I had pulled my intake manifold in the process of reworking the entire exhaust system (i'm getting Timevalve setup next week!) and I noticed quite a lot of carbon built up throughout the manifold. Mostly very thick black oily build up around the cold start valve area but down by the cylinders it was also quite thick and looked like charcoal, just what you would find on a chimney wall or an oven that hasn't been cleaned in 50years. See the before and after cleaning pictures below and note the size of the mound of carbon I pulled out, and that's not even half of it.
I estimate it was about 1mm thick everywhere and very rough, which must reduce available airflow volume. I'm sure the fuel injection pump is optimally adjusted assuming a clean intake, so effectively this sort of airflow impedance must also cause a rich bias to the mixture which further reduces power and efficiency (milage) .
The openings into the head are slightly oval (39mm x 35mm) so if you assume an average of 37mm and reduce that to 35mm when dirty, surprisingly it works out to over 10% reduction in cross-sectional area, at least for my case. I'm sure air velocity increases, but so does turbulence with the dirty rough walls, so there's probably no way to know the true full impact without some careful experiments that I would love to do if it didnt add to my downtime.
In any case, the bottom line is that a clean manifold should run more efficiently than a dirty one and allow for up to 10% more air intake and power output, especially when your foot is planted on the floor as mine is not so infrequently. I dont know that I would pull it just to clean it, but if you have other reasons to remove it, then I would highly recommend allowing time to properly clean your intake. Please comment if you have ideas other than elbow grease to get it clean. I've scraped it out pretty well and plan to soak the whole thing in a special aluminum friendly carbon cleaner next.
