<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Lorin Budzinski:
The early, closed combustion chamber is more efficient in power production than the later open chamber heads. Tests done during the 50's show a 5% increase in power from a closed chamber to a fully open chamber. On the other hand, the airflow is enhanced slightly by getting that part of the chamber out of the way. The additional airflow raises the horsepower and the net result is pretty much a draw. The reason for the change in shape was mostly for pollution considerations. If you look at the aftermarket race heads, you find that they are produced with the minimal combustion chamber philosophy.<HR></BLOCKQUOTE>
I agree with Lorin's post!
With the closed chamber cylinder head, you have a much larger quench area than an open chamber head. When the air/fuel mixture is compressed with a closed chamber head, the air/fuel mixture becomes much more turbulent. The more turbulent the mixture, the quicker and more efficient the air/fuel mixture will burn.
An engine set up properly (Zero Deck) with closed chamber heads and all other parameters equal, will require LESS ignition lead than an open chamber head.
An added bonus! Closed chamber heads, again with a zero deck, are also more knock resistant than an open chamber head.
Lorin is also correct about the open chamber head and emmisions control. You will notice that all American V8's began to use open chamber heads starting in about 1968. The same approximate time that Federal Smog Laws were being put into place.
I also would not modify the factory 670 chamber. It is an efficient combustion chamber. By changing the shape, you also alter the mixture swirl patterns. Without extensive dyno testing, I personally would not touch the chambers.
If you want to lower compression, properly dish the pistons.
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