Stromberg Carburetor Development
In 2008, I was contacted by the owner of Stromberg Carburetors,Clive Prew (in England), about the possibility of helping them develop a higher flowing version of their tried and true Model 97 carburetor. The goal was to develop a carb that would have considerably more air flow than the 97, but would look identical to the 97 on the outside. This new carb model, eventually named the Big97, is now destined for those guys with the vintage hot rods who were looking for more power. The typical model 97 flows 160 CFM at 1.5"Hg (20.4" H2O) vacuum, and the production version of the Big97 flows 250 CFM. Incorporated into the Big97 design is a more efficient venturi shape, a throttle body feature known as the early plenum effect, power enrichment through the main metering circuits, and ported vacuum for engines with a vacuum advance feature in their ignition systems. The result is a healthy 56% increase in flow, better fuel conditioning, and improved throttle response and driveability.
So, for the typical tri-power setup , commonly used on American V8's, that would be 750 CFM at 1.5"Hg. The goal for horsepower, using currently available mass production tri-power intake manifolds with three Big97's, was 400HP. It was found in the development process, that the flow capability of those production tri -power intakes was somewhat restrictive to the flow capability of 3 Big97's.
Always a major factor during the development of the Big97, was the need to retain the good low speed engine characteristics that the original model 97 is famous for. After all, for any street driven hot rod, low speed/part throttle driveabilty is a necessity for the car to be fun to drive. During a development phase, in which the first functional prototypes were run on a small block Chevy on the dyno, much of the fuel calibration was determined. That relatively mild 355 Chevy made close to 400 HP, which was close to our original goal. And the engine had low speed manners that were as good as it had with a modified Holley 4 barrel carb on a typical street/2 plane/4 barrel intake.
Now that the Big97 has been in production for a while, I decided to see how much air flow I could get through a modified Big97, again with a strong emphasis put on retaining the good low speed characteristics. I found that it is possible to get the modified Big97 to slightly over 300CFM @1.5"Hg with no loss in low speed characteristics, and slightly over 320CFM with a slight amount of loss. Since the Big97 carb is intended for use in higher performing street driven vintage hot rods, the loss of any of the good low speed characteristics is not a good trade for the extra CFM. So, with 3 modified Big97's in a tri-power configuration, we now have 900CFM of available flow at 1.5"Hg vacuum. This is well beyond the flow capacity of the currently available 2 plane cast aluminum mass production intake manifolds, some of which were designed decades ago.
There has been some testing of tri-power sets on a modified tunnel ram , and on a modified Edelbrock dual quad manifold, both on some street small block Chevy engines. Peak HP was in the 420-430hp range, which is what was expected from those fairly mild engine combinations. I believe that it is possible to make 450hp, with good low speed drivablility, with a tripower BIG97 set. The engine combination would have to be carefully chosen, and the intake manifold would have to be better than the currently available production tri-power intakes. With the advances that have been made in aftermarket cylinder heads and camshafts, there is now more need for improved tri-power intakes, at least for the popular engines. © Norm Schenck