Choosing a carburetor
Selecting carburetor size
The higher the engine can rev, the greater the required CFM. However, bottom end response worsens with increased CFM.
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.
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).
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
this section needs additional opinions and confirmation
Opinions vary on the choice between vacuum and mechanical secondaries (aka "double pumpers"). However, generally speaking:
- Gas mileage is better with vacuum secondaries.
- 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. In some cases the fact that the secondary jets can be changed in the double pumpers and there's an extra accelerator pump to deal with, some may find it harder to tune than the simpler 4160 type Holley. But once the process is understood the tuning becomes much easier, plus the added adjustability that was a hurdle at first becomes an asset. The 4160 needs the metering plate replaced or modified to use jets if the secondary air/fuel ratio is off.
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.
*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.
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 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 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.
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.
The following will give some insight into the various makes and models of carbs that are readily available.
Holley carburetors have been around for a long time, and they have been a favorite among hot rodders through the years. Holley has many different models for different applications, including GM in the 60's for some select high performance Chevy engines, including the BBC Corvette 3x2-barrel carbs. Ford used Holley carbs basically from their beginning.
Here are a few List numbers (found stamped into the front of the airhorn along with a date code) of the more common or popular Holley carbs:
- 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 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.
- 0-80457 is a vacuum secondary carb with electric choke, and is in the 4160 family. 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 flows 850 CFM and is a model 1850. There is little to no economy with this bad boy; this is for use on race/high RPM or strong street/strip type engines.
- 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.
- 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.
- 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.
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 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.
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.
See Edelbrock for more information.
In 2011, BG underwent a corporation redesign. The "old" BG carb line (Demon, etc.) was replaced by a totally new design carb seen here.
This carb is still something of an unknown and untested by many hotrodders; whether it will be a hit or a miss remains to be seen.
- How to Choose your next Carburetor, Ford Truck Enthusiast forum
- How to Choose the Proper Carburetor and Fuel Pump, ThirdGen.org forum
- Carb Confusion, Shedding Some Light on the Dirty, Dark Secrets of Choosing the Right Carburetor, from the February, 2009 issue of Classic Trucks
- How To Build Horsepower, Volume 2: Carburetors & Intake Manifolds, by David Vizard. ISBN 1884089143