold plenum and inside the carburetor. The higher atmospheric pressure (14.7 lb/in at sea level) causes air to pass through the carburetor to fill the low pressure inside the plenum. The low pressure also causes fuel to pass through the carburetor and mix with the air flowing through the venturis.
"As the air travels through the carburetor's venturi, the velocity increases and the pressure decreases," Schriefer adds. "The decrease in pressure allows the fuel to be drawn easier from the fuel bowl through the carburetor's circuits. The increase in air velocity aids in the atomization of the fuel being drawn into the engine. The size and shape of the venturi will affect how much the air velocity increases and the pressure decreases.
"Most carburetors have some type of booster venturi installed within the main venturi to increase the vacuum signal and help draw the fuel from the bowl more efficiently," says Schriefer. The venturi adds velocity to the airflow through the air filter, and the booster increases that velocity even further. Essentially, the booster is a venturi inside the venturi. Since the final speed increase takes place inside the booster, this is the area where the fuel from the main circuit is added. Keep in mind that these are not idle circuits or boosters that activate for a short time when the carburetor reaches wide-open throttle (WOT).
You aren't limited to just...
You aren't limited to just jet changes. There are several points along the fuel's path (from the float bowls to the point that it mixes with the air in the booster) that you can use to influence the air/fuel mixture, from idle all the way to WOT.
Different booster designs have varying amounts of effectiveness. Consequently, changing the type of booster affects how much fuel can be drawn through it, and also how much air can be drawn through the carburetor. The most common booster style for street, or stock, carburetors is the "straight-leg" style, which uses a bar across the center of the booster to introduce fuel to the air. This works, but the bar greatly disrupts airflow.
A "downleg" booster is easily recognizable because the leg of the booster is on an angle. It eliminates the bar and introduces fuel to the air by a bleed hole on one side of the booster. While the elimination of the bar enhances the airflow, this type of booster uses a less efficient single bleed hole to get the fuel to the air.
Finally, the "annular" booster is the most effective style. Like a downleg booster, it eliminates the bar across the center of the booster's venturi, thus enhancing the airflow. However, it introduces fuel to the air at several points around the booster rather than a single bleed hole. In addition, it uses a channel inside the booster's venturi. The net result is a much better and more consistent air/fuel flow that helps to achieve the goal of creating as much air velocity, not necessarily volume, through the carburetor as possible.
The higher air speed atomizes the fuel better, and since small droplets of fuel burn faster than large droplets of fuel, engine performance is improved. This is why you don't always want to slap the largest carburetor you can buy on your engine when you have an open carburetor rule. A very large carburetor can help move more air into the engine and potentially produce better peak power numbers, but it will not boost the air's velocity as well as an intelligently sized carburetor, and overall performance-especially part-throttle response-will suffer.