Practically every day we get asked, "Which is better, the single or dual oil pressure (OP) case?" They are both the best! They are designed for different applications. The single OP case is to replace non-smog applications with the regular factory fan and cooler and the dual OP is for replacement of engines with the dog house cooling systems.
The dual OP case has larger galleries to flood more oil (larger pump) into the head to soak up the head temperature caused by the smog devices or improper tuning and to take it back to the also larger cooler and larger fan that pumps more CFM of air. Now all of the cooling parts can be retro-fitted either way with the GB 111E cooler adapter gasket kit, however, the pump and gallery size will have some affect on the results if mismatched. I, personally, would not have a dual OP case without the proper matching cooling system and oil pump.
The big problem today is getting stuck with a second or a reject. All of the engine cases we sell go through an extensive inspection before we accept them from our supplier. We reject almost 30 percent of the cases we inspect. We hear people saying they need a case with the back filled in behind #3. Yes, Germany did make the 1975-on dual oil pressure FI engines with the back filled and CB Performance (formerly Claudes Buggies) had a Brazilian single oil pressure case with the back filled which is no longer available. Do you need this on a high performance engine? We have never used one of these cases and have not had any such failures that people trying to sell you a filled case claim you will have if you do not have one.
First, the case will never break behind #3 unless you over heat the engine or have pre-ignition (detonation). I have ran 14.1:1 compression ratios for over 15 years on our race engine and over 180,000 miles on my 1968 (1900cc 138 HP) engine and have never had a case crack.
At this time only Brazilian AS41 cases are available.