Stromberg BIG 97 Primary Carburetor - Standard Finish
$ 492.99
DESCRIPTION
THE BIG97 PRIMARY
The BIG97 Primary features all of the typical 97 fuel circuits, though the power valve and accelerator pump circuits are significantly modified to improve fuel conditioning. It also provides ported vacuum for your modern aftermarket distributor (see more below). BIG 97 Primaries can also be used on their own or in multiples (with a direct or progressive linkage).
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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.
Everything that has made the regular ‘new’ Stromberg 97 so popular over the past ten years has been built into the all-new BIG97: An early-style cast iron base for low heat transfer. A reinforced air horn casting to eliminate warping and leaks. A 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.
Everything that has made the regular ‘new’ Stromberg 97 so popular over the past ten years has been built into the all-new BIG97: An early-style cast iron base for low heat transfer. A reinforced air horn casting to eliminate warping and leaks. A 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.
BIG97s move up to a 1.175in venturi with super-efficient 20 degree entry angles. The accelerator pump discharge jets are smaller and well out of the booster airflow. And flared exit cones build power by slowing the mixture down to reduce turbulence into the manifold plenum. The result? 250cfm per carburetor – some 55% more than our own regular 97s.
New ‘early plenum effect’ boosts top end power. A new, oval cut-out, just under the throttle plates, allows any cylinder to draw from both barrels, combining the easy drivability of a dual plane intake with a boost in top end horsepower more usually associated with a single plane intake.
A new power valve (Primary only) feeds top end enrichment directly into the emulsion tubes, significantly improving fuel conditioning for more top end HP on less fuel. How? Because the more it emulsifies the fuel, especially at high rpm when manifold pressure is low, the easier it is to atomize, distribute and burn
BIG97 Primaries come with a swap-in fitting to provide ported vacuum for your aftermarket distributor. 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 (typically 3 to 4 ft-lb).
IMPROVED FUEL CONDITIONING BUILDS POWER AND ECONOMY
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.
We achieved this through new power valve circuits for improved fuel conditioning and retuned boosters for perfect signal.
BIG97s also come with new emulsion tubes and revised jetting, compatible with their greater airflow capability. Along with a bigger bore, the 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 Tri-Power is factory jetted to suit a typical Small Block Chevy/Ford crate motor. Jetting for Primary and Secondary carburetors is different, as marked on the carburetor box. Alternative jetting may be required for altitude, other and non- standard engines; different multiple carburetion; forced induction or special fuels; and local and seasonal differences in fuel formulations.
As with a regular 97, idle air bleeds and high speed air bleeds are fixed, though for the BIG97, a wide range of main jets and power valves is joined by a small range of alternative idle jet sizes for more sophisticated tuning.
The BIG97 Primary carburetor has a new power valve. Located in the fuel bowl and activated by a new lever off the accelerator pump, it 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 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. By directing that power valve fuel into the emulsion tubes, the BIG97 Primary improves the condition of the total fuel delivery, meaning more top end HP on less fuel. BIG97 Primaries retain a valve under the accelerator pump, but it shuts off once the pump has discharged. And BIG97 Secondaries also boast excellent fuel conditioning, as all of the fuel goes though the emulsion tubes.
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.
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.
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.
TUNING THE BIG97 TRI-POWER
On multiple BIG97 applications, we recommend that the idle circuits are kept operational on all carburetors (ie. do not blank off the idle circuits on Secondary carburetors).
BIG97s are shipped with the top edge of the throttle plates set at the bottom edge of the transition ports, ensuring smooth low speed transition as the Secondaries begin to open. On a Tri-Power with progressive linkage, aim to keep the Secondary carburetor throttle plate positions set this way, linked so that they all close simultaneously. The idle speed is then set using the Primary carburetor throttle stop screw.
Once the idle speed is set, check that the Primary throttle-plate position is still below the transition ports. If it is too high (at or above the transition ports), this may indicate low idle manifold vacuum – typically seen with longer duration camshafts – and some additional initial (static) ignition timing may be required. In extreme cases, the addition of throttle-plate air holes may be required to get those plates below the transition ports. This is even more critical where the vacuum port is used as this port MUST not be exposed to manifold vacuum at idle.
Idle mixture is set in the usual way. Aim for matching idle mixture screw positions on both Secondary carburetors and not too much difference between Secondary and Primary.
NOTE: Traditional carburetor balancing can help with a multiple carburetor setup on a direct linkage. However, on a BIG97 Tri-Power with progressive linkage, secondary throttle plate position is more critical than absolute balance to ensure smooth running (see above).
MODIFYING YOUR INTAKE MANIFOLD
The BIG97 ‘early plenum effect’ brings the strong torque/easy drivability characteristics of a dual plane intake with a power curve more usually associated with a single plane. The simple intake manifold modifications explained in our Tech Center article ‘How To: Maximise BIG97 intake flow’ enhance that effect, adding even more potential to your BIG97 Tri-Power.
Put simply, BIG97s have bigger throttle bores than a regular 97 so some intake manifolds may need opening up a little on the carburetor mounting to match the carburetor. You can use the supplied manifold gasket as a template. The deeper you can take that porting, the better your BIG97s will flow.
