Here are the results of my testing of Scott’s new reduced diameter (4.9”) water pump pulley.
Both tests were conducted with the engine at idle, radiator fans cycled on, and A/C on (cycling).
Deck lids were closed, trunk installed, engine bulkhead cover installed.
Test vehicle was a ’71 Pantera
Engine: ’71 351C, 127K miles (never rebuilt), original cam, 11:1 advertised C/R
Water Pump: Edelbrock High Volume
Thermostat: 180*F Robert Shaw 333-180
Temp sensor location: front of block
Temp gauge: mechanical, 270 deg sweep
Radiator: Fluidyne (4.3 yrs old), mounted in the stock position (vertical)
Fans: Dual Flex-A-Lite sucker, 2500 cfm (unverified)
Factory timing & idle specs: 6* BTDC, 800 rpm
Pulley: 5.5” (stock)
Belt contact with pulley: 92*
Duration: 46 minutes
Ambient temp: Start: 91*F; End: 92*F
Relative humidity: Start: 54%; End: 54%
RPM: No load: 765; w/fans on: 728; w/fans & A/C on: 658
Max temp: 202*F
Pulley: 4.9” (reduced diameter)
Belt contact with pulley: 87*
Duration: 47 minutes
Ambient temp: Start: 92*F; End: 93*F
Relative humidity: Start: 46%; End: 45%
RPM: No load: 766; w/fans on: 728; w/fans & A/C on: 657
Max temp: 197*F
Summary of this test: After continuous idling for approximately 45 minutes, with the cooling fans
running and the A/C cycling, the reduced diameter pulley showed a reduction of 5*F in the coolant
temp. Note that there is a 5* loss in belt to pulley contact with the smaller pulley, which should not
be a problem.
The belt that was used was a NAPA #7405 (13/32 x 41-1/8”). There was enough adjustment left
to reuse the existing belt. Approximately ¾” is required at the adjustment arm slot. If you don’t
have that much adjustment left, you will need a slightly smaller belt.
Edit: I forgot to mention that prior to the test, the car was driven 10 miles (9 on the freeway). Upon returning, the car was left idling and the test time was started.
As a clarification, the initial ignition timing for a stock '71 351C is six degrees BTDC. In '72, the initial ignition timing for a stock 351C is sixteen degrees BTDC.