How CSU Builds Blow-Through Carburetors

All gearheads think about adding more performance to their machine, and in the case of adding boosted power to a carbureted engine, you’ve got your choice of turbocharging and supercharging. Turbocharging works best upon fuel-injected engines, but supercharging has also been a fave of the carb’d crowd. A centrifugal supercharger is one of those power-adders that plants you back in the seat with an ever-increasing pull from idle to redline, but having a supercharged, carburetor-equipped engine that performs properly both on the street and at the track has sometimes been a dicey proposition.

If you’ve got a supercharged blow-through combination and plan to street-drive the car with some regularity, good manners and high-RPM fueling both have to be taken into account when preparing the carburetor.

Here's what we started with - a box-stock Holley 750 HP Ultra carburetor and a list of hopes and dreams for CSU to fulfill.

Here’s what we started with – a box-stock Holley 750 carburetor and a list of hopes and dreams for CSU to fulfill.

Kevin Van Noy and Dennis Carlson at Carburetor Solutions Unlimited (CSU) are just the guys to achieve exactly that. The team have been building carburetors together for over a decade and have built a following of customers that have had success in many forms of motorsports.

Once disassembled, all parts are laid out on the table. Some of them will be reused, some will be modifed to CSU's specs, and some will be discarded.

Once disassembled, all parts are laid out on the table. Some of them will be reused, some will be modifed to CSU’s specs, and some will be discarded.

With that in mind, we headed out to CSU’s Fontana, California facility where Van Noy took a box-stock Holley 750 cfm Ultra Double Pumper carburetor and prepared it with all of the latest blow-through carburetor tricks to work well with our combination. “You want to treat a carburetor like this just as they are. When you’re down low, just driving around, when you’re idling, you’re just a naturally-aspirated engine with a drag on the crank, like a big A/C compressor, basically. You have to have a balance, where the carburetor is clean during driving and be able to transition to wide-open throttle as it transitions into boost. Once the car transitions into boost, you need to ensure there is enough fuel to keep the engine happy,” Van Noy explained.

In order to have all of those things take place as they need to within the carburetor, the CSU team goes through and modifies nearly every single aspect of the carburetor.

Before we even got started and because we had a fresh-out-of-the-box carburetor, Van Noy needed to disassemble it – and we ended up with a pile of parts on the table that looked like they may never resemble anything race-ready again, but he assured us that it was just part of the process.

Left: In the first step after disassembly, Van Noy drills out the throttle shaft bores for bronze bushings. In his experience there is too much play from the factory, and this is just one of the steps he completes in the blueprinting of the carburetor. Right: The bushing in place. The secondary throttle shaft end has been modified and is no longer protruding through the baseplate.

Once the shaft bushings were installed, Kevin replaced the throttle shafts and began working on the rest of the carburetor modifications – changes learned from years of testing and research with all sorts of different combinations, from drag racing to off-road and marine applications.

The main body takes a trip through the mill to remove the choke horn for better airflow. In an application like ours that will reside solely in southern California and won't ever need to use a choke, along with a racing application, it's completely unnecessary and only serves to hinder performance. Airflow is affected into the critical venturi area as well.

Left: The factory hollow brass floats are removed in favor of CSU's Nitrophyl pieces. These are required in a boosted application so they do not collapse with boost pressure. Right: By slightly opening up the venturi only at the top side of the hole, it helps with airflow over the venturi and improves atomization.

One of the important steps that he performed for us was to remove the choke horn. “Getting the horn out of the way in this application is critical. With the air bonnet on top of the carburetor, the choke housing is going to be a serious restriction to airflow. We knock it down level to where the gasket sits underneath the bonnet,” he said.

When you’re down low, you’re just a naturally-aspirated engine with a drag on the crank. You have to have a balance, where the carburetor is clean during driving, and be able to transition to wide-open throttle. – Kevin Van Noy

The factory brass floats are discarded and replaced with floats that are constructed of a solid Nitrophyl material. Kevin said, “The stock brass floats are hollow, which will implode when subjected to boost pressures. The Nitrophyl floats are solid and will not compress in those conditions,”explained Kevin. Since we’ll be pushing upwards of 20 pounds of boost pressure, this modification is absolutely necessary.

CSU also uses custom power valves that are billet and built to their proprietary specifications, which requires the floats to be clearanced, as the power valve has an extension tube that projects through where the float would typically ride. “Power valves are used on both sides of the carburetor so that we can draw a little bit of fuel from each side, and that allows us to stagger the opening points to gain more tunability from the carburetor,” he explained. In our application, he installed a front power valve that opens with five pounds of boost pressure, while the rear valve opens at eight pounds of pressure. This helps to keep the engine clean down low and in no-boost conditions, while enriching the tuneup as boost pressure comes in.

As the Holley main body is a cast piece, Van Noy takes the extra time to remove the casting flash from the tops of the venturi and smooth out the bores a bit, as this helps to clean up the incoming airflow before it gets to the booster level.

Speaking of boosters, CSU removes the stock downleg boosters from the main body and wedges a set of their custom billet annular boosters in place. The annular-style booster is much more sensitive to the metering signal, which helps with tuning, in addition to making more horsepower. The small holes in the annular booster (as compared to the one large hole in a downleg booster) produce a smaller fuel droplet and provide for better overall atomization.

Proprietary billet power valves are installed in both end of the carburetor. The rear metering block receives jet extensions as well as a power valve extension tube.

The stock downleg boosters are moved in favor of a two-piece, billet annular booster. These are specifically designed for CSU’s specifications.

Other tricks employed include opening up the main well exits to ensure that there is no restriction there for the amount of power we’re trying to make with the engine, along with replacing and resizing the idle feed restrictors. “There are a million different combinations as far as idle feed restrictors and idle air bleeds, and we have found a range of combinations that work really well,” Kevin said.

The change to a 50cc accelerator pump arm is also made for a number of reasons, chief among them the strength of the lever design so the arm does not flex back under boosted conditions.

Left: Van Noy explained that boost pressure can sometimes cause the stock 30cc accelerator pump arms to bend (bottom), so he installed these super-thick 50cc pieces for us (top). Right: Modifying the metering block for proper fueling at idle - these holes are re-sized to a dimension CSU has established through years of testing and refinement.

There are a couple of other proprietary modifications made to the carburetor, especially with respect to the idle air bleeds and high-speed air bleeds, and then it’s time to put everything back together.

Van Noy stressed that it’s really the entire package built with years of tuning expertise, and not one specific modification that makes a “blow-through” carburetor work properly.

Converting your standard Holley carburetor to blow-through spec will cost you between $500 to $1,500, depending on application. If you do not have a core, Van Noy can build one for you, for typically a little less than the cost of a new carburetor. There is much more going on with a blow-through carburetor versus a standard naturally aspirated carburetor and one should consult a professional, like CSU, to build an application-specific version that will require minimal tuning once bolted onto the intake manifold. This will allow you to get on the track and enjoying your car faster!

The final steps of reassembly - is setting a zero lash on the booster arm.

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About the author

Jason Reiss

Jason draws upon nearly 15 years of experience in the automotive publishing industry. Collaborating with many of the industry's movers and shakers assists him in the creation of compelling technical articles and high-quality race coverage.
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