Sizing the Carb Based on Usage.
For street motors and/or city driving, it's better to start with a smaller carb, which provides maximum torque, throttle response, and fuel economy. If you do a lot of highway driving you may want to go with a carburetor that's a little bit bigger. A slightly larger carb will sacrifice some low-end torque, throttle response, and/or fuel economy, but it will give you more top-end horse power for passing and cruising. If you do a combination of highway and city, it's better to size the carb on the smaller side, rather than larger, especially when fuel economy is a primary concern.

Next lets talk about drag racing, where low-end torque, throttle response, and/or fuel economy are not a concern. In this situation a larger carb is required to achieve maximum horse power at wide open throttle. However... while bigger is better, you still don't want to oversize the carb. This is where dyno testing comes in handy, or for those on a tight budget, numerous test-n-tune passes. It also helps to have a several carbs on hand.

Finally we have the mild performance motors, or those used for Street/Strip applications. These motors usually require a carb sized somewhere in-between Street and Strip. Basically, you simply need to decide what size carb best suits your own particular needs. You may even decide to run two carbs. Maybe you'll run a 32/36 Weber during the week, driving to and from work, as this will give you the best economy and overall driveability. Then swap to a 38/38 Weber on Friday night, so your ready for that long weekend cruise with your local Mustang or Falcon club. Or maybe you'll swap to a Holley 500 in preparation for a weekend of drag racing. In any case, remember it's always better to go smaller when selecting a carb for street use.

VE or Volumetric Efficiency
Volumetric Efficiency must also be figured in to your CFM calculations. A stock small log head with a 1-7/16 carb bore (commonly referred to as a 1.5" bore) will normally see about 65% efficiency do to the restrictive nature of the log intake. The later large log heads will do slightly better at 75%, and fully ported may reach as high as 85%. An aluminum head should get around 90%, while a fully ported version can reach 100%. Turbo charging the log heads, large or small, will net huge improvements in the efficiency, as the short comings of the asthmatic log manifold are greatly diminished. In such cases the VE may reach upwards of 120% efficiency, depending on the amount of boost.

In the chart below, we have done the CFM and VE calculations for a daily driver or weekend cruiser. If you're building a race motor, increase the size by 50-100 CFM, as the motor will require more airflow when operating in the higher RPM ranges. It's easy to see how the CFM requirements change, based on the various modifications.

Once you've determined your CFM requirements, the next step is to select a carburetor. The biggest decision is whether to use a one barrel (1V) or upgrade to a two barrel (2V). With all the different carburetors that are available, this may be very confusing, but its really pretty simple and straight forward.

The other thing you need to think about when doing a carb swap, is the ignition system. If you're replacing the stock Autolite 1100-1V carb, you'll need to swap out the distributor as well. The early (63-67) 1100's came with a "Spark Control Valve" and were designed to work hand in hand with the stock "Load-O-Matic" distributor. Even if you have a Petronics Ignition system, which eliminates the points, you still haven't resolved the problem. And while the later (68-69) 1100's carbs eliminated the "Spark Control Valve", you'll still want to ditch the stock distributor in favor of a Duraspark or DUI. Simply put, the early distributors were a piece of junk. Do yourself a HUGE favor and get rid of the stock distributor. It will be the best money you've ever spent, guaranteed. I have a lot of customers call in asking how to resolve their carb issues, and the first thing I tell them is to replace the stock distributor. Nine times out of ten, a good ignition system will resolve their carb issues, or should I say, what they assumed were carb issues. If your going to replace the carb, why not start with the ignition system first, since it has to be replaced anyway. You may be surprised.

Autolite 1100 carb with a "Spark Control Valve" (circled)

CARB CHOICE
In general, if you want a little more performance, but still want to maintain throttle response and mileage, a smaller carb will work better. On the other hand, if your not overly concerned about mileage and/or driveability, and you want maximum top-end performance, a larger 2V carb may be a better choice.

Holley and Autolite carbs can be installed on stock or fairly mild performance motors using a 2-1 adaptor, however they may not be the best choice. While you might get a little more top end power, you can almost certainly expect to see hesitations and flat spots under hard acceleration, at lower rpm's, and off idle. They certainly won't be an out of the box installation, which means you'll need to re-jet the carb to get it to a point where it is satisfactory, if this is even possible. In fact, some suppliers won't even sell you a two barrel carb if they know it's going on a stock log style cylinder head via an adaptor. In most cases, Holley and Autolite carbs should only be used on large log cylinder heads, and preferably on a head that has had some work done to increase airflow, such as larger valves, porting, or a 2V Conversion. A performance cam and headers are also recommended, as well as milling the cylinder head (reducing the size of the combustion chamber) to increase the compression ratio.

A far better choice for stock or mild performance motors would be the progressive 32/36 Weber DGV, or the Holley/Weber 5200, which is a Weber carb manufactured by Holley. The progressive Weber is a two barrel carb, but it differs from Autolite's and Holley carbs, in that the secondary throttle plate doesn't open until the carb is at 2/3 throttle. The advantage of this is simple. As a progressive carburetor, it operates as a one barrel carb most of the time, with the secondary only opening when its needed, under hard acceleration and/or wide open throttle. This reduces the likely hood of hesitations and/or flat spots under normal operation, and it greatly improves throttle response and fuel mileage. Yet the added airflow of the secondary venturi increases acceleration, passing power, and overall top-end horsepower. If you want to maintain throttle response and fuel economy, while adding a little more top-end horsepower, the progressive Weber's are an excellent choice for stock or mild performance 170/200ci motors with a stock small log cylinder head.

If you insist on installing a synchronous carb (both throttle plates open at the same time) via an adaptor, on a stock or a fairly mild performance motor, you'll be better off using one of the smaller Autolite carbs or the Weber 38/38 DGS. You can also use a Holley 350cfm, but only when the carb has been properly tuned and adjusted, which includes the use of a custom metering block, which we'll cover in a separate tech article (coming soon).

The reason we like Autolite carbs, is because they are available in several different sizes, making them much easier to size to your motors specific CFM requirements. The Autolite 2 barrel (2V) carbs also incorporate Annular Fuel Discharge, which helps to diminish the effects of over-sizing the carb. In other words, while you may still have hesitation and flat spots under acceleration, they may not be quite as noticeable. In most cases, the 1.01 (240cfm) or 1.02 (245cfm) carbs work best. On the other hand, if you have a large log head which has been reworked and converted to a two barrel carb, what we call a 2V Conversion, the larger Autolite's and/or a Holley carb may work nicely, providing they are properly tuned.

It should be noted that there are kits are available to add annular discharge to Holley carbs, however they may diminish the CFM ratings slightly, as the annular boosters are larger in diameter and take up more space inside the venturi, thereby reducing airflow.

Holley 2V
Holley offers two sizes, 350cfm and 500cfm, which are available with manual, electric, or hot-air chokes. Both carbs are synchronous, which means both barrels open at the same time. Unfortunately this also means they are more prone to hesitation and flat spots under sudden acceleration, especially if your using a 2-1 adaptor. Therefore in most cases, the smaller the carb the better. However if your motor has a performance rebuild and/or a modified cylinder head (2V conversion), the likely-hood is greatly reduced or eliminated all together. It all depends on the CFM requirements of your specific motor. Normally the 350 has better throttle response and low end torque, with less top end power. The 500 offers more top end power, but it sacrifices lower end power, throttle response, and mileage, and there is a greater chance for hesitation and/or flat spots.

For more information on Holley carbs, please see our tech article (coming soon).

Autolite 2V
Autolite carbs, also synchronous, are available in seven different sizes, making them much easier to size to your motors CFM requirements. Ranging from 240cfm to 424cfm, they are available with manual, electric, or automatic hot-air chokes. Not only can you can select a smaller carb, which is better suited to your CFM requirements, they also incorporate a feature called Annular Fuel Discharge, which greatly reduces the likely-hood of hesitation and flat spots under acceleration. For stock or mild performance applications, we usually recommend the 1.01 (240cfm) or 1.02 (245cfm). For performance motors we recommend one of the larger Autolite's, a Holley, or the 38/38 Weber, depending on the situation, and the CFM requirements of your specific engine build.

For more information on Annular Fuel Discharge, please see our tech article on Autolite Carburetors. It should be noted that kits are available to add annular fuel discharge to Holley carbs, however the larger annular boosters slightly diminish the CFM ratings.

Weber 2V - 32/36 DGV or Holley-Weber 5200
Weber offers several carbs, in various styles (side-drafts, downdrafts, etc) and sizes, with manual, electric, or hot water chokes. The most commonly used carb on a stock or mild performance engine is the Weber 32/36 DGV, which is a progressive down-draft carburetor. As a progressive carburetor, it operates as a one barrel most of the time, using the smaller primary venturi during light acceleration and cruise. The larger secondary venturi only opens when it's needed, at 2/3 throttle, under hard acceleration and/or wide open throttle. This vastly improves throttle response and mileage, and it greatly reduces the likely-hood of hesitation and flat spots during acceleration.

Weber 2V - 38/38 DGS
If your not overly concerned about mileage and driveability, the synchronous 38/38 Weber is a great way to increase performance. The advantage Weber carbs have over other carburetors, is that they are infinitely tunable. In the hands of a qualified tuner the Weber carb can be tuned to out perform any other two barrel carb, when it is sized correctly for the application. The only drawback, is that it takes a qualified tuner to get them running at peak performance. Hence the reason so many backyard mechanics prefer the Autolite and Holley carbs. In the coming months (as time permits) we will be adding several tech articles on the Weber carbs, simply because very little information is available on the internet.

Classic Inlines stocks 32/36 and 38/38 Weber carbs, as well as carb adaptors, linkage kits, fuel regulators, and air cleaners. For more information on Weber Carbs, click here.

CFM RATINGS
One final point to consider is how 1V and 2V carbs are rated, as compared to how 4V carbs are rated. All one and two barrel carbs are rated at .3 inches of pressure drop through the carb, while all four barrel carbs are rated at .15 inches of pressure drop, which is more accurate. For example; a Holley 500-2V carb flows 380cfm when it's rated like a 4V carb. As such a 480-4V actually flows considerably better than a 500-2V (380cfm), even though the Holley 500 appears to be the larger carb.

NOTE: For more information on sizing your carburetor properly, please see our tech article on Calculating CFM.

INSTALLATION
The most common method for installing a two barrel carb is accomplished using a 2-1 adaptor. The adaptor merely changes the bolt pattern and bore size of a one barrel manifold so it accepts a two barrel carb. They are several different adaptors available, some work with Autolite and Holley carbs, others with Weber carbs. Carb orientation also varies depending on the design of the adaptor, however most adaptors install the carb with the throttle plates parallel with the crankshaft. Bottom line, make sure you purchase the right adaptor for your application. Classic Inlines now offers billet adaptors for most applications. If we don't have what you need, let us know and we'll see what we can do.

Note: Classic Inlines also offers phenolic spacers, or insulators, which provide a thermal barrier to prevent heat soak (transfer of heat to the carb). This is a common problem with our sixes, with the carb sitting directly over the exhaust manifold or header, as heat transfers upward to the intake manifold and carburetor, resulting in vapor locks and/or diminished performance.

The other option for installing a two barrel carb, is using a special conversion plate or adaptor, which allows you to hog out the intake making it a true two barrel opening. This method, called a 2V Conversion, requires machining the log intake. As such, the cylinder head must be removed for the modification, then reinstalled when the modification is completed. However this isn't all bad, as its a great time to install larger valves, new valve guides, hardened seats, and if your budget permits, a little port work . It's also a good time to CC the chambers and increase the compression ratio, which is accomplished by milling the head (reducing the size of the combustion chambers) and/or installing a thinner head gasket. You can also install deck the block and/or install flat top pistons to increase the compression ratio, however this requires pulling the motor and disassembling the short block. In most cases we recommend using dished pistons and zero decking the block on 200ci engines, as this improves the quench. However zero decking isn't possible on 250ci engines, as zero decking would result in compression ratios that are way to high. As such, we normally recommend flat top pistons for the 250ci, then decking the block and/or milling the head accordingly, to archive the desired compression ratio.