Carburetor Dyno Test - Six To One

We've all heard rumors and opinions about what size, style, or manufacturer's carburetor is better for the engine in your Mopar. At Mopar Muscle we've tried just about every combination on the cars we drive and race. And while we feel we have a pretty good idea what size carburetor works well for a given combination, a comment we often hear at cruise night is that a certain engine is "over- or under-carbureted." You may have your own opinion about what size carburetor would be best for your Chrysler engine, but an engine dyno is the only real way to see which size carburetor makes the most torque and horsepower. This month we'll test six different sized Holley four-barrels on a Mopar big-block to see which performs best.

Since we had a 383 Chrysler big-block on the Superflow engine dyno at Auto Performance Engines, we decided it would be a good time to settle an age old question: What size carburetor is best for the Mopar 383, the most widely produced high-performance big-block? From the factory, the 383s with the highest horsepower were rated at 335 horsepower and came equipped with a 650-cfm Carter AVS. Our 383 is basically the 335 horsepower combination, with flat-top pistons and 906 casting cylinder heads. At just over 10.0:1 compression, this is a great pump gas engine, which we've modified in a similar way many 383s are modified.

In stock form, the big-block Mopar engine is one of the toughest engines around, and pre-1972 versions all have a forged steel crankshaft. Our 383 is a '68 version, with a forged crankshaft, windage tray, stock oiling system, and stock connecting rods with ARP bolts. The only deviation from stock is the .030-inch overbore and appropriately sized flat-top pistons. We topped this engine with 906 heads that were treated to a multi-angle valve job, new valveguides, and Comp valvesprings to match the .500 inch lift flat-tappet camshaft. Knowing this combination would like the upper rpm flow of a single-plane intake, we chose an Edelbrock Victor 383 intake manifold.

A carburetor's size is dictated by a rating called cubic feet per minute (cfm), which is the amount of air the carburetor is capable of flowing at wide-open throttle. The cfm requirements for any given engine can vary depending on a lot of factors, so choosing the correct carburetor from the huge variety available can be a daunting task. It would also be a daunting task to test all of the carburetors available for your Mopar on the dyno, so we narrowed it down to six different Holley models between 650 and 1050 cfm. In our experience, carburetors from Holley, Carter, Edelbrock, Quick Fuel, and other manufacturers all work very well if properly tuned. For the purposes of this article, we'll be testing all Holley carburetors since we had a variety of that brand on the shelves. Follow along and we'll show you which size four-barrel carburetor performed best on the big-block we tested.

1. Holley 650-cfm vacuum secondary

Starting small and working our way to carburetors with higher cfm ratings, we installed a Holley 650 vacuum secondary unit on our 383. It may look a little rough, but it works fine. This is the smallest carburetor we'll test, and is the same size that the factory installed on '68 383 HPs like the one we're testing. Using an Innovate Motorsports oxygen sensor to monitor air/fuel ratio, we made our first pull, but power was down from our initial break-in pulls. This is a clear indication that our 383 wanted a larger carburetor, like the 750 we used for break-in. On its best pull with the 650, our 383 made 368 lb-ft of torque and 356.9 horsepower.