People always ask and talk about putting an 'i' cylinder head on an 'e' block to improve performance. You would think this makes sense because the 'i' and 'e' are both M20s and the parts are interchangeable. This would work out great if it wasn't for one thing; the 'i' and 'e' combustion chambers are completely different.
On the 'i' chamber (head) you have a large, high volume, dome design. This works well on an 'i' block because an 'i' has pop up pistons. When the raised dome piston reaches top dead center (TDC) it fills the dome chamber and results in a compression ratio around 8.8:1. Think of it as two puzzle pieces. The piston fits the shape of the chamber to make a match.
Then there's the 'e' head. The 'e' chamber is a flat chamber and the pistons are also flat (the 'i' design is superior) so when they are at TDC you get solid compression, and a ratio that is actually higher than an 'i' at around 9:1.