Currently in the process of restoring my father’s ‘69 Cutlass S convertible. Pulled the engine with 130,000 miles & sent it off to the machine shop for evaluation. Block needs 0.030” bore so new pistons are in order. However the aftermarket availability for this is limited & will require custom parts. Looking to warmup the 250 HP engine with some higher compression pistons but retain the stock heads. Car is just a cruiser with TH350 trans & 2.73 gears fueled by a Q-jet & a new Erson cam,E540121, 214 duration & 0.478 lift. Real question is what will be the octane requirement if I go with 10.5 CR? Or should I dish the pistons to get 10.25? Want to be able to run on pump grade premium.

 

It's good that you are asking about the pistons as the common aftermarket replacement cast pistons have HUGE 24cc dishes that result in around 7:1 compression ratio, so avoid those at all costs. There are multiple forged pistons available; you just need to determine what compression ratio you want.

FYI, flat top pistons such as the SpeedPro L2320F (10.5:1 replacement pistons) along with Fel Pro head gaskets, no block deck milling, and typical heads with ~68cc combustion chambers will give around 9.8:1 compression ratio. If you use the L2321F with 6cc dishes (10.25:1 replacement pistons), you'll get around 9.2:1 compression ratio.

 

Use the Mahle Motorsport pistons. They’re available in 4.065, 4.100 and 4.155 sizes. With a stock head you’ll get about 9.25-9.5:1 compression.
You may also want to add an RPM Intake. It’ll accept the Qjet and offer a performance advantage.