I just did a compression test on my 77 400. I pulled the BAT wire from the HEI. I do have a 78 Pontiac Service manual - but , I did not check the manual
HEI started in..1974/75? So, I'd guess later manuals might discuss.
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For me, I tested cold
(i) removed all spark plugs
(ii) disconnected HEI BAT terminal
(iii) held open throttle with twist tie (and held open qjet choke blade and secondary flap)
(iv) screw gauge into spark plug hole (#7 cyl hardest for me)
(v) cranked until pressure did not go up