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I am wondering what variations of the gas engine there are.

My parts book mentions:

1. Tropical with compression ratios of 7.2:1, 83.5 HP
2. Tropical with compression ratio of 7.5:1, 85.5 HP
3. Normal with compression ratio of 7.5:1, 87 HP
4. Normal with 7.8:1 compression ratio, 90 HP

My shop manual gives jetting for four carburetors, one to meet ECE 76, one to meet ECE 79, one exempt from ECE due to truck being over 3.5 tons, and one for "hot climates" which also seems to be exempt from ECE. The "over 3.5 tons" and "hot climate" seemed to be jetted richer and have the "big screw" throttle body.

There are 4 camshaft numbers in the parts book. No clear indication on how they are different. The torque curves of the "standard" and "hot climate" engines look as if there is something else going on other than richer jetting so maybe they had different cams.

Can anybody make any sense of this?

I can only assume the factory adapted to a variety of fuel qualities.
The worse the fuel the less compression and horsepower .....

makes sense to mill offf some material from the heads if you live an a high quality fuel area ...

I think, but am not sure, that the compression was to be varied by shortening the cylinder.

I also think it is strange that up in front they list the different compression ratios, but in the tabulated data do not show parts for the different ratios. Maybe they planned for different ratios but never used them.

In another thread someone mentions there was a 2.7 version from the factory. I believe this is different from Jim L's 2.7. I cannot find any info on the factory 2.7. Anybody know anything about it?

Cylinder length varies 128-130mm. The shorter cylinders are more commonly found on 6x6 variants. Jetting is used for the rest.
On cams, the steel billet itself varies, they can have 1 of 3 different sized bore end plugs, all the ones I send for re grinding/profiling have the same starting profile.
The factory modded engines were for the most part 1 offs, so little data is out there.

I have been struggling with my engine and i am starting to make it run ok with hot climate jetting and definitely more advance than 7 degrees. With 7 degrees the engine spits and backfires significantly. Also I noticed my carb bases only have 4 holes by the electro idle cutout valve, and not 5 like the manual describes. It looks like I may have a hot climate carb set?
Cheers,
JC