The
1962 261 (page 128 of the pdf) seems to have come with the same cam as in the
1954 261 (page 115 of the pdf).These same references also seem to show that 1962 truck 235 had the same cam is in the 1954 truck 235, milder in both duration and lift than for the 261. However, as Tim pointed out, the Master Parts catalog shows the 1956-57 trucks use the same cam for the 235 and 261 engines; the GM data packs also do indicate this for the 1956, 57 and 58 trucks. However, in 1959 the 235 engines went back to the milder cam. Maybe if someone examined all of the torque vs RPM curves this might be obvious, unless the performance curves came from the marketing department instead of a dynomometer.
If one does believe the performance curves, a comparison of the 235 and 261 data in the
1955.1 data book (pages 92-99 of the pdf) does seem to show that there is a benefit at higher (>2500 rpm) engine speeds for the 261 cam (and a larger carburetor?). For example, at 2000 rpm the 261 puts out 10% more HP than the 235, just about what the change in displacement and compression ratio would predict. However, at 4000 rpm, the 261 generates 21% more HP than does the 235, indicating at least another 10% for the cam and carb. The benefit may be less on a 235, as the airflow demand and pressure drop will be lower. If I can find my engine simulator program, I might try to see how much of the extra 10% is due to the carburetor with the larger venturi diameter (1-15/32" for the 261, 1-11/32" for the 235).
The 1954 Corvette data pack indicates (page 2 of the pdf), as Tom pointed out, that the cam was changed during the 1954 run to increase HP from 150 to 155. However, the GM data packs
for the 1953 Corvette (page 6 of the pdf)and
the 1955 Corvette (page 10 of the pdf) show the cams for both years to have the same timing and lift. The 1954 book did not contain any cam data that I could find. Another surprise was in comparing the cam timing for the Corvette and the 261; same duration and lift, but the cam in the Corvette seems to be advanced eight degrees.
Has anyone ever had a chance to check the profiles of any or all of these cams?
Hoyt
