Choosing a carburetor
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On the primary side the carb has the duty of supplying air/fuel to allow the engine to start up cold (choke), to idle, to accelerate the vehicle up to the point to where the secondary circuit is called on. Along with that is the accelerator pump circuit, the power enrichment circuit and the main circuit. Of the commonly used carbs on the market, the Q-jet has the most responsive primaries of them all. This is due to the relatively small size of the primaries along w/the triple boosters that enhance the vacuum signal to the circuits controlling the air/fuel mixture at the various loads/speeds. | On the primary side the carb has the duty of supplying air/fuel to allow the engine to start up cold (choke), to idle, to accelerate the vehicle up to the point to where the secondary circuit is called on. Along with that is the accelerator pump circuit, the power enrichment circuit and the main circuit. Of the commonly used carbs on the market, the Q-jet has the most responsive primaries of them all. This is due to the relatively small size of the primaries along w/the triple boosters that enhance the vacuum signal to the circuits controlling the air/fuel mixture at the various loads/speeds. | ||
| − | The non mechanical secondary carbs' (Q-jet, Holley 4160, etc., Edelbrock/Carter to name a few) secondaries all work on a "as needed" basis. That's to say they won't (if properly adjusted) allow any more secondary opening than the engine actually needs. This is unlike a Holley double pumper mechanical secondary carb, which opens up the secondaries regardless of the engine demand, if you floor it. | + | The non mechanical secondary carbs' (Q-jet, Holley 4160'''*''', etc., Edelbrock/Carter to name a few) secondaries all work on a "as needed" basis. That's to say they won't (if properly adjusted) allow any more secondary opening than the engine actually needs. This is unlike a Holley double pumper mechanical secondary carb, which opens up the secondaries regardless of the engine demand, if you floor it. |
You may hear a Q-jet being called a 'mechanical secondary' carb. And to some extent this is true- the secondary throttle blades WILL open when you floor it if the choke is off. But- and this is an important 'but'- it's the air valve at the top of the carb that works on the 'as needed' basis. The Edelbrock/Carter is a similar design in that the secondary throttle blades will open if floored, but the Edelbrock/Carter uses a air door that works by the amount of air flowing into the secondary side and it too, won't give the engine any more airflow than it needs. | You may hear a Q-jet being called a 'mechanical secondary' carb. And to some extent this is true- the secondary throttle blades WILL open when you floor it if the choke is off. But- and this is an important 'but'- it's the air valve at the top of the carb that works on the 'as needed' basis. The Edelbrock/Carter is a similar design in that the secondary throttle blades will open if floored, but the Edelbrock/Carter uses a air door that works by the amount of air flowing into the secondary side and it too, won't give the engine any more airflow than it needs. | ||
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4776, 4777, 4778, 4779, 4780, 4781 - These are the traditional "double pumpers". Ranging from 600 cfm to 850 cfm, they are a mainstay of racing and hot street engines. If matched correctly to their application they can be used on the street but economy will not be as good as a vacuum secondary carb similarly chosen. These are 1850 type carbs, having both a primary and secondary metering block and accelerator pumps on both the primary and secondary sides of the carb. | 4776, 4777, 4778, 4779, 4780, 4781 - These are the traditional "double pumpers". Ranging from 600 cfm to 850 cfm, they are a mainstay of racing and hot street engines. If matched correctly to their application they can be used on the street but economy will not be as good as a vacuum secondary carb similarly chosen. These are 1850 type carbs, having both a primary and secondary metering block and accelerator pumps on both the primary and secondary sides of the carb. | ||
| − | BTW, the model 4160 is a vacuum secondary carb- w/few exceptions, one is the 0-4224 660 cfm "center squirter" carb, originally designed to fit on a dual four barrel intake end-to-end. Another is the 450 cfm 0-9776 Model 4160 mechanical secondary carb. It has no secondary accelerator pump. These carbs are designed to be used in a multiple carb application. They have no choke flap, they have side hung float bowls and single fuel inlet. The model 4150 can be either a vacuum secondary or a double pumper carb. | + | '''*'''BTW, the model 4160 is a vacuum secondary carb- w/few exceptions, one is the 0-4224 660 cfm "center squirter" carb, originally designed to fit on a dual four barrel intake end-to-end. Another is the 450 cfm 0-9776 Model 4160 mechanical secondary carb. It has no secondary accelerator pump. These carbs are designed to be used in a multiple carb application. They have no choke flap, they have side hung float bowls and single fuel inlet. The model 4150 can be either a vacuum secondary or a double pumper carb. |
4604 - This is one of the Holley 3-barrel carbs rated at 1050 cfm. The 3 barrel Holley was originally designed for NASCAR stock car racing and was used on the 426 Hemi in 1964. It preceded the Dominator and offered in several versions. It never really hit its stride, suffering from issues below WOT because of the less-than-ideal secondary venturi shape (and booster style in the 1050 cfm version). It was made basically from a Holley 850 w/the center taken out of the secondaries. It used an oversize secondary vacuum diaphragm housing (which the diaphragms are no longer available for) and an early version of the "cathedral" center hung float bowls made famous later, on the Dominator. A carb kit for a 850 can be used w/some modification to rebuild them. Today it has a following among collectors and some actually still use them. They can fetch high prices for good examples. | 4604 - This is one of the Holley 3-barrel carbs rated at 1050 cfm. The 3 barrel Holley was originally designed for NASCAR stock car racing and was used on the 426 Hemi in 1964. It preceded the Dominator and offered in several versions. It never really hit its stride, suffering from issues below WOT because of the less-than-ideal secondary venturi shape (and booster style in the 1050 cfm version). It was made basically from a Holley 850 w/the center taken out of the secondaries. It used an oversize secondary vacuum diaphragm housing (which the diaphragms are no longer available for) and an early version of the "cathedral" center hung float bowls made famous later, on the Dominator. A carb kit for a 850 can be used w/some modification to rebuild them. Today it has a following among collectors and some actually still use them. They can fetch high prices for good examples. | ||
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