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Building A Blow-Through Carburetor - Feeding The FireBuilding A Bulletproof Blow-Through Carburetor From the June, 2010 issue of Chevy High Performance By Henry De Los Santos Photography by Henry De Los Santos
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When it comes to supercharging, carburetors have been given an ill rap for a number of years-at least until recently. Most naysayers would spew out the negative connotations with little knowledge of how carbs really worked. The truth of the matter is, carbureted blow-through technology is much more advanced than they're given credit for and The Carb Shop has proven time and time again that its carburetors flat-out work. If you weren't aware of how far back this technology goes, we'll start by saying The Carb Shop first tried the blow-through setup back in 1977 on a twin-turbo boat application. In the automotive sector, it was in the early '90s they outfitted a '69 Camaro with a ProCharger system. On the dyno, the numbers were impressive, but The Carb Shop later learned through extensive testing on the road and at the dragstrip that it's important to size up the intercoolers properly, as they themselves can be an added restriction. Over the years, they've had an extremely successful track record with a number of fast heads-up cars, all of which has trickled down to the street-level consumer. Can you say 1,000-plus horsepower with no driveability issues whatsoever? For those of you who are still up in the air, we're more than happy to report that we've used nothing but The Carb Shop's carburetors on our blow-through builds, including a 383ci small-block along with a 496ci and a 502ci big-block, all of which have generated big numbers on the engine dyno. Once it's down to the bare... Once it's down to the bare housing, every carb is checked for casting flash. In our case, everything looked good (no flash), so on to checking the sides for a smooth surface. This month, we're going to go through the motions of how these carbs are built; granted, some of it's proprietary information, so you won't get the exact dimensions on certain components. However, it's a good overview to showcase the amount of detail that goes into them. If you're looking for a complete kit to build one for yourself-you can forget it. Honestly, there are way too many variables and you're much better off purchasing one custom-built for your application. For those of you who are looking to put in your orders, expect to wait 7 to 14 working days and pricing is contingent on the power level you're trying to achieve. Of course, you can save a few bucks if you already have a carburetor to send in, but even if you don't, we're sure you'll agree that the price is more than reasonable for the results that can be achieved. Quick Notes
What is it
Custom-built blow-through carburetor by The Carb Shop What we did
Modify a Holley 950 HP Price (Approx)
Starts at $480 if you supply the carb; $1,600 for a ready-to-run, custom-built piece  Most of the blow-through setups...  Most of the blow-through setups are built with these annular boosters, which have been proven to move a lot of air. The signal is good, and responds well to jet changes. For lower-horsepower applications in the neighborhood of 450 hp, The Carb Shop uses dogleg boosters in the primary with the annulars in the secondary. For 700-plus-horsepower applications, they use nothing but the annulars.  Both primaries and secondaries...  Both primaries and secondaries are installed using a special tool. This tool secures the booster in place by expanding its pot metal.  With the boosters in place,...  With the boosters in place, the body is then machined to eliminate any rough edges from the sides. Once completed, the top and bottom areas were also checked to ensure a perfectly flat surface; Often times, cutting 0.007-inch seems to be the norm, but this will also promote a better seal for the gaskets. Note: this process is vital on older carbs, especially since you never know how much the bolt holes have been pulled in from constantly removing the bowls. This ensures no leaks, which is detrimental to a blow-through setup.  For the next step, each hole...  For the next step, each hole is deburred for a nice, smooth edge.  A little epoxy is used on...  A little epoxy is used on the edge of the new boosters to keep them in place.  Next, the throttle shaft entrance...  Next, the throttle shaft entrance on the base was machined. The machined portion will allow seals to be pressed in, keeping the fuel from getting out under boost. Without this proces when lifting off the throttle, the leftover pressure will force fuel out of the throttle shaft. People tend to think it's leaking, but the fuel is actually forcing its way out. Also, it's a good idea to spray a little WD-40 occasionally to help keep the seals fresh. With the seals in place, the throttle shaft itself has to be machined on both ends to fit properly.  During reassembly, the factory...  During reassembly, the factory throttle blades were swapped out with a set from The Carb Shop that eliminates the holes in the factory setup. Originally, the holes were there to make tuning easier by the idle circuit and helped the transition to be a little cleaner, especially to keep the end user from opening the throttle blades too far. Removing the holes makes the idle air bleed a bit more sensitive but allows the caburetor to be custom tailored based on engine specifications.  From here, it's time to secure...  From here, it's time to secure the base back on by placing the six Phillips screws with Loctite.  Here, the idle fuel restrictors...  Here, the idle fuel restrictors are removed, which gives a general fuel curve with a little adjustability. In place of it went in an allen set screw that will allow for additional adjustments when needed.  Another modification is to...  Another modification is to change the size of the main well where the fuel is emulsified. This allows the carb to move the amount of fuel it needs. Think about it, these carbs were never designed to support 2,000 hp.  At this time, the booster...  At this time, the booster exit on the metering block is plunge-cut for a smoother transition.  To reassemble the carb, new...  To reassemble the carb, new air bleeds were installed, featuring a different orifice size that are calibrated specifically for our engine combination.  Idle mixture screws were put...  Idle mixture screws were put back into the metering block. To set them, they're first turned all the way in and then backed out by a turn and a half as a starting point.  Next, the four power valve...  Next, the four power valve restrictors were enlarged to a proprietary size, one for each main jet.  From here the boost-activated...  From here the boost-activated power valves were installed. Unlike standard power valves, which work off of vacuum, the new power valves are boost activated, allowing a more linear fuel curve as the boost comes in.  The needle and seat used is...  The needle and seat used is different in where the fuel exits. The standard design has the fuel coming out of the window just below the seat (right), where as the down-feed design (left) exits the fuel as it sounds-from the bottom. The needle then rides on the float and helps minimize aeration.  To provide maximum fuel under...  To provide maximum fuel under hard acceleration, The Carb Shop utilizes a press-fit power valve extension that gets its fuel from the back of the bowl, similar to jet extensions. Below that are the two jet extensions. The horsepower level and engine combination determine which jets are placed into the metering blocks; in this case, 84/89 jets were installed as a starting point for its new home-a 540ci big-block.  The billet aluminum float...  The billet aluminum float bowls shown here are unique in that they feature AN-6 fittings, dual needles and seats with dual floats, and significantly larger fuel bowls to prevent starvation.  As a comparison, here's the...  As a comparison, here's the latest design that The Carb Shop will be converting over to. Mechanically, the newer design works the same; the change is simply for a smoother modern look.  With both bowls installed,...  With both bowls installed, the upper AN fittings and braided-stainless line are put in place. This is the balance tube, which helps to equalize the float bowl pressure.  Before leaving the shop, every...  Before leaving the shop, every carburetor is tested with the in-house mule, a 532ci big-block with a ProCharger F2 huffer. Old reliable here has been proven to hit 1,100hp figures with ease. They also have a 535ci big-block with a set of Brodix heads that generates 1,660 hp around 7,500 rpm. Boost levels are around 16 to 17 psi for the 532ci, whereas the 535ci peaks in around 18 psi.
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