the common myth, it aint as easy as you think it is.
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.
So here's the problem with swapping... The 'i' chamber needs a pop up piston in order for it to get the compression ratio it needs to make good power, and the 'e' pistons aren't of that design. When you put the 'i' head on top of a stock 'e' bottom end you are LOWERING the compression ratio, lowering equals bad, lowering equals less power. By doing this the ratio becomes LESS than the 'i' stock ratio of 8.8:1, I believe it's something like 8.4:1.
Now hopefully this clears that up.. if there are further questions email me. So what does yield more power? I'll be posting another article soon with different methods of building up M20s. 'e' and 'i'.
If for example (b27block,crank,rods,b25pistons) there would be a clearence issue where the b25 pistons need to be shaved ~3mm have a good fit.
