One of the most popular modifications to increase the performance of the small inline six, is to replace the stock one barrel carb with a two barrel. In this article we discuss the different ways to install one, carb selection, and the results you can expect to get.

Installation Method

2V Adaptors:
The easiest and cheapest way to install a two barrel carb is by using a 2V carb adaptor, which can be purchased from numerous suppliers, including Classic Inlines. The 2V adaptor is a bolt on modification, which works by funneling both barrels of the carb into the one barrel hole in the manifold (as shown in the picture below). One company claims gains of 20HP are possible, others says you can expect small improvements in gas mileage and throttle response, and a slight gain in performance. However another company states the 2V swap is a waste of money, offering little or no benefit for the money spent. In their opinion the swap will, in most cases, result in hesitation and flat spots during acceleration, with little or no gains in performance. Classic Inlines has yet another opinion, however our thoughts are on a slightly different path. We question the value of the modification, rather than the validity. In other words, we believe additional power gains may be possible using a 2V adaptor, but question the amount of power gained verse the amount of money spent? Simply put, are the gains worth it or is the money better spent elsewhere, as in high ratio rocker arms?

2V Conversion:
The other way to install a two barrel carb, is what we call a "2V Conversion". This method requires removing the cylinder head, milling the intake manifold, opening up and widening the carb bore, installing a 2V conversion adaptor (as shown in the picture below). While this method offers huge benefits over the 2V adaptor, it also has a big drawback, as in removing and re-installing the cylinder head. Of course, if your rebuilding the cylinder head or the motor, then the only drawback is finding a machine shop that's willing to do the modification. Most shops charge anywhere from $100 to $200 to do the conversion (labor only). Some shops may offer to make an adaptor, but usually charge more to make it, than what you'd pay for it online. You'll also have to figure in the expense (parts and/or labor) to R&R the cylinder head, which makes this method more costly than the 2V adaptor, and more labor intense. However I think you'll understand why we recommend this method, over the 2V adaptor, by the time you finish reading this article.

NOTE: For more information and "how to" details, please read our tech article.

2V Adaptor (Weber carb)	2V Conversion (Holley or Autolite)

Carb Choice

The next item to consider is carb choice. Not only do you need to decided the size, or CFM rating, you also need to determine which manufacturer makes a carb best suited to your needs. The three most popular 2V carbs are Autolite, Holley, and Weber (or Holley-Weber). Each has advantages and disadvantages over the other, so it's very important to know the differences between them, and the applications they are suited for.

Holley:
Holley only offers two sizes, 350cfm and 500cfm, which are available with manual, electric, or hot-air chokes. Both are synchronous, which means both barrels open at the same time. Unfortunately, this also means they are more prone to hesitation and flat spots during quick acceleration, especially if your using a 2V 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 likelyhood is greatly reduced or eliminated. 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. While the 500 offers more top end power, it offers less bottom end power, reduced throttle response, and a greater chance for hesitation and flat spots.

Autolite:
Autolite carbs are available in seven different sizes, making them much easier to size to your motors CFM requirements. Ranging from 240cfm to 351cfm, they are available with manual, electric, or hot-water chokes. For most applications we recommend either the 240 or 245 CFM carbs. Autolite carbs are also synchronous, however the likelyhood of having hesitation and flat spots is greatly reduced. Not only can you can select a smaller carb, which is better suited to your specific CFM requirements, they also incorporate a feature called Annular Fuel Discharge. For more information, see our tech article on Autolite Carbs.

It should be noted that there are kits are available to add annular discharge to Holley carbs, however they usually diminish the CFM ratings slightly, as the annular boosters are larger in diameter.

Weber/ Holley-Weber:
Weber offers several carbs, in various styles (sidedraft, downdraft, etc) and sizes, with manual, electric, or hot water chokes. The most commonly used carb on a small six is the 32/36 DGV, which is a progressive down-draft. A progressive carb operates on the smaller primary venturi most of the time, during light acceleration and cruise. With the larger vacuum operated secondary only opening when required, under hard acceleration or wide open throttle. This improves throttle response and mileage, and greatly reduces the likely-hood of hesitation and flat spots during acceleration, as the secondary only opens enough to supply the air flow needed at that moment.

NOTE: For more information on carb selection, please read our tech article.
NOTE: For information on Calculating CFM, please read our tech article.

Value (gain verses cost)

Which method gives you the best bang for your buck? If your just comparing HP gains to cost, this is a no brainer. But what if you want to take mileage and driveability into account? I like to make a list, with the most important item on top and the least important on the bottom, then compare the results to figure out which modification best suits my needs.

Horsepower Ratio: This is an easy one, add up your total cost and compare it to the HP gained. For example: If a modification results in 5 HP, at a total cost of $500 dollars, the ratio would be 1HP per $100 dollars.

Mileage: This ones a little tougher, as you won't know the increase in mileage until the modification is done. However you can use estimations or ask other guys that have made a similar modification and get their results. You can also calculate the payback period, or the money saved per year, for comparison.

Driveability: This one is a given for the most part. Either it's important, or its not. Someone looking for maximum horsepower, as in a race car application, has no problem with a motor that idle rough, has poor throttle response, or one that stumples under quick acceleration. All they are concerned with, is how it responds at full throttle.

In summary
When we compare the numbers from dyno #3A, verse dyno #3B, we see some very interesting results. First and foremost the advantage of mounting the 2V carb directly to the intake, verses using a 2 to 1 adaptor, are quite evident. We gained a maximum of 5RWHP when we used an adaptor, and 24RWHP when the carb was mounted directly to the log intake, or a gain of 6% verses 24%, respectively.

All three carbs produced nearly the same maximum HP when using the 2V adaptor, with the Holley producing the highest average HP, besting the Autolite by 4HP and the Weber by 2HP. Both the Weber and Autolite carbs accelerated smoothly, while the Holley had a definite stumble around 2100 rpm. The Holley 350cfm may be just a tad bit to big for a stock 250ci, while the Autolite 240cfm was probably a little small, which would explain the lower HP gains. The Autolite also peaked sooner than the other two, which are also signs that it was a bit to small.

Prices on the Weber carbs can vary greatly from one supplier to another, so check around. We have found that Langdon's Stovebolt offers the best price on the Weber Carbs, and the adaptor needed for this swap. Holley and Autolite carbs can be purchased from several online suppliers, including Classic Inlines, or you can buy a used carb at just about any swap meet for $50-$100 bucks.

All things considered, we feel the Autolite carbs are the best solution for those using an adaptor, or directly mounted, providing they are sized properly. Not too small, but not to big either. The Autolite's, unlike Weber and Holley carbs, come in seven different CFM ratings, so it should be pretty easy to match the proper sized carb to your specific engine build. Plus they are the only carbs that were designed with Annular Discharge, which we feel is far superior to other designs. However, that's just our opinion. They also have a one piece float bowl and main body casting, which eliminates leaks and maintenance.

Annular fuel discharge, by definition, is where the fuel is discharged into the bore of the carburetor from a series of small holes around the perimeter of the booster venturi, rather than a single hole. This method of discharging the fuel breaks up and partially atomizes the fuel for better throttle response, improved fuel economy, and more power. It's also as close as you can get to fuel injection, using a carb.

Dyno: #1 - #2 - #3 - #4 - #5 - #6 - #7 - #8 - #9 - #10 Track: #1 - #2 - #3 - #4 - #5 - #6 - #7 - #8 - #9 - #10