I'll try and find the article the gm engineer who built the 400 firebird ho motor wrote. even got to speak to him on the phone great 2 page tech read on how the vacuum lines connect and how the setup helped prevent overheating. Great guy to talk to. you might find it in a Google search. he describes how it works to reduce heat. I went and put it back to original per his article and yep never overheats anymore. if my uncle who I gor the car from was still with us I would have kicked him in the button as like alot of the backyard mechanics in 68 removed them thinking they were like emissions and would reduce performance. and all it did was cause my Uncle to try bigger and bigger radiators. I'm now using the original factory radiator and I live in az and last summer at 118 degrees it never overheated. so glad I found not just the article but the engineer so much info not in the books. now I have to figure out why she suddenly started stalling at fourth today. new pump lines clean tank all of that same issue tomorrow I'm gonna replace the coil as it seem like power to plugs drop. we'll I will get the article tomorrow and post it I meant to do it a long time ago. I think it would help alot of folks with hot or overheating 400 HO's