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Car guys speak a strange and unique language. There are thousands of colloquial references to cars, engines, parts, and circumstances along with shortcuts, abbreviations, euphemisms, and a seemingly limitless volume of vehicular vernacular. Buried deep in this language are certain terms that everyone should know.
Among the hundreds of internal combustion subcultures are those who speak the prose of the Holley carburetor. It is a separate language from those who only converse in EFI tongues. But, for those who want to walk the walk while still using the terms wisely, there are a few verbal Holley morsels that you should commit to memory. Most old car guys know what a double pumper is, but the young guns may have only heard the term in passing, so consider this a short cut into the simple world of calibrated orifices, jet sizes, and the four barrel Holley carburetor.
As succinctly as possible, the double pumper is a Holley carburetor with 4 barrels, two accelerator pumps (hence the double pumper monicker), and a mechanical linkage connecting the primary and secondary sides of the carb. The double pumper was used on many performance cars in the glory days of muscle cars and plays a significant role in racing to this day. Simply put, it was the carb to have. Over time, the double pumper name began to be applied to lots of Holley carbs and it's not uncommon to stroll down a swap meet lane, peruse the internet for parts, or overhear a fellow gearhead refer to a double pumper that well, isn't. It happens all the time and below, we'll show you what makes a double pumper, and what carburetor features have people getting tricked!
Both of the carburetors in this picture are vacuum secondary style. The carb on the right uses center-hung, dual-feed float bowls, whereas the carb on the left uses side-hung float bowls with a single fuel feed. The most common misidentification of a double pumper carb is because it has center-hung floats.
The reason double pumpers (mechanical secondary carburetors is their proper name) have these twin accelerator pumps is to prevent a momentary lean situation during quick throttle blade movement. All carburetors use an accelerator pump circuit, however many only have one on the primary barrels. These pumps apply a quick shot of fuel into the engine that compensates for the short delay in delivering main circuit fuel when slamming the throttle open. For all double pumper carbs, there is an accelerator pump circuit on both the primary and the secondary sides.
Vacuum secondary carburetors control secondary opening using a vacuum diaphragm that only opens the secondary barrels when there is sufficient airflow demand to warrant it. As such, this style carburetor does not require an accelerator pump on the secondary side. So, a vacuum secondary carb is only a single pumper carburetor – but don’t call it that – you'll get weird looks.
These vacuum secondary carbs make excellent street carburetors. They open the secondaries progressively and are designed for stock-ish to mildly built engines that are heavy, have taller (lower numerically) rear end gears and automatic transmissions with tight converters. By keeping the secondaries closed until considerably higher load, these carbs also help to improve fuel economy over a similarly sized mechanical secondary carb. Holley has a plethora of vacuum secondary carbs to choose from in small and large CFM sizes.
Mechanical secondary carbs (double pumpers) are the better option for race cars, hot rods, and performance vehicles with manual transmission, automatic transmissions with looser torque converters, and lower (higher numerically) rear end gears. They are designed for engines that rev quickly, demand air flow more quickly, and are more focuses on quick acceleration.
This is a secondary metering block. Some carburetors use blocks like these on the secondaries as they accept interchangeable main jets and some metering blocks have adjustable idle provisions for the secondaries. Other carbs use metering plates which require complete replacement in order to tune. Secondary metering plates can be used on both vacuum secondary and mechanical secondary carburetors.
Vacuum secondary Holley carburetors, such as this one on the right use a vacuum diaphragm housing (right) to open the secondary throttle blades. A mechanical secondary Holley (left) uses a mechanical linkage.The linkage is tied directly to your right foot to signal full throttle opening.
The Double Pumper gets its name from the twin accelerator pump outlets (arrows) for both the primary and secondary throttle bores. This is an HP version carburetor where the choke housing is removed for smoother air inlet into the carburetor.
Among the most popular 750 cfm Double Pumper Holleys is the traditional PN 0-4779C. It offers a 4150 configuration with metering blocks on the primaries and secondaries and also offers four-corner idle control, which is preferable for engines with big single plane intake manifolds and long-duration cams. The manual choke can easily be upgraded to electric control at a later date.
The Ultra XP line positions the double pumper in between a street and full race carburetor with billet metering blocks as well as billet aluminum base plates. While street carburetors use fixed metering points for the less often accessed restrictors, the Ultra XP employs screw-in bleeds that allow for fine tuning the carb for all load conditions.