In the earlier days of emissions controls, it was felt that an open chamber, lower compression ratio was beneficial to these efforts. The exhaust temperatures between our '66 Newport 383 2bbl and our '72 Newport 400 2bbl are significant. The '66 is much cooler, by comparison. Keeping the chamber surfaces hotter and more heat in the exhaust helped "cook" the mixture better. Both engines did not have AIR pumps, just normal optimized tuning and specs.
So open chamber heads were employed, using the larger exhaust valves for better scavenging of the exhaust mixtures. Valve sizes are determined by the largest bore size the head can fit onto. Larger exhaust valves can help, BUT the flow capabilities of the ports can or cannott make that beneficial. As the size of exhaust piping relates to this too. As does the normal rpm level of the motor.
As mixture flow came to be much better researched and understood, plus combustion chamber dynamics, it was discovered that the original wedge closed chamber heads could actually produce more power and torque. "Active mixtures" were operative. On our '66 Newport 383 2bbl, its base timing spec was 12.5 degrees BTDC. Moving it to 15 degrees BTDC would increase the idle speed in direct proportion to the timing increase. My '70 Monaco 383 N, set at 5 degrees BTDC, did not have such a relationship as increasing the base timing did little to increase base idle speed, by comparison.
When I discovered NGK V-Power spark plugs (with the v-notch in their center electrode), they made the most noticeable difference in the open chambers and little in closed chambers, by observations. They still worked well, just not the same degree of improvement in throttle response, off idle and otherwise, as the open chambers had. Chrysler engines or not. When I put them in my '80 360 2bbl, it was a difference I noticed immediately upon first start, although the prior Autolites were burning well with a normal plug gap.
Remember, too, that many cyl heads were designed "by sight" and what was suspected to work, back then. Chrysler had some decent heads back then, but I also suspect they had few resources to verify that other than gauge what Chevrolet was doing with their ports and such. There were few efforts, it seems, to really maximize Chrysler heads for great low-to-mid lift flow, only focusing on max exh sizes and such. Can't forget the DC Porting Templates, either!
What I'd like to find is an aluminum cyl head for Chry 383s that has great combustion dynamics and air flow to support an easy 5500rpm and make another 50lbs-ft of torque at 3000rpm, with the most efficient combustion chamber dynamics for efficient burns with normal flat top pistons and 9.5CRs. Yet still have ports that don't "stall" until about .550" lift with great low-to-mid lift port flow and velocity. Then add some Mahle narrow-ring "MM" costed pistons to round out the combination. With Plateau Honing for good measure. That ought to make a nice motor.
Just some thoughts,
CBODY67
