I'll admit that I concur that the advertised CR CAN always be a bit less than actual CR. But my orientation is that it can't be a big lot of difference OR that all will measure the exact same.
Let's consider "advertised" as "blueprint specification", which is what NHRA and other major racing racing bodies use to determine if an engine is "legal" in the post-win teardowns. So, getting the block decked to spec, using pistons with the ORIG spec deck height (i.e., compression distance), getting the heads CC'd for correct combustion chamber volume (which can include how much the valves are "sunk" into their valve seats), and using the orig ..018-.020" compressed distance steel shim head gaskets will be the only way to get to that orig advertised/blueprint CR.
In my experiences, the '66 Newport we bought in early '67 with 7100 miles on it (383-2bbl) would NOT run on regular fuel back then. Timing was at factory spec. Had to run Premium in it to keep it quiet. With premium, I could bump the timing to 15 degrees BTDC from the spec 12.5 degrees BTDC for a slight bit more off-idle response. This was with the rated 9.2CR.
When I got my '67 Newport CE23 in 1981, it had the optional 383 4bbl engine (10.5 advertised CR). Had to run 91 pump octane super unleaded in it. No problem. It had a burnt valve on #7, so my machine shop operative had a set of abandoned 906s laying around, so we/I did the bronze heli-coil guide work and I had a set of NOS 440-6bbl valves, so we reconditioned them. When assembled with .040" compressed thickness head gaskets, it would "trace rattle" at WOT in 2nd gear. Again, with the timing at factory specs. Otherwise, it was quiet and ran well. No changes when I upgraded to the Mopar Perf electronic ignition kit later on. I had computed that the former 10.5 engine should be in the mid-9sCR range. When we pulled the heads from both 383s, there was no large accumulation of deposits on the piston tops and certainly not in the combustion chambers. just "normal" for an engine driven on the highway, it seemed.
The fuels used in the '66, earlier on were full-leaded fuels, when Regular was 95 Research Octane and Premium was 97+ Research Octane. Later, the Phillips 66 Flite Fuel I was using in it in the 1973+ time frame had a pump octane of 95, which should have been approx 99+ Research Octane, still leaded but with reduced lead from the earlier days.
So, the 9.2CR engine would not run on 94 Research Octane fuels, quietly. and the 10.5 engine would run well on 94-95 Super Unleaded fuels with 906s on it.
In a SAE Transaction Paper I found in the automotive engineering section of the Texas Tech Library, for the original RB 413 in 1957, it stated that the spec fuel used for calibrations was 97 Research Octane "premium" fuel. So that wouid have been what was used to determine the spark advance curves, I suspect, as that was pretty much what was available, back then, for Premium gasolines. Of course, there could have been variations of probably plus/minus 1 Research Octane number, in the real world. I'm sure they used not only a spec fuel but also bought fuel on the open market for actual road testing, back then.
I also observed, in the earlier 1970s that the TX DPS only purchased Regular-grade fuel for their enforcement vehicles. Which were (except for 1968) Plymouth Furys with 383 4bbls and some Ford Galaxie 390s. As they saw little/limited WOT use, the regular fuel apparently worked decently well.
In some cases, unless you are going to competively race the motor, the CR makes less of a real power difference than "feel" does. Is off-idle response good enough to burn rubber a good bit? Will the engine eagerly send the speedo needle into the triple digits without a lot of coaxing? AND will it run quietly, start quickly, and get decent fuel economy?
We all want the most effieicent engine around in what we rebuild, understandably so, but I also suspect that the real "feel" between a 9.5CR engine and a 10.5CR engine will be more negliable than some might suspect. Considering that I computed, back in the later 1960s, that a Chrysler TF powertrain comsumed about 15% of engine power before it got to the rear wheels. Which can make 20 engine horsepower difference much less when the rear wheel turn. Which can make the "feel" more important, in the real world. BTAIM
There are other side issues with replacement pistons since the earlier 1980s when it appeared that the best fuels we'd get were 91 pump octane super unleadeds. That's for another thread, but important. One thing that not many people might know about, but my machine shop operative did as he did lots of circle track and drag racing engines.
BUT back to the original issue of using the 1973 pistons with closed chamber heads, the spec for those pistons is still .125" "in the hole" at TDC, which will not build a good CR, although with a cam choice, they can work decently well.
The other thing is that from my research on pistons, until you get into forged pistons (rather than cast), you are pretty much stuck with low CR pistons. With .040-.060" compressed distance head gaskets. Sometimes you can't win for losing!
Just some thoughts and experiences,
CBODY67
