If you talk to ARP they recommend 625 over 800hp. I ran the 425 for years with about 80psi of boost making 1000rwhp and had no problems. I even reused them twice. The motor I'm running now has the 625 just to be on the safe side. LOL
I do not know what pressure it is but I did think about this. I figured if the tank was too small, it would have done this from the get go. I believe Russ runs this same tank on his nova with no issue.
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I always re-torque head studs on my race truck after running it hard for the first time (1 event usually). Even running 625's and now 14mm studs, it's amazing how much torque they take. They then are good for the season.
Things settle, re-torqueing is a royal pain but I feel it is needed when running the type of boost and cylinder psi needed for over 1000HP.
I don't have an answer on what 425's will take, sorry. I do know that they can benefit from a re-torqueing also though.
I am running the same canton overflow tank you are, just stock engine with a 475/stocker and do notice the occasional dribble, or splash of coolant from it. I keep mine around the halfway full area, and Im not sure how much the coolant levels raises when warm, but the tank is small. Might just be a side effect of running a small capacity tank?
When I stud my motors I torque and let them sit over night then retorque. Wish I could have with my girdle also.
Why? I've done this and seen zero difference they are still at the same tq the temp swing is so infinitesimal