66 389 LOW COMPRESSION
 

Have a 1966 Pontiac 389 and from doing some research looks like the compression ratio is something like 10.5 and I was wondering what cylinder heads I should be looking out for to give me the lowest compression possible.

For 1966, the low compression heads were casting 091. They bought the compression down to 7.6 or 8.6 to 1 depending on the particular engine. There were different casting numbers for other years; if you have a copy of Pete McCarthy's book you can find them there, or you can probably figure it out using the information on the Wallace Racing site.

Before you go trying to put a blower on a 66 engine, be forewarned that they have thin wall cylinders on 65, and 66 389s. That block is probably the worst choice to stick a blower on of all Pontiac blocks.

If you want the largest combustion chamber the 71-76 455 smog heads are the largest combustion chambers. The valve angle changed in 1967 so the only way you could use the 455 heads on a 389 is keep the valve lift under .450 so the valve doesn't kiss the piston due to different angle valve reliefs. The valves are larger valves on the 67 and later heads. The 389 notches aren't in the right place, they're the wrong inclination angle, and they're too small for the later heads.

Any of the 67 and later 400s have much stouter cylinder walls, as do 428s and 455s. These blocks are better choices for a blown engine. Properly prepped 2 bolt blocks are good for 600 HP, 4 bolt mains, or an aftermarket block should be considered over that point.

Lets go back from 66 to 1960.
Chevy turbocharged the the horrible Corvair spider motor and it lived 6 psi of boost with the aid of water and Alky injection.

My 2 cents is if this crap Chevy motor can live with 6 or so psi of boost then surly a 389 can live just fine on 8 psi, assuming forged Pistons and with the right tune the water Alky mix will not be needed.

Those 60s era 421 SD blocks that Micky Tompkins supercharged and feed nitro did not have cylinder walls all that much thicker then a production 389!

The Corvair Spyder didn't use alcohol injection, that was the Oldsmobile Jetfire, which was the turbocharged aluminum 215.

Sorry, you are correct!
Thinking back to those days the Spider owners made there own or adapted the Buick one to there motors.

If can afford a roots blower you can afford the proper forged pistons and dish em as need or valve angle change for later normal heads... plus if ya can afford a blower you can just built a 400 or 455 block and move on....

for just driving no blower.... stock low loft 066 cam I put 71 400 2v small valve #99 heads on our 66 Starchief 389 10.25:1 motor that had 092 heads.. 56k mile original short block runs great now at about 8.0:1 on 87 octane all day no issues hot weather no issues.

My experience is with a 65 GTO, 389 that was naturally aspirated that ended up with a piston inside of the water jacket when the cylinder wall collapsed. After that experience I would never recommend someone use one of those blocks for a supercharged application. Pontiac had warranty claims back then due to failed blocks due to their "Thin wall casting process". which covered 65, and 66, 389 engines.

Put new pistons in one of those blocks would usually include boring the engine, further weakening the cylinder walls. Mine was bored, with forged pistons, and at roughly 3000 RPM the cylinder wall just collapsed, completely destroying the engine. I was just cruising along when it happened.

The earlier 389 blocks had much more beef in them than the 65, and 66 blocks did. Anyone else that would recommend using a 66 block, probably didn't have the same experience I had. It is fairly well known that those 2 years had thin cylinder walls, I read about it after mine failed in one of the monthly car enthusiast magazines back in 1971, too late for me though.

X 2, Brad. I have personally seen two 389s that had cylinder walls of less than .100 (.090-.095) in two different virgin ‘389 blocks. One was the dealer installed 389 in my uncle’s ‘66 Tempest, the other was an old friend’s ‘66 GTO. Neither engine had been raced ( up to the point of the cylinder walls failing).