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 confiuration, 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.
To address this mismatch in carb flow vs. intake manifold flow, a prototype tri-power 2 plane intake manifold was prepared and tested on a stock 330Hp 350 Chevy crate engine. This manifold was a simple prototype, more like a proof of concept, and was somewhat lacking in the area of the smoothness of some of the new passage turns. To get a comparison, we first tested a moderately ported mass production tri-power intake to get a baseline power curve for the engine. Since we're talking about a street engine here, we will compare the torque curves, because it is torque that accelerates a street driven car, not top end HP.
Starting at 2600RPM(full throttle), the prototype intake started making more torque, and by 3600RPM, was making 13.9 ft-lbs more torque than with the ported mass production intake. That gain in torque started to taper off slowly as the RPM's went higher, up to 4700RPM, where it was up by only 2.7 ft-lbs. From 4800RPM up to the peak RPM of 5200, the prototype intake showed a slight loss in torque. It became obvious that the abrupt nature of some of the new passage turns in the prototype intake was causing too much turbulence in the runners at the higher flows, and that turbulence was causing a loss of flow.The air flow meter on the dyno, in fact, showed a 20 CFM loss at 5000 RPM, compared to the ported mass production intake.
We are confident that this prototype intake showed us that our concept is good, and that the next prototype intake will benefit from smoother(longer radius of of curvature) turns in the runners. This will be done with an emphasis in retaining the good torque gains in the 2600-3600 RPM range, and building even more torque in the 3700-6000 RPM range, by reducing the turbulence and increasing the flow in the internal runners of the manifold. © Norm Schenck