I am starting the disassembly process of my 87's engine and just took off the lower manifold last night. The runners, port openings, sleeves, injector diffusers, and actuator forks were all coated with a thick, flaky black gunk that must have stayed in there even after running fuel injector cleaner through the engine.

I am rebuilding due to a stuck apex seal on the front. Compression is all 90ish on the faces that read. It is amazing that Mazda could design an engine to feed off of its own burnt, contaminated, dirty crankcase oil. I am amazed that the engine ran as well as it did before the seal got stuck. It is DIRTY inside that manifold! I'll attach a pic in a while . . . didn't have the camera with me last night when it all came apart.

 

So clean it

 

No if I leave it all dirty, the gunk causes a restriction in the runners which increases intake velocity thereby increasing low end torque!

 

If you have the time, equipment, skills, and money, you can experiment with some removeable venturis in the intake runners to increase intake velocity.