The Model 4M was redesignated...
The Model 4M was redesignated Model M4M in '75, and it contained the same 17/32-inch primary bore as the SD-455 Model 4M. The secondary air valve was used to restrict total airflow for the myriad combinations it was used on. Our testing shows that once modified, the Model M4M contains airflow capacity similar to an SD-455 unit.
A Superflow 110 flow bench...
A Superflow 110 flow bench was used to measure the airflow capacity of the carburetors. We created an adapter from thick pine and modified a generic drop-base air cleaner to smooth entering air. All primary measurements were recorded at 9 inches of water and mathematically converted to 20.4-inch values.
The Model 4M castings used...
The Model 4M castings used on most '74 Pontiacs contain a primary diameter of 13/32 inches. Our testing indicates an approximate airflow capacity of 180 cfm.
Airflow Testing
Before our flow test could begin, we had to create an adapter that would allow us to mount a carburetor onto the flow bench. Using a bare Quadrajet throttle body as a template, we drilled four correct-size holes into a piece of 1.25-inch-thick soft pine. By applying tape over a hole, we could effectively seal off any portion of the carburetor we wanted to eliminate during the flow test.
After properly warming and calibrating our Superflow 110 bench and eliminating secondary flow through the adapter, we placed each carburetor on the bench and recorded airflow. The testing procedure included ensuring the choke and throttle valves were wide open, checking port alignment and for air leaks, and installing a modified air-cleaner base to smoothen the incoming air. The results were recorded at 9 inches of water and mathematically converted to 20.4-inch values.
Secondary airflow was a little more difficult to measure. As previously mentioned, Rochester used the secondary air valve to limit total airflow of its 4M and M4M models for applications that required less. Surprisingly, this includes most standard-performance Pontiacs. It seems that the air valve of high-performance units like the high-flow '71 and SD-455 open furthest. So we used them to establish base flow.
After eliminating primary airflow and replicating the testing procedure, we found peak secondary airflow of the high-performance castings to be around 600 cfm. Maximum airflow through the remaining castings varied from 520-580 cfm (depending on the application), which indicates that engineers spent time determining the optimal amount of total airflow for each application.
Simply removing a slight amount of material from the secondary air-valve tab can increase airflow. Using a carbide cutter, we applied this modification to a test carburetor. The goal was to remove enough material that the rear half of the secondary air valve opened nearly 90 degrees from its closed position. Subsequent flow-bench testing revealed that secondary airflow had significantly increased. But keep in mind that the added airflow won't necessarily translate into additional performance. Typically only engines that are airflow-restricted will gain from this.
| Approximate Primary Airflow Converted to 20.4 inches |
| | 7028263 | 7044262 | 7041270 | 7044270 | 17057262 |
| Primary airflow | 176 cfm | 182 cfm | 228 cfm | 212 cfm | 210 cfm |