the p90 uses a 3/4 ported cylinder and is good for barrels upto about 400mm. Anything larger, you'll need a non-ported cylinder.

"M4 Cylinder = "3/4 hole" aka Type-2 17cc - 229mm(min) 430mm(max) barrel length"

quoted from acme airsoft.

So does anyone know if taping the port will do anything?

It would probably block the port therefore making it act like a non ported cylinder but there has to be something you're missing.... I'm not entirely sure if it will block it or if the pressure from the compression will make the tape leak air eventually. Try it and see what the results are I'd like to know.

This may help match your cylinder volume to your barrel.

http://arniesairsoft.co.uk/?filnavn=...andbarrels.htm

no, because the port extends past both sides of the piston oring as the piston moves by, it just means the air would move to behind the piston as opposed to outside the cylincer

Not true as the port is designed to vent off a large amount of air. Even if it does allow a small momentary airleak as the o-ring passes over the port cut-out, taping a port will restrict the amount of air vented out substantially over a completely open large sized port. Make sure you do it properly and don't have a sticky wall facing in.

Your milage will vary but I've had 60 FPS increases from taping ports when matched with an appropriate length barrel. This is only a short term solution though and an actual unported cylinder should be purchased if you want to do this reliably.

Regarding the issue of SRC cylinders being unported - that's just a thing I've seen on most SRC cylinders. Maybe they don't make any ported cylinders. If you watch Ken's video, that one's unported as well.

http://www.youtube.com/watch?v=cQkMb...embedded#at=28