Join Date: Dec 2012
Location: Gastonia, NC - Born & raised in Connecticut - 31 years
Mentioned: 1 Post(s)
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YS code is for the 1969 400CI/350HP automatic engine. This engine calls for the #16 or #62 heads in a GTO.
Have you checked the date code at the rear of the block near the distributor hole to verify it is a 1969 block?
The #48 heads could certainly be original to the engine as Pontiac did not always follow their own rules and mixed and matched on occasion. It is possible that a previous owner purchased them when the car was "newer" and installed the #48 heads off of the 350HO IF the chambers were the smaller size and he/she wanted to raise compression for a little more kick. It could have been blueprinted for racing purposes. They could have been replaced at a dealership for warranty purposes.
So, it is hard to tell what the true story is if you do not have the car's personal history. If the engine is still original and has not been rebuilt, I would take a look at the cam. The factory cam for the 350HP automatic was the "067" cam. If someone was blueprinting it to race in a specific class, they may have upgraded to a more aggressive factory cam. Thinner head gaskets would be another "trick" to raise compression as well.