Classic Inlines - Selecting the right carburetor for your inline six

Choosing the right carburetor is critical if you want to achieve the maximum power potential of your inline six, as such it's one of the most important decisions you'll make. Selecting the wrong carburetor, to big or to small, decreases power and driveability. An improperly sized carb may also result in hesitations and/or flat spots, poor idle quality, reduced mileage, and increased emissions. The most common mistake when building a performance motor, V8 or Inline, is selecting a carb which is simply to big (or oversizing the carb). Don't fall into this trap, bigger is not better.......

The idea is to choose a carburetor that has a large enough bore and venturi to allow maximum airflow, yet one that's small enough to maintain a high-speed airflow signal through the primary venturi(s), which the carburetor reads in order to determine how much fuel is required to achieve the proper air/fuel mixture. Lets say that again..... The idea is to choose a carburetor that has a large enough bore and venturi to allow maximum airflow, yet small enough to maintain a high-speed airflow signal through the primary venturi(s), which the carburetor reads in order to determine how much fuel is required to achieve the proper air/fuel mixture.

In theory, if the bore and venturi are too large, the signal will be weak at low engine speeds. As such, fuel will not be pulled from the carburetor's discharge ports accurately and the air/fuel mixture will be lean. The opposite of this effect, is when the carburetor's bore and venturi are too small for the engine's airflow demands. While throttle response will be outstanding, the engine's maximum power potential will be diminished. The ideal carburetor will move the maximum amount of airflow through the carburetor and intake manifold, while maintaining the strongest possible signal. This was very evident when we spent a day at Pony Carbs, testing various carbs on a small six (see the tech article on our dyno results).

It's important to use a CFM calculator to select the proper size carburetor for your CFM requirements. However, the calculated results are just that, calculated. Once you have done the calculations, consider the following "Rules of Thumb" before you make your final decision. For street motors, its better to start with a smaller carb, for more torque. On the other hand, it's preferable to use a bigger carb for race applications, which give you maximum power at wide open throttle. Street/strip motors require a carb sized somewhere in-between.

First, lets go over your carb choices for a one barrel. In the early years Ford used Holley-1904-1908-1909 carburetors from '60-63, and the more common Autolite 1100 from '63-69. All of these carbs had 1-7/16 inch carb bores. The Autolite 1100's were offered on the 170 and 200ci, with three venturi sizes (1.00/1.10/1.20). Mustangs normally had the 1.10 or 1.20 depending on the model year, configuration, and engiine size. In '69 they switched to the larger Carter YF carbs, which first saw use in California during the '67-'68 model years, to lower emissions, however '70-'73 models used the more common Carter RBS, which came on the 250ci engine. The 1101, YF, and RBS carburetors had 1-11/16 inch carb bores.

The Autolite 1100's are fine for stock applications, but they obviously come up short in the performance arena. Many inline enthusiast prefer upgrading the early log heads and smaller Autolite or Holley carburetors to a late model cylinder head with the larger Carter-YF and RBS carbs. While this increases the engines power output, most of the additional power is the direct result of the of the late model large log cylinder head, with it's larger valves and increased manifold volume, rather than the carburetor. However the larger carburetor, which increases CFM, certainly doesn't hurt.

While Autolite carbs are commonly considered to be the best carbs ever built, the Autolite 1100's were designed and manufactured before Annular Fuel Discharge technology was developed. As such the Autolite 1100's don't measure up or perform nearly as well as their bigger brothers (2100/4100 series carbs), which incorporate annular discharge boosters. The early Autolite 1100's (63-67) also used a Spark Control Valve (SCV), which was designed to work hand in hand with the Load-O-Matic (LOM) distributor. However, the later 1100's (68-69) were designed without a Spark Control Valve. Please read our tech article on the Load-O-Matic distributors for more information on this subject, as it's important to know how they operate, especially if you are planning a carb swap.

Another option for a 1V carb is the new "Vaporizer" carburetor, which was introduced by Pony Carbs in 2008. Basically the Vaporizer is an Autolite 1100, however it has been redesigned to incorporate Annular Fuel Discharge. This gives the new Vaporizer superior performance, drivability, and mileage, over the original 1100 series carb. The Vaporizer is rated at 200CFM and offered with or without the Spark Control Valve. It also comes with an automatic (hot air) choke and an optional extra long header choke tube, which is needed if you install aftermarket headers. This hollow aluminum tube clamps to the outside of the header with a hose clamp, and provides hot air to the automatic choke.

For stock applications or mild performance engines, the Vaporizer is an excellent choice and tough to beat. Not only will you realize an increase in horsepower, you should also see small gains in your gas mileage. Both are a result of the Annular Fuel Discharge boosters, which atomize the fuel better than the older carbs ever dreamed of doing, especially at idle and lower rpm's. Dyno test have shown the Vaporizer to increase power by as much as 20HP, however for most applications one should expect to see gains around 10-15 HP.

As we said earlier, the stock Load-O-Matic distributor is designed to work with carburetors that have a Spark Control Valve. Like the stock Autolite 1100's, the Vaporizer comes with, or without the Spark Control Valve. Therefore, if your planning to keep the stock Load-O-Matic distributor, you'll need to order the early (63-67) version. But if your going to swap out the distributor with a Duraspark or DUI, you'll need to order the late (68-69) version.

One of the most popular modifications for increasing horse power and performance, is to install a two barrel carburetor on the existing log manifold. This can be accomplished with a 2-1 adaptor, or by machining and modifying the log manifold. Each method has advantages and disadvantages, so you'll need to know the differences to make the best decision.

The most common mistake when installing a two barrel carburetor, is installing a carb which is simply too large for the application. The engine might actually make less power, and may stumble and fall flat on its face when you push the peddle to the medal. Most DIY, backyard mechanics simply ignore CFM calculations and install the biggest two barrel carb they can get their hands on, thinking they are going to haul ass once it's installed. They couldn't be more wrong, and they are most certainly going to be disappointed in the results.

CARB CHOICE
In general, Holley and Autolite carbs don't work well on stock or fairly mild performance motors, when using a 2-1 adaptor. While they may give you 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, and you'll probably need to play with the carb quite a bit (re-jetting), to get it to a point where it is satisfactory. If this is even possible. In fact, Pony Carbs won't 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, etc). A performance cam and headers are also recommended, and it's a great time to increase the compression ratio by milling the cylinder head.

A far better choice for stock or mild performance motor would be the progressive 32/36 Weber DGV, or what some call the Holley/Weber. The progressive Weber is still 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. The progressive Weber 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 additional venturi increases acceleration, passing power, and overall top-end horsepower. In short, the progressive Weber's are an excellent choice for stock or mild performance motors, if you want the maintain throttle response and fuel economy, while adding a little more top-end horsepower.

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'd be much better off with an Autolite carb for two reasons. First and foremost, the Autolite's are available in several different sizes, making them much easier to size to your motors specific CFM requirements. We recommend either the 1.01 (240cfm) or 1.02 (245cfm) carb. Secondly, Autolite carbs incorporate Annular Fuel Discharge, which helps to diminish the effects of oversizing the carb. In other words, while you may still have hesitation and flat spots under acceleration, they may not be quite as noticeable. 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.

On the other hand, if you have a large log head which has been converted to a two barrel carb, what we call a 2V Conversion, the larger Autolite and Holley carbs will work nicely. If you want a little more performance, but still want to maintain throttle response and mileage, the smaller carbs work better. On the other hand, if your not overly concerned about mileage and/or driveability, and you want maximum top-end performance, the larger carbs will work better.

REMEMBER: The idea is to choose a carburetor that is large enough to allow maximum airflow, but small enough to maintain a high-speed airflow signal through the venturi(s). Use a CFM Calculator, then select an appropriately sized carburetor. In general, smaller carb for driveability and mileage, larger carb for top-end performance. It's that easy.....

Holley:
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 even 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 lower bottom end power, reduced throttle response, and a greater chance for hesitation and/or flat spots.

For more information on Holley carbs, click (tech article coming soon).

Autolite
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 normally recommend one of the larger Autolite's, based on the CFM requirements of your specific engine build, however we may recommend a Holley or Weber carburetor depending on the situation. 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. For more information on Autoilte 2V carbs, click here.

Autolite 1V Carbs		Autolite 2V Carbs
Size	CFM	Size	CFM	Size	CFM
1.00	120	.098	190	1.14	300
1.10	150	1.01	240	1.21	351
1.20	185	1.02	245	1.23	356
Vaporizer	200	1.08	287	1.33	424

Weber or Holley-Weber
Weber offers several carbs, in various styles (sidedrafts, 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 32/36 Weber 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.

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, given 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.

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 500cfm is rated as 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 of installing a two barrel carb is accomplish using a 2-1 adaptor. The adaptor merely changes the bolt pattern and bore size of a one barrel carb to that of a two barrel. 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 makes phenolic spacers, or insulators, which serve two purposes. First, they help to funnel the air in our 2-1 adaptors, for improved air flow. Secondly and more importantly, the phenolic material provides a thermal barrier to prevent heat soak, which is a common problem on our inlines. With the carb sitting directly over the exhaust manifold or header, heat transfers upward to the 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. This method, which is called a 2V Conversion, requires modifications to 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 guides and seats, do a little port work, and CC the chambers. It's also a good time to increase the compression ratio, which is accomplished by milling the head 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 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 the block 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.

The 2V Conversion method is by far the best way to install a 2V carb on a log intake manifold. Regardless of which carb you select, the power gains are substantially increased when the log intake is opened up to accommodate a two barrel carb. Classic Inlines did a lot of dyno testing last summer and the results verified our suspicions. Adding a 2V carb via a 2-1 adaptor generally netted 4-5HP, while a converted head bumped the power by 20-25HP, or more. That's a huge difference considering we used the same carburetors. The only difference was the method used to mount the carbs. For more information, see our tech articles on 2V Conversions and Dyno Testing.

In the near future Classic Inlines plans to offer modified large log cylinder heads, which will be rebuilt and converted to use a two barrel carb. You can save a $400 core charge by sending us your cylinder head, however keep in mind that we only modify large log heads. Classic Inlines also offers a wide range of services, including new or oversized valves, unleaded valve seats, valve guides, valve springs, retainers, viton seals, and freeze plugs. We also do multi-angle valve jobs and back cutting, CCing and reshaping the combustion chambers, 2V Conversions, porting and polishing, flow testing, and any other machine work required. We also offer furnace brazing, aluminum and iron welding, and we can repair most cracked or damaged cylinder heads. All cylinder heads are cleaned, hot tanked, and magnafluxed before we start work, and pressure tested when the work is completed.

If your building a stock or mild performance motor, and you want to maintain the stock appearance, the Vaporizer can't be beat. This is especially true if your building a concours restoration, where you need to keep the motor stock, or when you don't want to modify (temporary or permanent) the existing carb linkage or a numbers matching cylinder head.

If your building a stock or mild performance motor, and you want the added punch of a two barrel carb, there are a couple ways you can go. The Progressive Weber is a good way to go when you want to maintain mileage and throttle response. The only modification required to complete the install, is to modify the existing carb linkage, however that's usually pretty simple. A lot of guys just switch over to a throttle cable. The only problem is that no one makes a kit, yet. If you leaning more towards performance, rather than mileage and/or driveability, then the 38/38 Weber, or a smaller Autolite or Holley carb will work great.

If your building a performance motor, and you can't afford the OZ head or our aluminum head, a late model, large log cylinder head with a 2V conversion is the only way to go. The best way to do this, is to purchase a second cylinder head, and work on it as time and money permits. This way you won't be rush and wind up taking short cuts, and you can work on it when your pocket book allows, rather than trying to do it all at once. If you already have a carb sitting on the shelf, grab a 2-1 adaptor and install the 2V carb on your existing log head, while your working on the conversion head. The experience you'll gain tuning the carb will be invaluable when you go to install the converted head. And if you get frustrated tuning the 2V carb, or run out of time, you can simply remove the 2V carb and adaptor, reinstall the one barrel, and go back to it when you have more time and patience. For carb selection, a larger Autolite, a Holley 500, or the 38/38 Weber, are recommended. Another option is to go with multiple carbs. Maybe triple one barrels, or twin Webers?

Finally, if your building a performance motor with an OZ-250-2V head or Classic Inlines aluminum head, you have several options. You can use a large two barrel carb, a four barrel carb, twin two barrels, triple two barrels, triple one barrels, a throttle body, or multiple throttle bodies. The sky's the limit, providing you have a large pocket book.

If you really want to add some punch to your small six, add a turbo or super charger. Of course that's a whole new subject, which requires a tech article all in it self.