Reply to "Closed Chamber Heads"

We haven't figured out which heads are installed on your Pantera's motor, there's three possibilities:

ARD1AE head castings (Australian 302C)
new compression ratio = 10.4:1
additional horsepower = 15 bhp minimum

D0AE head castings (1970 M code)
new compression ratio = 9.8:1
additional horsepower = 10 bhp minimum

D1AE head castings (1971 M code & Boss 351)
new compression ratio = 9.5:1
additional horsepower = 8 bhp minimum

The minimum horsepower increases "assumed" there were no other changes to the motor (such as the camshaft), the heads weren't milled, and the motor had 8.6:1 compression before the heads were swapped.

I normally didn't just bolt heads on as-is, it was my custom to mill the heads to achieve a 10.0:1 compression ratio. I have a rule of thumb, taking a 8.0:1 motor to 10.0:1 will result in a 20 bhp minimum increase; taking a 8.6:1 motor to 10.0:1 will result in a 12 bhp minimum increase. A head swap on a Cobra Jet motor was usually accompanied by advancing the camshaft timing, Boss 351 valve springs, Boss 351 crankshaft damper, plus re-tuning the carb and ignition. That was a 320 bhp motor equipped with the factory cast iron exhaust manifolds; headers and good mufflers brought that up to a solid 350 bhp. And contrary to modern sensibilities, that is quite a powerful motor.

Even though Ford advertised the compression ratio of the early 351C 4V motors at 11.0:1 in 1970 and 10.7:1 in 1971, the actual compression ratios were 9.8:1 and 9.5:1, respectively. Its hard for some to believe, but Ford lied that much.

The compression ratio numbers I've quoted are "static compression ratio". The number that actually impacts the performance of a motor is the "dynamic compression ratio". Among the variables that alter the dynamic compression ratio is the point at which the intake valve closes as the piston is rising to compress the air/fuel mixture. The later the valve closes, the lower the dynamic compression ratio. It should be obvious that the camshaft specs heavily influence dynamic compression ratio, power output and the octane of the fuel required by the motor.

It is my experience the last 40 years that a Cleveland motor with 10.0:1 static compression, equipped with a camshaft that closes the intake valve around 70° ABDC, has a dynamic compression ratio of around 7.66:1 and will operate on 91 octane fuel quite readily, with a reasonable margin of safety. 10.0:1 is actually a conservative compression ratio for the 351C running on 91 octane pump fuel.

Among the benefits of swapping to quench chamber heads is an increase in mixture turbulence within the combustion chamber at low rpm and therefore an improvement in low rpm torque. This is felt as a little extra "kick" or improved throttle response at low rpm.

You can "hear" increases in compression ratio as a louder "pop" in the Cleveland's exhaust note when the exhaust valve opens. It sounds more "angry". The charging rhino snorts louder. Puts a smile on my face. Sound of a Charging Rhino

-G