I am considering making a centre air filter set up for my dual 40's.

The engine is 1600 capacity and I am running 28 vents.

It appears that there is a lot of turbo induction material about and it would be easy to make a system from 3" dia pipe work.

I have a feeling that with 3"pipe work I may have air velocity trouble?

Is there a maximum size I should be looking at or will 3" intake tubing be OK?

I would think that the 3" would be ample

I used CB Turbo Tops and they are 2"

Have a look at the CSP one

I made this et up on a 1775 with a engle100 cam, the pipe size was 2.5 inch, an over kill really when you consider the standard air clearer inlet size, this worked very well and made a good improvement

Thanks for the photos, I have started to fabricate a 3" system.

Have a look at Alfa boxer style engine set up.
Fan housing maybe a problem but could give you some ideas
They fit with some mods to T4 in a 914

here's what i made out of PVC pipe as a proof of concept. Works a treat at quietening it all down. I might eventually get around to getting it fabricated in steel or ali pipe

this is the one that we made ages ago its based on an excel airbox.

Thanks, that givers me plenty to work with.