What are the Differences in Big Holley 4150 Carburetors? Part 1

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Holley offers all sorts of different carburetors. They’re the standard for performance and racing applications, and the most common here has always been the conventional flange 4150 series carburetor. Over the years, the engineers at Holley have poked and prodded the 4150, adding features and, of course, improving performance. Some of those improvements have also had a trickle-down effect on older carburetor designs. For example, today’s 0-4781-C (the traditional 850 double pumper) is now equipped with four corner idle circuits.

What we’ll look at here are the differences in the big Holley 4150 carburetors – all with 1.750-inch diameter throttle bores. Our examples include the 0-4781-C (Classic Double Pumper), the 0-80514-1 (Classic HP) and the 0-80805-HBX (Ultra XP).

Externally, the easiest differences to spot are the lack of a choke on the HP series carb and the Ultra XP. Both of those have radiused venturi entries topside. Additionally, the Ultra XP isn’t gold dichromate. You have a choice of “Hard Core Gray” or polished finishes. The reason the Ultra is a different color is because the body, bowls and metering blocks are manufactured from aluminum. The Classic Double Pumper and the Classic HP carbs have bodies, bowls and metering blocks manufactured from zinc. And by the way, Holley has recently added aluminum bodied HP carbs to their product list. They’re easy to spot due to their vibratory polished finishes. See the Holley website (https://www.holley.com/products/fuel_systems/carburetors/hp/) for more.

Dig a little deeper and you’ll actually find there are considerable differences in the carburetors beyond cosmetics. There are also some similarities. All three are built with 1.750-inch diameter symmetrical throttle bores. Venturi sizes differ though: The 4781 and the Classic HP are built with 1.561-inch venturis. Meanwhile the big Ultra XP comes with a 1.600-inch venturi.

Boosters differ as well, although the Classic HP can be purchased with two different booster configurations. The 4781 Double Pumper makes use of Down Leg boosters. You purchase the Classic HP with either Down Leg boosters or Annular configuration boosters. The XP is equipped with Down Leg boosters.

So what’s the difference between the two types of boosters? Let’s back up a bit. A venturi booster is basically a smaller venturi installed within the main venturi. The reason it is tagged as a “booster” is because it increases the pressure drop as the airflow passes through the main carburetor venturi. This improves performance, particularly at lower engine speeds by moving fuel into the engine. The carburetor is calibrated for the specific booster used.

A Down Leg booster discharges the air and fuel emulsion from one hole. Meanwhile, Annular boosters typically discharge the main circuit air and fuel emulsion from eight to seventeen (or more) discharge holes. Annular boosters are usually larger than Down Leg examples. That in turn can have an effect upon the total CFM of the carburetor. Changing the size of the boosters can have a dramatic effect upon the carb signal. Basically, the larger the booster “banjo,” the higher the carb signal will be. Obviously, as the banjo becomes larger, it restricts the CFM of the carb more severely.

Since the Annular booster has many more discharge holes in comparison to a Down Leg booster, the fuel droplets are considerably smaller and, as a result, much easier to atomize. As the carburetor size increases, the effect of the annular booster becomes more noticeable. There are a lot of variables to take into consideration when moving from one booster configuration to the another and as mentioned above, the overall calibration of the carburetor changes with the type of booster.

For a closer look, check out the accompanying photos. And stay tuned for the next few issues – there’s a lot more to come in our “Carb Comparo.”

Although all three Holley carbs shown here are 4150 examples with dual accelerator pumps, they’re actually very different carburetors.

Of the three carbs, the 4781 is the only example with choke. As you can see here, the other two have radiused entrances into the venturis.

Classic Double Pumpers such as the 4781 and the Classic HP shown here are manufactured from zinc. That isn’t the case with the Ultra series carbs (next photo).

Ultra series carburetors are manufactured from aluminum. That aluminum construction extends from the base to the body to the metering blocks to the fuel bowls. Additionally, you can opt for two different colors on Ultra series carbs – Hardcore black as shown here and a mix of polished components and red anodizing.

All of the carbs in our comparo make use of four corner idle mixture screws.

This wasn’t the case in years gone by. Earlier conventional double pumpers had idle mixture screws on the primary side only.

All three carburetors are manufactured with 1.750-inch throttle bores.

Upstairs, the venturi diameters differ. The 4781 and the Classic HP are built with 1.561-inch venturis. Meanwhile the big Ultra XP comes with a 1.600-inch venturi.

Boosters differ too. For example, the 4781 Double Pumper is fitted with Down Leg boosters.

The Classic HP shown here incorporates an Annular Discharge booster configuration.

Meanwhile, this Ultra XP carb makes us of Down Leg boosters. See the text for more info on the differences in boosters.