The earlier motors (69-71?) had hi-compression pistons (9:1) and solid lifters. Then the compresson ratio was dropped for the 72-75 motors (7.6:1) and hydralic lifters. Then in 75, only, F.I. was used instead of a solex carb. You can tell the difference between the hi-comp and lo-comp pistons, by the fact the hi-comp pistons have 2, 1/2 moon valve reliefs cut in the top, whereas the lo-comps have a dish in the entire top of the piston.
you can pull the starter and look at the casting numbers on the block, which will tell you if the the high comp or low comp motor, or what most people do i just pull the plugs and have a look inside when the piston is at TDC. 68-70 had high comp pistons, solid lifters and forged crank, 71 and up had dish pistons, cast crank, hydrolic lifters, 75 switched to fuel injection.
i just bought a '72 gt, and i'm wondering about the difference in engines between the early and late models. can one put hi-top pistons (9:1) in a 72 block? would i have to change to an early cam and lifters?
also, are there any real performance differences between hi-comp and low comp? both 1.9 liter engines, right?
Aside from the obvious, most parts are interchangeable.
The obvious? Well, don't put a 3 bearing cam in a 4 bearing head, as the empty bearing race will dump all the engine oil pressure. But I believe you CAN do the opposite, in a pinch. And I believe you CAN use the three bearing cam in a 4 bearing head if you remove the empty bearing and install it with the oil hole intentionally misaligned. And beware of using a solid lifter cam with hydraulic lifters, as the ramp-up is much steeper. The opposite might work, but why would you?
Pistons are interchangeable, and it's VERY desirable to get the flat-top pistons. I believe that the difference in advertised horsepower was 102 HP down to 76 HP. Even factoring in advertising exaggeration, you're looking at a 25% increase with just HC pistons. So long as the rest of the engine is up to snuff. It won't help to have the flat-top pistons if your cam is worn flat, or you have burnt valves, or the carb is flooding (spelled Solex), or ....
I believe the heads are interchangeable, and you can even use a later ('72 and later) head gasket with the two extra (12 in total) bolts. Preference seems to use the '71 head, which has the 4 bearing cam but is less prone to cracking than the later flame hardened valve seats (get stelite seat inserts installed).
There are a couple TERRIFIC threads on the topic of engine performance. Try: