Choosing a carburetor
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[[File:Blackgoldcarbs1.JPG|right]] | [[File:Blackgoldcarbs1.JPG|right]] | ||
==Selecting carburetor size== | ==Selecting carburetor size== | ||
| − | The higher the engine can rev, the greater the required CFM. However, if the primaries are too large the drop in vacuum caused when the throttle is opened can hurt the bottom end and drivability. | + | The higher the engine can rev, the greater the required CFM or Cubic Feet per Minute. However, if the primaries are too large the drop in vacuum caused when the throttle is opened can hurt the bottom end and drivability. |
===Based on displacement and use=== | ===Based on displacement and use=== | ||
| − | For a street engine, a general rule is 1.5 to 2 times the displacement equals the necessary carburetor CFM. For a competition engine, it can be from 2 to 2.5 times the displacement. | + | For a street engine, a general rule is 1.5 to 2 times the displacement equals the necessary carburetor CFM. For a competition engine, it can be from 2 to 2.5 times the displacement or more. |
===Based on RPM and volumetric efficiency=== | ===Based on RPM and volumetric efficiency=== | ||
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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. | ||
| − | The vacuum secondary Holley uses a different approach to secondary actuation, where the secondary throttle blades are controlled by engine demand via the secondary vacuum diaphragm assembly. But like the Q-jet and Edelbrock/Carter carbs, the vacuum secondary Holley is also self-regulating. And because of that, the Edelbrock/Carter and Holley vacuum secondary carbs are somewhat more forgiving if a too-large carb is used. | + | The vacuum secondary Holley uses a different approach to secondary actuation, where the secondary throttle blades are controlled by engine demand via the secondary vacuum diaphragm assembly. But like the Q-jet and Edelbrock/Carter carbs, the vacuum secondary Holley is also self-regulating. And because of that, the Edelbrock/Carter and Holley vacuum secondary carbs are somewhat more forgiving if a too-large carb is used. I say ''somewhat'' more forgiving because even though a 750 Holley w/vacuum secondaries will self-regulate the secondaries, the primaries- what you drive on 99% of the time- are also larger on a 750 than a 600 Holley. And what that can mean is on a mild or smaller displacement engine the 750 cfm carb won't give as crisp of a throttle response and possibly less MPG and vacuum because of the bigger throttle bore and venturi. This is not an issue w/the Q-jet; its primaries are small and have primary booster rings to provide a strong vacuum signal. The 800 cfm Q-jet castings are used on 4.3L truck engines to give an idea of the flexibility of a Q-jet. |
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| − | I say ''somewhat'' more forgiving because even though a 750 Holley w/vacuum secondaries will self-regulate the secondaries, the primaries- what you drive on 99% of the time- are also larger on a 750 than a 600 Holley. And what that can mean is on a mild or smaller displacement engine the 750 cfm carb won't give as crisp of a throttle response and possibly less MPG and vacuum because of the bigger throttle bore and venturi. This is not an issue w/the Q-jet; its primaries are small and have primary booster rings to provide a strong vacuum signal. The 800 cfm Q-jet castings are used on 4.3L truck engines to give an idea of the flexibility of a Q-jet. | + | |
So all in all, there's definitely something to be said in favor of sizing the carb as close as possible to what the engine ''actually'' needs. The Q-jet is damn hard to beat but if it has a downside it's that you have to learn how to tune it. The secondary side is easy as pie to tune- everything needed is external. It's the primary side that you have to open the carb up to tune almost anything except the idle mixture and the APT (as long as you've got a removable APT plug). And there is more to taking the top off a Q-jet than the front bowl off a Holley. | So all in all, there's definitely something to be said in favor of sizing the carb as close as possible to what the engine ''actually'' needs. The Q-jet is damn hard to beat but if it has a downside it's that you have to learn how to tune it. The secondary side is easy as pie to tune- everything needed is external. It's the primary side that you have to open the carb up to tune almost anything except the idle mixture and the APT (as long as you've got a removable APT plug). And there is more to taking the top off a Q-jet than the front bowl off a Holley. | ||
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*0-80532-1 - This represents the Dominator series of Holley carbs, this one is 1250 cfm. The Dominator first was used on the high banks of NASCAR to feed the biggest engines ever used in that form of motorsports. They became just as popular for drag racing and today are available as small as 750 cfm (Holley 0-80186-1) for the "look" w/o the huge flow so it can be used on the street. Because the bolt pattern and footprint is considerably larger than the traditional Holley "squarebore" pattern, a Dominator requires a dedicated intake manifold. Adaptors are available, but should only be used in special circumstances if at all. | *0-80532-1 - This represents the Dominator series of Holley carbs, this one is 1250 cfm. The Dominator first was used on the high banks of NASCAR to feed the biggest engines ever used in that form of motorsports. They became just as popular for drag racing and today are available as small as 750 cfm (Holley 0-80186-1) for the "look" w/o the huge flow so it can be used on the street. Because the bolt pattern and footprint is considerably larger than the traditional Holley "squarebore" pattern, a Dominator requires a dedicated intake manifold. Adaptors are available, but should only be used in special circumstances if at all. | ||
| − | *[ | + | *[[Media:HolleyCarbList.pdf|Aftermarket Holley carb numerical listing]] by carb number |
| + | *[[Holley HP series carb specs]] by carb number | ||
* [http://www.holley.com/data/Catalogs/Holley/90.pdf Holley 4160 metering plates to jets] | * [http://www.holley.com/data/Catalogs/Holley/90.pdf Holley 4160 metering plates to jets] | ||
| − | * More on Holley carbs [http://www.crankshaftcoalition.com/wiki/How_to_rebuild_automotive_carburetors#Holley_squarebore_carb '''here'''] | + | * More on Holley carbs [http://www.crankshaftcoalition.com/wiki/How_to_rebuild_automotive_carburetors#Holley_squarebore_carb '''here'''] |
[[File:AVS Manual E.jpg|right]] | [[File:AVS Manual E.jpg|right]] | ||
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=='''[http://www.edelbrock.com/ Edelbrock]''' Performer/AVS Thunder== | =='''[http://www.edelbrock.com/ Edelbrock]''' Performer/AVS Thunder== | ||
Edelbrock is another well recognized name in carburetors. They make carburetors for different applications: street, racing, marine. Edelbrock carbs have become a favorite among hot rodders. They manufacture good quality carburetors. | Edelbrock is another well recognized name in carburetors. They make carburetors for different applications: street, racing, marine. Edelbrock carbs have become a favorite among hot rodders. They manufacture good quality carburetors. | ||
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*[[Quadrajet]] | *[[Quadrajet]] | ||
*[http://www.cliffshighperformance.com/ Cliffs High Performance Quadrajets] | *[http://www.cliffshighperformance.com/ Cliffs High Performance Quadrajets] | ||
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| + | ==Carter AFB== | ||
| + | *[[Media:Carterafbtuning.pdf|Carter AFB Selection and Tuning Guidelines]] | ||
==References== | ==References== | ||
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