(The following article by Larry Brumfield appeared in the Jan/Feb 1994 issue of MODEL A TRADER)
From the letters I receive, people seem to be confused about high compression and the Model "A" Ford. We call the head that we manufacture "High Compression" but that is really a misnomer. It is actually just higher compression. Ford made the Model "A" a low compression engine for maintenance reasons. Let me explain.
In an engine that is operating in a normal manner, the ignition flame consumes the charge at a steady rate of about 100 feet per second until the charge is consumed. However, if the first part of the charge burns in a normal manner and the last part burns almost instantaneously, an excessive momentary pressure unbalance is created in the combustion chamber. This abnormal combustion is called detonation and engine efficiency is decreased.
If detonation is severe, structural damage can occur to the piston and cylinder head. This can be a real problem in aircraft engines because the audible knock cannot be heard over the rest of the other sounds.
After many miles of driving and excessive carbon build-up, detonation would be a problem especially with old time gasoline. Ford avoided the problem by making the compression ratio low but still high enough to give the engine "pep." He knew that the average person was not a mechanic. However, with modern gasoline a higher compression ratio is no problem. In fact, most of the automobiles of the Model "A" era had ratios of about 5.5 to 6 to 1.
Nevertheless, more compression does mean more pressure on bearings. We have experimented with various ratios and we have determined that ratios of 6.5 to 1 and greater tend to be hard on stock babbitt bearings. An engine bored .125 over is still less than a 6.5 to 1 compression ratio with our head. During one of our tests, we held the accelerator on the floor in second gear until the engine was "screaming." Upon disassembly, inspection revealed no damage to the bearings. We don't recommend this procedure to our customers and since we have no control over the condition of their engines we cannot make any guarantees.
But we can honestly say that we have had no complaints. After one of our other tests, inspection revealed that the babbitt began to flatten at ratios of 7 to 1 on a hard run. Unless a person has gone to the expense of modern bearings, they are asking for trouble with extremely high ratios. (Or they have more money than brains!)
Higher compression can also mean better cooling. It is true that the higher the compression ratio, the higher the combustion chamber temperature. But by comparison between two alike engines with different compression ratios, the higher compression one will tend to run cooler. The high compression delivers more of the energy of the fuel as power and less for heat. Consequently, it will run cooler at the same speed and load.
We conducted an experiment on a High Country Tour in Colorado in 1979. At high altitude, the boiling point of water is lower. On a steep grade, our Model "A" never lost a drop while the rest were losing water with a steady trail behind them. We passed them in high gear with smiles on our faces!
In conclusion, once you experience the difference that our high compression head makes, to go back to a stock one would be like going back to a burnt valve or running on three cylinders when you can have four!