I inherited a 1968 Cougar XR7 with a 390 2-barrel. The car was in storage for quite a while, and the engine was seized. The VIN says it has an "X" code engine with 10.5:1 compression ratio and requires Premium fuel. My research showed that Premium fuel in 1968 was 100 octane. This is not practical for me. I would like to rebuild it to run 93 octane pump gas. So the question is: What is the best way to do this? I assumed different pistons would be in order but I am also wondering about the cam, rods, crank, etc. I read somewhere (forget where) that 9.6:1 compression is a good fit for this. The engine is completely torn down, so all options are on the table. Any advice would be appreciated, and part numbers would be awesome.
P.S. The heads are C8AE-H.
Likely you have C8AE-H heads?
And that 10.5cr "2v premium fuel" should be with flattop pistons, valve reliefs only.
The 1961-63 390-4v pistons with their shallow dish would give you about 9.7 CR with those heads. TRW L-2291F etc forged versions come pretty cheap, and forged can run a little more CR so 9.7 would be fine on 93 octane, even ok on 87 octane if you keep it clean & don't overjet.
The 390-2v FoMoCo pistons have a dish you could use as a salad bowl and will give about 9.0-9.2 CR. Theyre cast, and I've seen a lot of them punched through, which seems odd with the CR that low. Maybe the dish is so deep it weakens the piston. Much less forgiving than the early 61-63 TRW forged ones above.
The cam in there is 192-199-111 LSA and is fine if your goal is not to add power. We used to get 17mpg with that cam in a big C-6 Galaxie.
Depends on your goals. If the cam/lifters are good it saves you breaking in a cam.
If not, the Comp 252H is a 206-206-110 cam, about like the early 390gt cam, or the Melling "RV" cam is 204-214-112 which is about like the 428CJ cam. The Edel Performer (NOT RPM) is 194-204-112? which is about like the 66-up 390-410-428 4bbl vanilla cam.
I don't know why any big block EVER came with a 2bbl!
Yes, get a 4bbl intake, even a stock 66-up iron or Edel Performer. The 428pi alum intake also works well with that stock cam or the ones I mentioned.
All that and keep the CR about 9.5 & you're fine.
That engine can get away with 10.5 CR partly because of the 2bbl- the cylinders just cant fill, not enough carb, so it's more forgiving. Slap a 4bbl on there & it will spark knock like crazy, BTDT. It's too high, unless you run a monster cam which then needs headers, way better heads, etc.