The porting of the 12v CTD head can be very costly with an ample engine build budget. A properly ported head goes way beyond simply hogging-out everything. A true head-porting professional will have and use a flow-bench to measure actual improvements in flow, at varying valve lifts, with each round of whittling. It's not just making each port flow a lot, it's also making them all flow the same consistent figure.
Right off, the 12v head has the intake log (manifold) cast integral with the head. That means that while you can port / clean up the intake bowls, you can't do anything with the intake runners for the intake log being in the way.
Now days, it's common practice to cut the integral intake log off. Machine the remaining head surface, drill and tap bolt holes, and install an aftermarket intake log. The design of the aftermarket intake logs are all over the place. Some being better than others for a given application.
- The point of cutting the integral log off is to have full access to the head's intake runners for porting. You don't necessarily need some fancy voodoo log to bolt back on either.
The stock/OEM cast integral intake log has a rather nasty bolt hole boss cast into things such that the boss is well in the way of intake flow to cylinder #1 . . .
The exhaust side of things is easily accessed for porting.
A few years ago, I got the funds together to buy a mildly ported head and was lucky enough to call PDR at the right time. They had a head that fortunately had all the casting molds in the right places resulting in port walls thick enough to allow a lot of very effective whittling. The customer who the head was built for backed out of the deal and as it happened, that head was the only one they had on the shelf. They let me have it for a good deal.
At the time, PDR advertised the typical stock/OEM 12v head flowed 145CFM (intake), and 153CFM (exhaust). Of course, this is at a given valve opening (lift). The head I got from them flows 195CFM (intake), and 220CFM (exhaust). It included O-rings.
Intake valve bowl . . .
Exhaust valve bowl . . . .
Exhaust port (ceiling) . . .
Exhaust port (floor) . . .
Valve unshrouding . . . .
Back-cut valves . . .
The above head combined with a Hamilton 188/220 cam, a Hellman intercooler, conservative twins, etc, flows a lot of air (volume) with a lot less restriction to flow (which means a lower "boost" number changing nothing else).
I like it a lot!
I got it from these folks in Washington: http://www.pdrdiesel.com/cart/catalo...eads-31-1.html
Hope this helps.