Choosing a carburetor
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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 where the throttle blades are | + | 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 vac. 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 | + | 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, | + | 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. |
Regardless of what carb is used, if using a GM OD automatic tranny the trans TV cable adjustment is super critical. It has to be right or the trans will fail. | Regardless of what carb is used, if using a GM OD automatic tranny the trans TV cable adjustment is super critical. It has to be right or the trans will fail. | ||
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