I got my plenums back from my local machine/performance engine shop. I was curious so had it flow tested. The owner of the shop did the flow testing, and was polite, but not impressed with the results (they make highly tuned and blueprinted vintage muscle-car and dragster engines).

I was initially impressed that things were pretty close, until i saw cylinder #6.
http://i1168.photobucket.com/albums/...psqzqrzdnz.jpg

#1 261 CFM (cubic feet per minute)
#2 251
#3 252
#4 245.5
#5 248
#6 238

Lets run some numbers...
- The largest spread is between #1 and #6: 23 CFM. That is a near 8.8% difference.
- The mean CFM is 249.25.
- Thus deviations from the mean are as follows...
#1 +4.7%
#2 +0.7%
#3 +1.1%
#4 - 1.5%
#5 - 0.5%
#6 - 4.7%

Now, my engine will be punched out to a 3.4L (actually 3367cc, or 205.47 cubic inches). Lets calculate the total CFM demands of my engine...
- max rpm (6760) x cubic inches (205.47) = 1,388,977.2
- 1,388,977.2/3456 = 401.9
- 401.9 x 0.85 (presumed volumetric efficiency) = 341.6CFM
- Thus, an ideal intake should provide about 342CFM to meet my 3.4L engine's intake needs. In other words, 56.9CFM per plenum!!!
- My intake (at the plenums) provides 249.25CFM on average per plenum. The worst is 238CFM (#6). That is still over 4 times the flow needs of the cylinder at max rpm.

Such info in my mind suggests that extrude honing of the plenums makes little difference if any, as this simply is not an area of flow limitation... not by a long shot. This would support an opinion shared by me from a well known tuner that they had never seen dyno proof of gains from extrude hone.

I welcome thoughts, counter-points. I will also look to see if there is a more accurate way to estimate the total CFM needs of an engine.

That is not how it works, only one cylinder pulls at a time.

I have taken discussion here... http://forums.pelicanparts.com/porsche-911-technical-forum/862450-3-2-intake-plenum-flow-testing-interesting-results.html#post8594915