smokinmx6 said:
i am completely uneducated about port polishing and what it does, but a friend of mine that has a v6 firebird(which i smoked) is going to have a guy do the port polishing for him. from what little i know it is just widening the intake or something like that. if anybody knows anything about this i would love to know, and can it be done to our mx6's. i am about to replace my valve cover gaskets and figured that it wouldn't be too much to take off the heads as well. i am also thinking about phenolic spacers. thanks for the help.
Think first about large scale air movements: air rushes much easier and with less turbulence across a prairie plane than it does through downtown Chicago owing to the lack of obstructions the prairie has over the buildings and whatnot in the city.
Now scale this down to the intake. The intake runners and ports are generally produced by sand-casting in molds that are put together roughly in halves. When the molten metal is poured into the mold, it comforms to the shape and texture of the mold (the top of your intake manifold shows this sand-graininess) and also, some seeps into the area between the mold halves; this is called "casting flash". Manufacturers don't, in general, have the time to remove the flash and smooth down the rough surface resulting from the casting process. The result is millions of microscopic "skyscrapers" sticking up into the airflow a bit that can reduce the bulk velocity, cause turbulence etc. Additionally, the casting flash can sometimes be quite large which can also disrupt airflow.
These phenomena occur in both the ports (in the heads) and in the runners (the intake manifold). Porting, in addition to opening up the overall port size a bit, also serves to smooth out the sand-cast roughness which usually leads to better airflow. A mirror finish isn't always best (do you know why golf balls have dimples instead of being perfectly smooth?) but a smooth finish is better than the sand-casting legacy finish.
I added this paragraph during the edit...
You might have noticed that some car makers are going to composite intake manifolds. The Corvette's LS1, for instance, uses one. It looks like black plastic, not unlike the factory airbox in the MX6/PGT. Two of the biggest benefits of going this route instead of cast aluminum is that (a) the composite is lighter and (b) the runner finish is much smoother than casting gives, resulting in incrementally higher VE (volumetric efficiency).
It can be done to the V6 (and 4-banger too). The intake on the 6 is tough because it's hard to get deep-down inside it due to the way the VRIS casting is and the length and shape of the runners. You can usually reach an inch or two into the runners and you can clean up the area directly behind the throttle body, but to get deeper than this means cutting parts of the plenum off to gain access to the insides and welding covers in once done porting. The head ports on the V6 aren't too bad but can benefit a bit from cleaning up and a multi-angle valve job (which smoothes the radius the air must take between the valve seat and the open valve).
Removing the heads is more work than you might think, even after removing the intake...
Remember though, bigger is not always better when it comes to head ports and intake runners, especially on little 2 and 2.5 L engines. The general principle is to try to get the maximum possible airflow through the smallest possible runner. Air column velocity is as important as air column volume, if not moreso.
[Edited by Mike 94PGT on 02-09-2001 at 04:40 PM]
