I have used a Vortec pump with the Vortec accessories and a TBI pump with TBI accessories. Stepping back nearly 20 years ago, used the V-belt pump with V-belt accessories prior to that. I have not hooked the external bypass up on any of them unless it has a Vortec pump. When I use an older pump setup for internal bypass I drill the RH head for the bypass. I have yet to run across a Vortec block without the bypass passageways drilled from GM, but I do know they exist. The TBI era stuff has constant bypass through the heater core and I always drill 2 x 1/8" holes in my thermostats. The latest budget L31 build I did a write up on, has the Vortec pump, I plumbed up the external bypass. It warms up quickly and runs a very consistent temperature, despite having a double bypass and drilled thermostat. I say double bypass because the heater control valve is open unless the A/C is in recirculate on the 1987 G20 van I set it up in. The van was setup factory without a heater control valve and I added a 70s style valve and connected it to the vacuum line running to the recirculation door. When you move the a/c controls to recirculate, vacuum shuts the heater valve.
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