Choosing a carburetor
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===Based on displacement and use=== | ===Based on displacement and use=== | ||
| − | For a street engine, a general rule is 1.5 to | + | 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. |
===Based on RPM and volumetric efficiency=== | ===Based on RPM and volumetric efficiency=== | ||
Another way to figure carburetor size is with this formula: RPM x CID/3456 x VE. (VE = volumetric efficiency). | Another way to figure carburetor size is with this formula: RPM x CID/3456 x VE. (VE = volumetric efficiency). | ||
| − | A basic, run-of-the-mill engine might have 85% volumetric efficiency. A well planned-out motor can achieve 100% volumetric efficiency, and finely-tuned racing motors can reach up to 120% volumetric efficiency. | + | A basic, run-of-the-mill engine might have 85% volumetric efficiency. A well planned-out motor can achieve 100% volumetric efficiency, and finely-tuned racing motors can reach up to 120% volumetric efficiency in a narrow band. |
==Selecting carburetor type== | ==Selecting carburetor type== | ||
''this section needs additional opinions and confirmation'' | ''this section needs additional opinions and confirmation'' | ||
| − | Opinions vary on the choice between vacuum | + | Opinions vary on the choice between vacuum and mechanical secondaries (aka "double pumpers"). However, generally speaking: |
*Gas mileage is better with vacuum secondaries. | *Gas mileage is better with vacuum secondaries. | ||
| − | *Mechanical secondaries are generally best suited to light, manual transmission vehicles. | + | *Mechanical secondaries are generally best suited to light, manual transmission vehicles or automatic vehicles with the correct stall speed torque converter and correct rear gears to match the engine and carb. |
*Mechanical secondaries are preferred for racing applications. | *Mechanical secondaries are preferred for racing applications. | ||
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==Selecting carburetor brand== | ==Selecting carburetor brand== | ||
| − | + | Modern day carburetors- regardless of the brand- are designed to accomplish the same basic things: Meter and atomize the fuel, mix it with air, and deliver this air/fuel mixture in the correct ratio to the engine to allow it to run efficiently over a broad range of load, conditions and engine speeds from idle to WOT. How these things are done by each particular brand of carb can differ somewhat. | |
| − | + | 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. | |
| + | 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 what is controlled by engine demand, but the vacuum secondary Holley is also self-regulating. And because of that, the Edelbrock/Carter and Holley vac. secondary carbs are somewhat more forgiving. | ||
| − | Holley carburetors have been around for a long time, and their quality is fairly good. | + | 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 the 750 won't give as crisp of a throttle response and possibly less MPG 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 you an idea of the flexibility of a Q-jet. |
| − | They manufactured carburetors for GM in the 60's, and have been a favorite among hot | + | |
| − | rodders through the years. Holley has several different models for different applications. | + | So all in all, I would have to say there's definitely something to be said in favor of sizing the carb as close as you can 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 APT (as long as you've got a removable APT plug) and the idle mixture. And there's a little more to taking the top off a Q-jet than the front bowl off a Holley. |
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| + | Regardless of what carb you get, the trans TV cable adjustment is super critical. It has to be right or the trans will fail. | ||
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| + | The following will give some insight into the various makes and models of carbs that are readily available. | ||
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| + | == Holley == | ||
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| + | Holley carburetors have been around for a long time, and their quality is fairly good. They manufactured carburetors for GM in the 60's, and have been a favorite among hot rodders through the years. Holley has several different models for different applications. | ||
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| + | Ford used Holley carbs basically from their beginning. GM began using Holley carbs in the '60's on select high performance engines, including 3x2 bbl on BBC Corvette engines. | ||
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Here are just a few: | Here are just a few: | ||
| − | -Holley 4412S -- a 2 barrel, 500 CFM carburetor that is ideal for a good ratio between CFM and economy. For a 2 barrel, its pretty large. Works well on a stock 2 BBL manifold for engines such as a 327 SBC | + | -Holley 4412S -- a 2 barrel, 500 CFM carburetor that is ideal for a good ratio between CFM and economy. For a 2 barrel, its pretty large. Works well on a stock 2 BBL manifold for engines such as a 327 SBC. |
| − | -Holley 80457S - Decent 4 BBL carburetor that | + | -Holley 80457S - Decent 4 BBL carburetor that flows 600 CFM. It uses mechanical secondaries, so economy takes more of a "backseat" on this one. Would probably be ideal for your daily driver/mild build. This is about the average 4 BBL sized carburetor. |
| − | The 0-80457 is | + | The 0-80457 is a vacuum secondary carb with electric choke, and is in the 4160 family. All 4160 carbs are vacuum secondary. I would use the 0-1850 as an example a basic 600 CFM carb, and the 0-3310 as a 750 CFM example. And I'd start by describing the various carb families, like 4150, 4160, 4175, etc. Newer and more popular carbs are Street Avenger series of 4150 carbs. By the way, the S or C after the carb list number designates the finish. S = shiny, C = zinc chromate. I can do a complete write-up here, or you can simply go to Holley's website for the facts. |
| − | -Holley 80531 - This carburetor is | + | -Holley 80531 - This carburetor is 850 CFM. There is little to no economy with this bad boy; this is for use on race/high RPM or strong street/strip type engines. |
| − | + | [http://www.usmotors.cz/old/tech_tips/HolleyCarbList.pdf Holley Carb Numerical Listing] | |
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| + | == Edelbrock Performer/AVS Thunder== | ||
| − | Edelbrock is another | + | 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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| − | Edelbrock Performer Series Carburetors | + | Edelbrock Performer Series Carburetors come in 500, 600, 750, & 800 CFM versions, having mechanical or electric chokes, depending on the model. These 4 bbl carburetors offer a wide range of versatility. The smaller 500 and 600 CFM models are suited for small block engines up to about 350 cid. Like the Q-jet and Holley vacuum secondary carbs, they are good for mild builds/daily driver. They have air valve type secondaries, so they can offer decent performance and economy. |
| − | Edelbrock Thunder Series (500, 650 and 800 CFM) | + | The Edelbrock Thunder Series (500, 650 and 800 CFM) are similar to the Performer series, but with a few variations such as an adjustable opening rate secondary. The 500 CFM model can be used for small to medium displacement small blocks. Used in a dual four configuration the 500 cfm carbs are good for medium to medium large displacement engines as well as mild supercharged engines. The big carbs are suited to applications calling for their size, obviously. |
| − | Edelbrock also makes the only carburetor that is designed for Lowriders; designed for extreme angles such as the popular "3 Wheel motion", it keeps the fuel in the float bowl there constantly so you | + | Edelbrock also makes the only carburetor that is designed for Lowriders; designed for extreme angles such as the popular "3 Wheel motion", it keeps the fuel in the float bowl there constantly so you don't have "pedal lag" when driving at severe angles. See Edelbrock.com for more information. |
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| + | ==Rochester Quadrajet== | ||
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| + | [[Quadrajet]] | ||
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| + | [http://www.cliffshighperformance.com/ Cliffs High Performance Quadrajets] | ||
==References== | ==References== | ||
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[[Category:Engine]] | [[Category:Engine]] | ||
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| + | [[Category:Quadrajet]] | ||
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