With a giant blower sitting atop your hot rod's engine, you're going to get more than your share of attention from enthusiasts and those who'd like to throw a huffer on top of their ride. But when it comes to superchargers, it might be easy for the average "Joe" to overlook what's feeding that big chunk of polished aluminum its requisite amount of fuel and air. We're talking about the guy that may want to take the plunge and cut a big hole in his hood, but more than likely doesn't know the ins and outs of how a blower works, let alone how the carburetors need to be set up. So this "Joe" character sends away for a fully polished 6/71 in hopes of pumping out mega horsepower.
He's told that he'll need a set of fuel meters made (or reworked) to work with a Roots-style blower. Being an intelligent person (obviously a SUPER CHEVY reader), he orders two Holley blower carbs. Sounds easy, but in reality there is much more to it than just bolting the four barrels on and hammering the go-pedal.
The real science is figuring out the correct combination of carb components (jet, accelerator pump, idle feed restricters, needle and seat size, air bleeds, emulsion main circuitry, venturri size, and so on) to get the desired results. This can be an overwhelming experience to say the least. That's just the start, now figure into your growing equation, blower specs, hp and torque, cylinder head flow capabilities, and so on.
Yeah, we know, it's hard enough to remember our own cell phone number without having to set up our own carbs, and this is why we contacted the gurus at The Carb Shop in Ontario, California, to help us understand just what is necessary when trying to build ultimate power from a blower motor. They were able to give us some insight to what makes a carburetor different when used on a supercharged engine.
The main idea here is to move a greater amount of fuel and air through the carburetor and still have a good deal of control over each. When the blower starts pulling on those carbs, it is going to use whatever the carbs give it. This is where the control factor is so important.
If you're planning to add a Roots-style supercharger (or even a centrifugal blow-through design), you need to learn about the specific requirements of the carb(s) you'll be using. Following are a few tips that O.J. of the Carb Shop taught us. Check it out and hold on when you mash that accelerator pedal.
Here are what jet extensions look like. These are simply used to make sure no matter where the fuel goes in the bowl, the jets will still be able to pick some up. Under severe acceleration the fuel obviously moves to the back of the bowl, thus they are only needed in the rear.
These holes are the emulsion circuit, or air bleeds. This is a very important adjustment because the air bleeds control how much air is mixed with fuel to emulsify or lighten the fuel to achieve the necessary air/fuel balance. The emulsion well holes also control how much and when the boost starts.
The Idle Fuel Restrictor controls the amount of fuel being delivered at idle. With a blower's needing more fuel, even at idle, this becomes a necessary adjustment. All of these changes are strictly done with a set amount depending on horsepower, size of blower, and carbs, in addition to about another half-dozen factors.
The Carb Shop builds its own annular boosters out of solid aluminum billet. The one on the left is the Super and the other is the Super Pro Billet Booster. These can be drilled to work with any application and have been used extremely well on the dyno and at the track. Since the booster controls the type of fuel spray, it can be necessary to alter them for any number of factors.
After the booster is pressed into its housing, it will need to be swedged back into the carb with this special tool.
With the boosters in place, both sides of the carb body are milled perfectly flat for a precise sealing surface. After this was done, the next step was to clean the edges of all holes with a countersink bit.
With all of the blower modifications complete, the carb can be reassembled. The Carb Shop installs only reusable gaskets. These carbs may need to be further adjusted internally to gain even more efficiency and power.
Here is The Carb Shop's finishing touch on a blower-ready carb. The next step will be to test it on the dyno and if everything is in order, ship to the waiting customer.