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  • Testing Demon Carburetors on a Small Block Chevy - Size Matters

Testing Demon Carburetors on a Small Block Chevy - Size Matters

With Carburetors, Bigger Isn't Necessarily Better

Mike Finnegan
Mar 1, 2008
Photographers: Mike Finnegan, Steve Brule
Photo 2/14   |   small Carburetor carburetors
When it comes to engines and our fiendish desire to make them more powerful, it's often easy to fall into a few traps. This is because there are a lot of wives' tales, myths, and misnomers out there that are all too commonplace. One of these myths that many of us subscribe to is the one that says bigger is better, especially when it comes to engines.
When it comes to carburetors, though, most of us know that bigger is not usually better; we just don't know exactly why. We wanted to find out why, so we grabbed a few carburetors and a pretty common small-block Chevy engine andn headed off to our favorite dyno facility to test the theory. The results were surprising.
The Test Engine
Our test engine is a 305-cid Vortec V-8, which is found in hundreds of thousandsof '80s and '90s GM pickups and SUVs. it's not known for being a superhigh- performance powerplant, but our goal here isn't to make astronomical power numbers but rather to find out which carburetor provided as close to perfect fueling for our particular motor as possible. The road to perfection should include excellent driveability and economy, as well as big power, and these are the criteria you should look at when choosing your own fuel metering device. Big power is useless if your engine won't run properly when the loud pedal isn't planted firmly on the floorboard.
The 305 is mostly stock, except for the addition of an edelbrock Performer RPM Air Gap intake manifold atop the Vortec iron heads, Crane 1.5-ratio roller rocker arms, and a Comp Cams Xtreme energy 268 hydraulic flat-tappet camshaft. We bolted Hedman headers to the heads, fired the motor using an MSD Pro Billet distributor and ignition system, and rotated through a set of Demon carburetors during the test.
Photo 6/14   |   small Carburetor carbs
The Test Carbs
The carbs ranged from BG Fuel Systems Road Demon to Speed Demon lineups.The smallest of the fuelers was a 525-cfm Road Demon. The Road Demon features high-density cast-aluminum metering blocks, vacuum-operated secondary throttle blades, adjustable jets, four corner idle circuits, a billet baseplate to resist distortion, large-capacity fuel bowls with patented sight glasses, and a contoured choke tower to increase airflow. The other carbs were 650-, 750-, and 850-cfm Speed Demons. The Speed Demon offers upgrades over the Road Demon that include billet metering blocks, optional electric choke or dual squirters, and a new idle-eze baseplate that doesn't require you to drill holes in the butterflies to fine-tune the idle speed and transition response.
Photo 7/14   |   small Carburetor superflow
Testing 1, 2, 3...
Our testing was simple. All we did was swap each carb onto the engine, warm it up, make a few dyno pulls, and then note the air/fuel ratio and the power numbers the dyno spit out. Because we used Westech Performance Group's Superflow dyno, we were also able to measure the airflow into the carburetor and fuel flow to the engine. These two parameters are what really told the story behind each carburetor's performance. instead of assuming why each carb did what it did, we were able to find out the truth.
The truth was the exact opposite of the misconception that overcarbureting would lead to pouring too much fuel down the intake runners into the engine. What really happened was the larger-cfm carburetors actually made the engine run too lean, consequently killing the power. Why? Because the larger venturi diameter and booster design of the 750- and 850-cfm carburetors slowed the velocity of the air being sucked into the engine. The slow air speed killed the signal to the booster, so not enough fuel was pulled through the main jet and booster into theventuri at slower engine speeds.
Sure, at high rpm the larger carbs worked OK, and the engine benefited from large doses of air, but at moderate rpm, from 2,500 to 4,000, the engine was down on power because the air/ fuel ratio was too lean. Check out the dyno charts, and you'll see that the 850 carb flowed more air than the rest by a large margin. But, the speed of that air was so slow that the fuel flow into the engine was down by an even larger margin, from 2,500 to 3,200 rpm. in fact, the 525-cfm Road Demon carburetor helped the engine produce 135 hp at 2,500 rpm, while the 850-cfm carb made just 108 hp. Up top, the 850 was good for 360 hp with an air/fuel ratio of 12.1:1, but at 2,500 rpm the air/ fuel ratio was 15.1:1, which was way too lean and almost undriveable. The engine bogged down hard and ran rough in the midrange, which is where, in a truck, the motor would cruise at. in the midrange, the 525 Road Demon maintained an air/fuel ratio of 12.7 to 11.3, which, in the minds of many engine tuners, is an acceptable range for building horsepower.
Photo 8/14   |   small Carburetor testing
The 750 Speed Demon wasn't much better. Power peaked at 357.2 hp at 6,100 rpm with a 13.1:1 air/fuel ratio. At 2,500 rpm, the air/fuel ratio improved considerably, compared to the 850-cfm carb. This time the engine ran smoother with a 14.2:1 ratio, which is still too lean, and the 750 carb made 124.9 hp. The 650 Speed Demon offered the best balance of power and air/ fuel ratio across the entire rpm band. At 2,500 rpm, the engine was making 135.1 hp with a slightly rich 12.4:1 air/ fuel ratio. At 4,000 rpm, the air/fuel was 12.6:1, and the engine produced 261 hp and peaked at 6,000 rpm with 356.9 hp and a 12.5:1 air/fuel ratio. The 650 carb proved to be the best all-around performer for our smaller, 305-cid V-8.
Tech Tip
Why So Small?

In order to figure out the why of our test, you first have to understand how a carburetor works. Here's the short answer as to why a carburetor provides a certain amount of fuel to a given engine. The fuel supply is housed in the float bowls and is introduced into the intake manifold at idle speed by transfer slots and idle discharge ports in the carburetor's baseplate. The pressure differential between the vacuum created by the running engine below the fuel supply and atmospheric pressure above the fuel supply is what sends the fuel into the intake, and the amount of fuel is restricted by the air bleeds and idle mixture screws.
The Demon carbs we tested of varying cfm each arrived at that flow numberwith a different combination of venturi diameter,throttle blade diameter, andbooster design.
525 Speed DemonThe 525 Road Demon had a 1.28-inch venturi diameter, 1-11/16-inch throttle blade, and down-leg boosters of varying size in the primary and secondary sides of the carb.
650 Speed DemonThe 650-cfm Speed Demon had the same-size venturi and throttle blades but larger down-leg boosters and no choke plate, which increased airflow.
750 Speed DemonThe 750 Speed Demon had larger, 1.40-inch venturi, 1-11/16-inch throttle blades, and annular boosters in both the primary and secondary sides of the carb. The 850 had 1.562-inch venturi and 1-3/4-inch throttle blades for even greater airflow. Annular boosters helped speed up the air in the big venturi but not enough for our small test motor to receive the right fuel curve at low rpm.
The Data Don't Lie!
The relationship between carburetor sizing and the fuel curve of an engine is illustrated best using the Superflow dyno data. The upper chart shows the 525-, 650-,and 850-cfm carbs' airflow and fuel curve. As you can plainly see, the red line of the 850 carb clearly flows more air, but if you look at the bottom of the graph, at 2,500 rpm, it's moving much less fuel to the engine, which is what resulted in the lean air/fuel ratio and lower power output. The black and blue lines representing the 525- and 650-cfm carbs are much closer together, and the carbs were much closer in performance because each one offered better fuel flow.
The lower graph shows the horsepower and torque output of our 305 engine at the extreme ends of our carburetor spectrum, the 525- and 850-cfm units. The 850 made more peak power, but the 525 was better everywhere else in the curve, which is where most engines will live in our sport trucks.
The Final Word
Size is everything when it comes to choosing a carburetor, and although this article will clue you into that relationship, it's still not a replacement for a phone call or e-mail to any carb manufacturer or tuner. The bottom line is that you can tune around most carburetor sizing issues, but it's a lot easier to start with the right size of carb for your application and just make a few jet changes than it is to tweak on a carb that's too big for your engine. Like we said before, a big carb can yield a lean air/fuel mixture, and that's a recipe for disaster when your foot is planted on the floorboard. in this case, the 650 Speed Demon carb proved to have the right mix of peak power output, as well as driveability and midrange power output for our 305-cid engine.


Westech Performance Group
Mira Loma, CA
Comp Cams
Memphis, TN 38118
BG Fuel Systems
Dahlonega, GA 30533



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