Back in the mid to late 80's, ............more likely Fel-Pro was recommending to increase the head bolt torque by 10 ft/lbs to prevent head gasket failure.
Now that you mention it, I think the recommendation was for an additional 10 ft/lbs for the upper row. Whatever the root cause was/is, I'm with you that the additional 10 ft/lbs solves the problem.
Though there is a wedging effect in the torquing of the intake manifold on the S.B.F. that does attempt to pry the heads off the block (therefore one should be only tightening the intake retention fasteners as required to hold the intake on, not attempting to hold the engine together! ), the main reasoning for the recommended greater staggered torquing values is due to the greater elasticity presented in the longer fastener on the intake side, this otherwise giving a lesser clamp-load if torquing equally to the shorter.
Generally, and particularly as stated by ARP Fastener, the torque value listed is in a range of approximately 80% of the yield value, so it is permissible to increase this torquing sum, 'some', for greater clamping loads. Just be aware you'll be testing the fastener manufactures consistency of production quality, and of the capability of the structures involved and that they will tolerate such force.
Scott.
The AFR heads I just used recommended the 10 # greater on the top bolts as well as recommending ARP bolts. They have the thickest of the decks that I have come across and am sure that helps the situation considerably. 5/8" thick decks I believe.
I had to shave the heads .020 and used a thin head Cometic head gasket to get the chambers to 53cc's.
I wound up going through three sets of intake gaskets on the C6OA intake with a continued water leak at the back near #8. The Fel-pro's couldn't deal with it well and the solution was a Mr.Gasket intake gasket with silicone on the "S's". They are the thickest of the group and the softest compound so they squish up nicely.
That back intake manifold bolt is the outboard one that you need to be careful of not breaking the manifold too.
All is well now. I was coming off of Clevelands which do not have those issues so I had to relearn the SB peculiarities...which they definitely do have.
My 427 aluminum heads were having issues in that exact same corner as well. Leaking water AND OIL from the drain back hole. In that case it was the aluminum squirming around and not enough material on the head gasket itself which also were Felpros. Hum? Coincidence?
On the 427 aluminum head you need to dial down the torque, NOT increase it to keep the head from turning into silly putty. I believe the most I could safely do on the bolts was 92#?
So much "tadue" over nothing. Something that should have NEVER been an issue at all.