S4 or S5?
 

What’s up guys,
So I have been recently looking at replacing
my center iron on my s4 NA. I blew the water jacket so I have been on a lookout. I want to get my 88 GXL back and running for now but I do want to eventually boost it in the future. I have heard that s4 NA blocks are weak, and the rear iron usually cracks at around 350hp. So I’m wondering should I just replace the center iron on my s4 or should I go with a stronger block. The strongest block is the FD block, but I don’t have FD money. So the second best choice is to get an s5 block. I’m wondering which is stronger the s5 NA block or the s5 Tll block? I’m also curious if I could just use the s5 irons and retain my s4 housings? Any help would be really appreciated, thank you.

so here are my thoughts.
the S4 NA housing is weird because of the EGR, so if you need to pass emissions, you need to keep the S4 NA iron
the FD or Cosmo have the engine mounts in a different place, so if you used those, the engine wouldn't bolt in the car anymore.

if you don't need to pass emissions, then any FC center iron works fine. strength isn't really an issue with the center iron.

Mazda added the same strengthening to the S5 replacement parts, so if you bought anything made after 10/89, you'd end up with the S5 NA and T2 about the same strength wise.

so if you're building the engine, and thinking about going turbo, maybe you just build a turbo engine?

Thanks for the response! So to recap I could use any s4 or s5 center iron and it would bolt up to my s4 NA block? Also why would the Tll engine be better for force induction? Is it because of the lower compression it has? It seems that the s4 Tll engine has 8.5:1 and the s5 Tll has 9:1 compression ratio. Could I use Tll rotors to yield that lower compression instead of having to get a Tll block? Sorry for all the questions I just really want to learn as much as I can!

the T2 engine has a lot of differences. the rotors are lower compression, 8.5:1 and 9.0:1 vs 9.4 for an S4 NA, and you can just use turbo rotors to lower compression.
the T2 oil pump is bigger, i think mostly to add oil for the turbo, again, easy bolt on

the big difference are the intake ports, the T2 uses a 4 port setup, like the older engines. requires replacing the irons, basically you'd just build a T2 engine.
the exhaust ports on the NA have a diffuser to help with noise, and the T2 doesn't so you can/ should replace the rotor housing with a T2 one, or if you're reusing stuff the sleeve

the FD adds a higher pressure oil regulator, which is like $32 so you should do it too
also better bearings and stationary gears which would be nice, but not needed.
you should build the engine to the FD specs though, they are more in line with higher power

Awesome thank you so much for the help you really helped me!

Out of curiosity, would you need a restrictor plate to do that? Stock turbo, I mean.

The current reg in my NA engine is probably aftermarket since I'm getting around 90psi during a long pull. I would just swap it over but don't want to damage the turbo seals.

EDIT: 90 indicated psi anyways. I should probably check with a different gauge to be sure.