In the mind of nearly every gearhead, the Holley four-barrel carb is tantamount to high performance. I know in my case, the first mental picture that comes to mind when I hear "four-barrel" is the image of a Holley 750 double-pumper. Sure, there are other makes and configurations of good performance carburetion on the market, but the legendary Holley still reigns supreme.
Over the last few years, one of the most prolific Holley carburetor designations for the performance-minded has been the HP-Series four-barrels-both the HP Street and Ultra HPs. These contoured venturi designs are extremely popular, and for good reason. They're both highly tunable and 100 percent wet-flow tested for reliability. The 4150 Street HP 750-cfm is a slightly tamer street version of the race-bred 4150 Ultra HP series that shares many of the same features, like the aforementioned contoured venturi inlet, screw-in air bleeds, power valve blow-out protection, and spun-in down leg boosters. The Ultra HP, on the other hand, is better suited for the track than their more docile brethren, and feature billet metering blocks and baseplate, changeable emulsion jets, and idle feed and power valve restrictors for nearly infinite tailoring and tuning to any engine.
Speaking of being tailored to any engine, the HP Series carburetors are available in a wide array of capacities, from 390 cfm up to a whopping 1,000 cfm; the 750 is available in both mechanical and vacuum secondary versions (check out www.holley.com for a great performance carburetor quick reference list). The availability of such a huge selection of carbs is great for those in the know, but for those of us who are not as thoroughly knowledgeable, picking the correct carb for our engine takes a little bit of investigation. What is confusing for many is that they tend to see all sorts of different-sized carbs used on engines of the same make and displacement. If we see 10 different small-block Chevys with 750s, then we might figure the same would work on ours, too. Unfortunately the bigger-is-better theory doesn't always work.
The optimum carburetor size is directly related to some important factors, like the engine's capacity in cubic inches, its efficiency (the average engine runs around 70 percent efficient), and its maximum rpm. An extremely efficient engine run at high rpm will most definitely make good use of a large carb to develop good top-end performance (6,500 rpm and over), but oft times at the expense of the low end (4,000 rpm and under). Thus, many carburetor-related performance troubles stem from over carburetion; at the other end of the spectrum, an under carbureted condition shows itself ostensibly at the top end of the rpm range. A lack of carburetor capacity becomes readily apparent, as the power flattens out abruptly because the flow capacity is insufficient to meet the engine's needs. But up to that high rpm barrier, most engines will perform relatively well.
Determining extremely precise cfm requirements is pretty difficult (for those of us who don't have access to flow benches and dynos anyway), as it's tough for us to accurately determine just how efficient our engines really are. To make an educated guess, we have to keep in mind that the efficiency of an engine is related to how close to maximum capacity its cylinders are after induction. For example, an average engine in stock configuration is about 70 to 75 percent efficient. A mildly modified engine with ported stock heads, tube headers, and a mild-performance camshaft, for example, should bump these figures up to near 80 to 85 percent. A completely professionally built engine with aftermarket heads, etc. could increase efficiency upward to nearly 95 percent or so. That said, there is a mathematical formula used to help calculate carb requirements for particular engine displacements that help get us get as close as possible under our circumstances-as long as we're realistic in our efficiency estimates.
The formula goes like this: Multiply half the cubic-inch displacement of the engine by the maximum rpm to be utilized. Divide the answer by 1,728 (the amount of cubic inches in a square foot). Multiply the answer by the engine efficiency rating (for example 0.75 equals 75 percent efficiency) and this answer is the cfm the engine can flow. You then use this figure to choose a carb, but keep in mind that there's another consideration, as well, and that's where you want your power in the rev range. If the cfm rating expressed in your calculations falls between two carb sizes and you're looking for top end, then go with the larger. If you're looking for mid-range performance, go with the smaller of the two.
Hopefully this info clears the air a bit, but keep in mind that there's a ton of useful info available on both the Holley Web site and in their Tech section, so don't hesitate to make good use of the information and awesome products Holley has to offer.