DIY Vortec Head Porting
DIY Vortec Head Porting
I got over my fear of ruining the valve seats and started porting the Vortec heads! This is the first head porting I've ever done but I did a lot of research in books, magazines and among professional head porters. I learned some interesting things. First off, it seems that "gasket matching" is among one of the LEAST worthwhile porting tasks (polishing the metal to a mirror shine is the #1 least worthwhile; turns out it's only for the sake of appearance to the customer...). That is - gasket matching gives barely any increase in flow, because the port inlets are not the bottleneck. Wise men advised me to spend my time porting the MOST restrictive parts FIRST, to get the biggest gains for the least amount of work.
That means opening up the valve throats, which are the true bottleneck as it's the smallest area the air flows through just prior to passing the valve. Here's an excellent diagram to get yourself familiar with head porting terminology:
http://en.wikipedia.org/wiki/Cylinder_head_porting
On the Vortecs (or at least, on my ~2004 #113 castings, although this likely applies to ALL cast iron production cylinder heads), the factory valve seat cutter leaves a big, sharp machined ridge at the valve throat. Take a look at my stock combustion chamber:
However, they say that "nowhere is the penalty greater than going too large in the valve throat". Everyone says you want the valve throat 85%-90% of the intake valve diameter. I took some measurements with my digital calipers I've been so eager to use.
Code:
[b]Valve throat narrowest point[/b]
Intake (1.94") | Exhaust (1.50")
-----------------------------------------------------------------
stock 1.65" 1.22"
85% 1.65" 1.275"
90% 1.746" 1.35"
Notice that GM's casting mark actually leaves the intake throat right at the 85% mark already. So I had to be careful not to remove TOO much material. However, an expert head porter said that to get decent gains from this "bowl blending", you have to actually remove some metal. I wound up right at the 1.74" figure after cleaning the intake up to my satisfaction. In the next photo, you can see I did the exhaust too (but haven't finished it yet). The exhaust short side turn had the sharpest ridge of all.
Here's a closeup of the intake short side turn:
Here's some bonus information. I noticed my stock heads had a shitty "one angle" valve job. Notice the Vortecs have a 3 angle on the exhaust, and what appears to be a 3 angle on the intake as well (the bottom angle seems to get lost (thin) in the cutter ridge on most of the ports, so maybe it's not really a valve seat angle cut?) Remember that's one of the key areas where air flow is most restricted. So you gain a nice little benefit right there, even on unported Vortecs with the factory valve job.
In case you want to look back, here are links to the images I already posted, of the
stock LB4 combustion chamber, and a
side shot of the stock vortec valve throat casting.
Editorial
I've been wondering why this seems like the first time anyone has posted head porting photos/advice on this site. Plenty of people must have done it (certainly the vendors all offer it), but I guess no one else had interest in assisting everyone trying to do the same thing. :? :dunno: I guess what I'm trying to say is, I'd like to
read articles like this (in addition to writing them.) Nonetheless, my truck is practically a product of the good advice I have received from other DIY owners on this site (and others) over the last 5 years, so I don't mind contributing my fair share. :thumbsup: I just hope I don't get edged out at a race some day by a guy who used all my tricks (on top of his own). :lol:
