Annular Vs. Dogleg Carb Showdown

Numbers Game
To begin with, the downleg Demon flowed an amazing 65 cfm better than the annular carb at 6,700 rpm and flowed an average of 19 cfm more air throughout the entire test (even though the annular carb used a larger venturi and butterflies). This is because of the restriction that the larger annular boosters present to the carburetor. Generally, the carb that flows more air will also flow more fuel to match that airflow. However, the annular-discharge carb flowed more fuel and also made more power. The weird part of this is that when we tried to jet the downleg carb richer to approach the fuel curve generated by the annular carb, the engine always lost power.

This requires some explanation. While airflow through an engine is certainly important, it is not the only factor. Fuel conditioning, or the state in which the fuel enters the engine is also important. Cylinders that experience extremely lean air/fuel ratios or a small river of fuel running into individual ports are the two extreme examples of situations that will not promote optimal power. Ideally, maximum power is achieved when all the cylinders benefit from a homogenous mixture of properly proportioned air and fuel.

The annular-discharge booster uses several oval-shaped holes (arrows) to more finely atomize the fuel as it enters the engine. These multiple small holes better atomize the fuel into smaller droplets that mix more evenly with air as it enters the engine.

What we learned from this exercise with this particular engine is that the annular-discharge boosters did a much better job atomizing and distributing fuel and air compared to the downleg boosters. This is especially true at lower engine speeds. If you look at the power differences in the 1,800-2,400-rpm test points, it's clear how well the annular boosters work. The annular discharge carb was worth a staggering 94 lb-ft of torque at 1,800 rpm mainly because the downleg-booster carb was extremely unstable at that speed.

Whenever we increased jet sizes in the downleg carb, the low-speed power improved slightly at the cost of big horsepower numbers at higher engine speeds. Had we experimented with larger high-speed air bleeds on the downleg Mighty Demon carburetor, perhaps we could have come much closer to the overall power levels generated by the annular-equipped carburetor. We also could have modified the bleeds in both the primary and secondary metering blocks to improve the downlegs. Of course, the same modifications could also have been applied to idealize the annular carb as well.

Our test engine was a GM 5.7L LS1 with a GM Performance Parts carbureted intake manifold and distributor conversion using a Performance Distributor HEI. The GM Performance Parts carbureted conversion kit includes an all-new casting to mount the distributor and mechanical fuel and water pumps. This also requires a new ATI balancer and pulleys.

It's also important to point out that this one test should not be taken as a blanket endorsement of annular-discharge boosters for all applications. In this particular case, the peak power difference was a solid 7 hp, but even more impressive was the 20-lb-ft average torque increase. This illustrates why Barry Grant recommends using the annular-discharge carburetor on any engine with a camshaft duration of 220 degrees at 0.050 or more. What you can expect is that an annular-discharge carburetor would be an excellent choice to improve part-throttle driveability on a street car equipped with a long-duration camshaft and lots of overlap while also promising a slight overall power improvement. Annular boosters could also be a great addition to a slightly oversized carb as a way to improve its part-throttle manners.

Current Speed Demon and Mighty Demon carburetors do not have replaceable boosters like the Race Demon carburetors. But this doesn't mean you have to buy a new annular-discharge carb to take advantage of these boosters. For under $100, you can send your Speed Demon or Mighty Demon back to Barry Grant and the factory will install the proper annular-discharge boosters and matching high-speed air bleeds to make the conversion.

So whether you're looking for a new carburetor for your steroid-induced big-block or just looking for a way to improve the boulevard manners on your street burner, an annular-discharge carburetor might just be the boost your street machine needs.