DRA September 2007
Split Carburetors Straighten Out Flow And Increase Horsepower
By John DiBartolomeo
As most any drag racers know, the straightest distance between two paths will always be the best route. This fact remains the same regardless of whether you’re talking about the actual quarter-mile, the route you take to the race track, or in this case, the path your air and fuel combination must take in order to get from your carburetor(s) into your cylinders.
| Split carburetors, or “splits” as they’re called, utilize the old adage that “the straightest distance between two paths will always be the best route.” By splitting a typical 4-barrel carburetor and moving them closer in line with the cylinder head intake port, it straightens out the air/fuel pathway, allowing for a better balanced and tuned engine. The end result: Horsepower!
Should the path of flow be under pressure, such as an exhaust port, a slight turn here or there is not that critical. However, in the terms of an intake port, where the air/fuel mixture must be pulled along by vacuum, the straight and true is always the best route.
Cylinder head manufacturers and head porters know this very well, and have machined/welded/cut many a cylinder head apart in an effort to straighten out flow characteristics. Of course, having to work around certain obstacles, such as pushrod clearance, water jackets and other things, have made their job rather complex. But through it all, straight has always been the number one priority.
So with a relatively straight cylinder head intake port, and an equally straight intake manifold, the next restriction had to come from the area below the carburetor. In most cases, when the air/fuel mixture drops out the bottom of a carb, the mixture has to make a turn to navigate its way into the intake manifold. Tunnel ram intake manifolds have long placed the carb higher up in an effort to make that turn as miniscule as possible, but still the turn remains.
Individual injection stacks are an attempt to place the throttle blades directly over an intake port and do a rather good job of it. However, fuel injection isn’t always the answer, and most people will continue to use a carburetor as their source of air and fuel. Therein lies the problem.
Doug Schriefer of Barry Grant says, “A traditional 4-barrel carburetor was built out of packaging convenience. The first engines started out with single barrel carburetors, and as they required more air, additional single barrel carburetors were added. Eventually, casting all 4-barrels into one unit made sense.”
|Even the best 4-barrel manifold tends to have four long and four short intake runners. A traditional tunnel ram two 4-barrel manifold moves the carburetors higher up and straightens out the intake runners somewhat, but still not as well as the use of a set of splits.
Marty Brown of Quick Fuel Technologies says, “With a conventional model 4500-style carburetor, the alignment of the throttle bores over an intake port became a problem. The only reasonable solution was to angle the carburetor to provide better alignment, but still the throttle bores were an unequal distance in relation to the intake valve. This required a lot of extra work in the intake manifold and plenum to get the air flow and distribution equalized.”
Therefore, some enterprising engine builder came up with the brilliant idea to “saw” a set of 4500-style carbs in half and place each (essentially) two-barrel system more in line with the runners of the intake manifold. Once the carburetor was cut in half, it was much easier to position the two throttle bores over a pair of cylinders. Having done so, the intake runners could be placed more upright, the runners could be positioned equal distance from each respective throttle bore and those intake runners became likewise the same cross-sectional area. Naturally because of this, the engine made more power. And isn’t that what it’s all about?
Brown says, “As a by-product of the intake alignment, plenum size could be reduced, so the signal through the carburetor is stronger, pulling both more air and fuel through each bore rather than pulling from a common plenum. As the split carb development evolved, manifolds have also changed, and the carburetor throttle bore and venturi have increased in size. By reducing the ‘restriction,’ it has helped to increase the power output which certainly pulls a lot more fuel through the carburetor as a by-product of the additional air flow.”
Unequal cylinder efficiency has also plagued engine builders. When one or more cylinders within the set of eight aren’t able
|As the name implies, splitting a carburetor first requires the use of a standard Demon or Dominator carburetor that’s sawed in half. The uniqueness of either carburetor is such that there are no fuel or air passages crossing between the front or rear two barrels. In essence, a 4-barrel Demon or Dominator carburetor is in actuality two 2-barrel carbs joined together.
to produce the same power as others, there becomes a potential problem. Or more accurately, it’s a more potential solution to horsepower should you be able to correct that deficiency. That’s the reason we see high horsepower output engines with intake ports spread apart. In the typical small or big block Chevy, there lies a “right and left” intake port. By way of cylinder head manufacturers spreading them apart, they end up with an equal set of intake ports within a set of heads. Equal ports eventually add up to equal horsepower derived from each cylinder.
Schriefer said, “The problem with a single 4-barrel intake manifold is that you’re going to have four long runners, and four short runners. There is no way to get an equal amount of air through these different length runners, so the dual 4-barrel intake was developed. This then helped equalize the airflow from cylinder to cylinder. However we still run into a few issues here.
“First off is dealing with all of the bends and turns between the base of the carburetor flange, and the intake valve,” Schriefer added. “To help straighten this out, cylinder head designers have rolled the intake ports on the cylinder heads of race engines to allow for a straighter path.”
Today we also have taller deck blocks, combined with cylinders that have been spread apart, in order to make room for a larger bore diameter. It wasn’t too long ago that the bore center of a race engine was in the 4.8-inch range. Now engine builders are faced with bore centers as much as 5.25-inches in an effort to accommodate large inch engines. However, by doing this, it spreads the ports even further away from a traditional carburetor.
|After sawing, the carb base is held in a fixture and flycut so that a smooth surface maintained in order to seal the backing plate.
“The more direct alignment of an intake port,” says Brown, “also helps sort out unequal cylinder efficiency much quicker since calibration changes for the throttle bore directly above a problem cylinder will have a direct positive effect on that cylinder. For example if cylinder No. 1 is running too hot (based on EGTs, plug reading, etc.), then changing the jet or air bleeds on the throttle bore directly above that cylinder will have a more predictable change and minimal impact on the adjacent cylinder, unlike should there be a significant shared plenum under the carb. This makes it a whole lot easier to balance out the cylinders and thus make more power with the engine.”
Obviously there are other tuning parameters that work in conjunction with the use of split carbs. In truth, like most other technological advances, it all has progressively evolved over time. Allied components, such as intake manifolds (runner cross-sectional area, configuration and plenum), hood scoops and scoop pans have likewise progressed as a compliment to split carburetor development.
Sonny Leonard of Sonny’s Automotive said, “It’s sometimes not uncommon for us to use air bleeds and jets that are quite a bit different from one another for each corner of the carburetor. We tune using three parameters, an O2 sensor, exhaust temperature and finally the actual plug reading. When the calibrations are correct with the electronics, you have to believe something such as the O2 sensor.”
Billy Carroll of Wilson Manifolds says, “The plenum volume needs to be considered when running split carburetors.
|In the case of the split Dominator, the backing plate produced also becomes part of the base plate for the back two mounting bolts.
Normally splits are used on larger cubic inch motors with a taller deck height and depending on the carburetor split and shift of the mounting, the plenum width might change by as much as three inches. We calculate the length and width and adjust the plenum height to compensate for the additional volume. The additional width of the plenum can also affect the width of the hood scoop needed to clear the carburetors and linkage. Any added width there will lead to additional (and unwanted) aerodynamic drag.”
The “splitting” of a carburetor might seem rather easy, but it does become a very labor-intense project. A 4-barrel casting is cut in half, after which time a backing plate must be machined and screwed on. In the case of a Barry Grant/King Demon carburetor, which uses bolt on base plates, a separate base plate is manufactured specifically for the splits.
In the case of one of Quick Fuel’s splits, the backing plate also becomes part of the base plate. In either instance, quite a bit of work is required to build a “split”. The carb must also be matched to the engine combination it’s intended to be used on. This is definitely not a case of simply pulling a carburetor off the shelf and installing it on the car.
However, when you’re looking for every ounce of horsepower from a particular engine combination, the use of a set of splits should certainly do the trick.
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