Can you get casting numbers of the heads in question? Or splits of the years? I might be able to cross reference something amongst my AMC information related to this topic...I've thought about taking the 258 out of my 76 AMC Gremlin and somehow integrating it with my Jeep stuff but the 70s vintage 258 has a few differences from my Jeep's 258, like the thermostat housing is different, I'm sure the timing covers and water pumps might be different. The heads could be a different casting but it's possible that the valve seats in the newer head might be more robust due to better metallurgy etc. The 4.0 head on the 4.2 block gets its reputation not just for higher-RPM flow and power but I believe offers a higher compression ratio... can anyone confirm this? I'm not sure if the 4.0 combustion chambers are smaller for the smaller motor and FI, or larger for better quenching...If you're going to lengths to change the cam and manifolds, the head flow theory goes out the window. The aftermarket cam and manifolds will require different air flow rates than factory and thus what may work for one combination may hold back another...
What years are we talking with old 4.2 heads? That engine was used for a good while.
I am uncertain of what exact years of the 4.2 head are preferred. I have a 1978 AMC 258- next time I am over at the shop I will clean the goo and get a casting number- Now I am interested in 'knowing' this, too
but IIRC anything before 1981 counts. Not sure. Maybe someone will chime in with real data?
Valve seats: this is another question I have had. Are those old heads acceptable for use with unleaded fuel? Leaded fuel used to keep the valve seats from wearing; unleaded causes problems for some older motor's valve seats.
As far as compression: the 4.0 head gives more compression because the combustion chamber volume is smaller in relation to cylinder volume. That doesn't make the 4.0 head special or better other than if you have the octane to use the higher compression, you *can* make a little more horsepressure with it. As far as the 4.0 head being superior: it flows a lot better at mid to high rpms. For my kind of wheeling and road use, this doesn't do much for me, but for mudding, bogging, or sand drags I don't think you can beat the stock 4.0 head even as an easy swap.
Both the 4.0 and the 4.2 stock cam lobe specs are the same at .253 and I believe they both have 1.6:1 ratio rockers, so a cam change WILL make a difference. There are one or two years of 4.0 cams that have longer duration cam profiles (I have been told) but other than that, stock-for-stock, a carbureted motor would probably love a mild cam and header. A small-ish diameter header would likely be an all-around improvement, but would especially benefit lower-rpm power.
Those are the things I know, some opinions based on non-jeep experience, and some questions raised about things I don't know. Loving the discussion