Test TimeWe tested a brand-new 750-cfm addition to the Mighty Demon lineup that now offers annular boosters to see if there was a fuel-flow or power difference between it and the more common downleg style. Our testing was performed by Ed Taylor on Ken Duttweiler's Digalog Benchmark series dyno using a carbureted LS1 engine.
The GM Performance Parts crate LS1 used a GMPP carbureted intake manifold conversion that includes an aluminum single-plane intake along with a complete new front casting that houses a new water pump, fuel pump, and distributor. The engine used a Crane PN 144HR00061 cam (222/234 at 0.050, 112-degree LDA), Crane 1.7:1 roller rockers, and GMPP CNC-ported LS6 heads.
To evaluate the strengths and weaknesses of both boosters, we began our wide-open-throttle dyno pulls at 1,800 rpm. That is the equivalent of nailing the throttle in a heavy car with a manual trans in overdrive at a slow highway cruise. While challenging, this isn't an unrealistic test. Each carburetor had to deal with a power range of 1,800 to 6,700 rpm
We started with the annular-discharge carb, which required substantially richer than the out-of-the-box jetting but made a solid 513 hp and 443 lb-ft of torque. Jetting on either side of this combination merely lost power, so we stopped and bolted on the downleg-booster Mighty Demon 750. With that carb, jetting changes merely shifted the curve around but did not increase power. Our final jet combination with the downleg carb made slightly less peak power at 506 at 6,600 and 436 lb-ft at 5,600 rpm. But when we really studied all the numbers, they revealed some interesting information.
Numbers GameTo 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.
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.