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BIG97. LIKE A 97. BUT BIGGER
NOW THERE’S A BETTER WAY TO GO FASTER
The Stromberg BIG97 takes everything that’s special about the classic 97 and improves it in so many ways, extending our traditional carburetor look to a whole new range of applications. Here’s a quick rundown of what makes it so special.
Everything that has made the regular ‘new’ Stromberg 97 so popular over the past twenty years is built into the BIG97: An early-style cast iron base for low heat transfer. A reinforced air horn casting to eliminate warping and leaks. A big 97 logo and all the other correct markings. High tech cellulose/nitrile gaskets. Stainless steel levers, linkage and springs. Original-style riveted throttle and choke shafts. And more. All three carburetors keep chokes, idle and transition ports for steady idle, a stable signal and good low throttle manners.
250CFM – 55% MORE THAN REGULAR 97s
Here’s the BIG difference. The Stromberg BIG97 flows up to 250cfm per carburetor – some 55% more than our own regular 97s (at 162cfm max). Stromberg worked closely with Norm Schenk at Competition Fuel Systems to provide more airflow than regular 97s without changing the classic 97 shape. Independent tests have rated BIG97s at 250cfm per carburetor. That’s 55% more than our own regular 97s (162cfm). Here’s how we did it:
BIG97s move up to a 1.175in venturi with super-efficient 20 degree entry angles encouraging maximum pressure drop into the venturi, with minimum flow loss and excellent signal stability. Bigger, thinner 1.34in throttle plates work within offset throttle bores, while flared exit cones ensure good pressure recovery and low turbulence in the mixture leaving the carburetor. BIG97 accelerator pump discharge jets (Primary carburetors only) are smaller and located higher up, well out of the booster airflow. Careful retuning of the BIG97 venturi booster shape and position has produced a very stable signal, whatever the cfm flow, with a curve that follows the mathematical equation to near perfection. The point in the venturi shape where the lowest pressure occurs (the vena contracta), is traditionally where the bottom of the booster is placed, but raising it higher in the venturi, with a radiused entry shape, helped air flow and made it easier to achieve a high quality booster signal curve. In addition, the mixture exiting the booster was then free to expand in the area where pressure is lowest, aiding fuel atomization and air/fuel distribution. A strong signal ensures the correct Air/Fuel ratio throughout the power band with good atomization even at low rpm. It can also boost fuel economy by allowing smaller jetting, especially in the Primary carburetor.
EARLY PLENUM EFFECT
In development, we added a unique oval cut-out, just under the throttle plates to boost top end power. Called the ‘early plenum effect’, helps the two separate barrels of air come together before they hit the intake plenum. This improves pressure recovery and increases airflow capability, but also builds power by slowing the mixture down to reduce turbulence in the transition from carburetor to manifold plenum. Coupled with our recommended intake manifold modifications, connecting the two manifold planes has further benefits as allowing any cylinder to draw from all six barrels brings the strong torque and easy drivability of a dual plane intake with a boost in top end horsepower more usually associated with a single plane.
IMPROVED FUEL CONDITIONING BUILDS POWER AND ECONOMY
The BIG97 Primary features all the typical 97 fuel circuits, though the power valve and accelerator pump circuits are significantly modified to improve fuel conditioning. Idle air bleeds and high speed air bleeds remain fixed, but revised to suit the higher airflow capability. And new emulsion tubes have a revised tip angle, which improves the signal curve and stops the airflow ‘curtaining’ off the tube discharge hole, helping prevent high end lean out.
The BIG97 Primary carburetor has a new power valve, located in the fuel bowl. Activated by a new lever off the accelerator pump, which feeds top end enrichment directly into the emulsion tubes, significantly improving fuel conditioning at higher rpm. Why is this good? Because the more it emulsifies the fuel, especially at high rpm when manifold pressure is low and vaporization is harder, the easier it is to atomize, distribute and burn. In a regular 97, the power valve simply pours raw fuel through the accelerator pump discharge tubes into the booster venturi, so when it opens (at around half throttle) total fuel conditioning gets worse because you’re using progressively less of the good emulsified ‘main jet fuel’ and more of the raw PV gas. There’s no air bleed in the standard 97 pump/PV circuit, either, so the mixture just gets richer as airflow increases. Worse than that, all this is happening at higher rpm when there is less intake manifold vacuum to help vaporization. It’s a triple whammy that can only hurt horsepower at higher rpm. BIG97 Primaries do keep a valve under the accelerator pump, but it shuts off once the pump has discharged.
These revised fuel circuits help BIG97s meter a high quality, atomized Air/Fuel mixture for excellent combustion efficiency. With fuel supply capability close to 90lbs of fuel/hour and a BSFC (Brake Specific Fuel Consumption) of just 0.414 lb of fuel/hr/hp, the BIG97 Tri-power has the potential to support significant horsepower.
PORTED VACUUM
BIG97 Primaries also come with a swap-in fitting to provide ported vacuum for aftermarket distributors like the Stromberg e-FIRE electronic distributor for Small and Big Block Chevy. Ported vacuum has been seen to improve combustion efficiency in cruising mode, helping engines smoother and cooler, with moderate improvements in fuel mileage and more torque through the mid-range.
IT’S ALL ABOUT THE TRI-POWER
BIG 97 Primaries can also be used on their own or in multiples (with a direct or progressive linkage). BIG97 Secondaries have no power valve or accelerator pump circuits. (Dummy accelerator pumps retain the classic 97 look). Secondaries are designed ONLY for use in multiple carburetor systems, connected to a Primary model with a progressive linkage. All three carburetors keep chokes, idle and transition ports for steady idle, a stable signal and good low throttle manners.
A BIG97 Primary with two Secondaries makes for the ultimate 750cfm hot rod Tri-Power. Add our progressive linkage and classic 3-into-1 fuel lines and you’re looking at old school carburetion with next level performance and drivability. The perfect bolt-on for your small block Chevy or Ford, you can cruise economically on the center Primary, then punch in the Secondaries when you step on the gas.
