What is the definitive bottom line answer. Heater Core Shut Off or Bypass Valve?
 

I want to block coolant going to the heater core in the summer. I read the same amount of articles for each approach:

1. block the heater hose coming off the rear passenger head with a single valve.
2. don't block the heater hose, keep the coolant flowing with a bypass valve, so the coolant keeps flowing from the head back to the water pump.

I know the factory did it like #1 above then stopped.

What should I do on my 77 400.

Thanks

On my 70 Firebird, I just installed a vacuum operated shut off valve. I love it. The interior stays way cooler compared to before. If I did it again I'd recommend the shut off valve.

Since it is vacuum operated, you can operate it in one of two ways. You can leave it disconnected in the summer, and only hook up a vacuum line in cooler weather. Or you can tap into your vacuum harness under the dash. The later years like yours don't have a dedicated vacuum port for the heater valve, but you can tap into the yellow line ( I think it's the yellow one). It activates the mode door when on the "heat" setting. If you do this, it would allow coolant flow when the heat setting is activated. However, it will not do so for defrost. I haven't figured out a way to do that without a dedicated vaccuum valve. Hope this helps.