Holley Carburetor Tuning - 10 Power Moves

6. Boosters Science
Here is an area where it is easy to step on your own toes unless you know what is going on. The purpose of a booster is to boost the signal seen at the carb's main venturi to allow better calibration and better atomization in conjunction with the air correction jet and emulsion well pattern. Fuel that is insufficiently atomized does not burn well, but like most things, having too much is also not good. If the fuel is too finely atomized, it may surprise you that it is bad for power.

If you are dyno testing in January, you may find the high-gain boosters, such as the annular discharge booster, work great. But don't be fooled into thinking that because you have tested a bunch of boosters and settled on the best, your homework is done. What worked in a winter test session can be way off what is needed in summer. Here is why. If the fuel is too finely atomized, a lot of it will turn to a vapor in the intake manifold. This takes up room and cuts the amount of intake charge weight drawn into the engine. In other words, it drops the engine's volumetric efficiency (VE). The result is one sweet-running engine with good brake specifics that is down on outright power. If you have to race at somewhat colder temperatures, you may want to check out results with a high-gain annular discharge booster-especially if you run an alcohol class motor. Other than this, years of testing with air temps from about 55 degrees to a little over 100 degrees strongly indicate that the stepped dogleg booster is the most versatile.

7. Emulsion Calibration
Most knowledgeable crew chiefs are aware that going up on the main circuit air corrector thins the mid- to top-end and visa versa. They also know that making the main jet or PVRC bigger richens the mixture and doing the reverse makes it thin. So far so good, but what happens when an emulsion well that can be calibrated becomes part of the equation? The answer is usually confusion. The purpose of this type of emulsion well is to allow the end user to trim the shape of the fuel curve. The term fuel curve is almost a misnomer here, as what we need is a flat line, but the word curve is applied regardless.

So here's the skinny toward a really refined carb setup that produces a near flat 12.8:1 air/fuel ratio from the bottom to the top of the rpm range. First, you need to appreciate that the top jet affects the bottom of the rpm range, the middle affects the middle, and the bottom affects the top. Now, if you are still with me, here is how the sizing of those jets comes into play. Making a jet bigger thins the mixture, and making it smaller, as you would expect, richens it. You now know all that is needed to get a near flat fuel curve.

8. Fuel conditions
Monitor fuel pressure to ensure that it doesn't get too high. Make any move possible to reduce the temperature of the fuel in the float bowl, especially in hot weather. The first move toward achieving this is not allowing fuel to remain exposed to sunlight. Keep it in a cool environment. Dropping the fuel's temperature by 5 degrees as seen at the carb is equal to dropping the air temp by about the same amount.

9. Monitor In Let Air Temperature
Hot intake air is one of the engine's worst enemies when it comes to making power. Worse yet, the power reduction comes about by a torque reduction throughout the entire operating range, resulting in less punch off the turn and less speed down the straight. If you are running a short circuit car (a 11/42-mile or less) and the rules call for an open 360-degree filter element (rather than a cold air cowl induction as per a Nextel Cup car), then you may want to reconsider the common practice of taping up the grill for qualifying. Unless the track is slick and creates a traction issue, an air temperature increase of about 10 degrees can offset any small advantage seen by taping over the grill for improved penetration and downforce.

10. A Weather Station
I learned long ago that understanding how atmospheric conditions affect optimal carb calibration was a definite advantage. A championship contending driver with whom I would have a door-handle-to-door-handle race on a cool dry weekend would be left standing the next weekend when the humidity was near 100 percent and rain was imminent. If you are an East Coast racer, be aware that there are not only substantial temperature and barometric changes, but also big humidity swings. Taking all these factors into account can be difficult, but 95 percent of the guesswork can be eliminated by using any one of a number of racers' weather station kits that come complete with a computer to calculate what changes are required from your original chassis dyno baseline. You did keep it, didn't you? The cost of these weather stations ranges from fair to phenomenal. Barry Grant sells an oval track weather station at a very fair price.