Some great information on porting copied and pasted from another thread long ago. Russell from NR-Racing speaks on porting.
Not sure I follow this. The chamber (on both styles) are bowl shaped. As you mill the head it removes part of the chamber and actually unshrouds the valves. A 18cc head milled .080" has much better flow potential than an unmilled head. I say potential because edge treatment can effect the flow. (i.e. round off the edges). We have a digital SuperFlow flow bench and have flowed 100's of heads in all kinds of different configurations with every type of porting one can imagine. We have backed up a lot of this with dyno testing (which does not always agree with the flow bench). In the end, unless major work is done to the chamber, the 18cc heads will always outflow the 14cc.
The only advantage to a 14cc head is that compression can be increased to around 10.3 to 1 by just bolting on ahead. You need to mill a 18cc head .050" to achieve that. However, a .050" 18cc head can make more power than an unmilled 14 cc head.
Here is some info from previous posting I did:
The key to making power with a Honda head is mild porting, smoothing out the short side radius, blending the bowl, and shortening the valve guides by .100". Together these will yield about 60% better flow than stock. Trying to remove any material (i.e. making the port larger) tends to hurt performance and there is really not enough aluminum to do so. In fact raising the floor of the port improves flow quite a bit. Honda ports (esp. the exhaust) are intentionally oversized to promote cooling. The ports should also be left rough since this creates a boundary layer that improves flow. Best is too sand blast the ports after porting/smoothing. This type of surface will flow much better than a polished surface, plus it helps fuel atomization. The exhaust is not so critical since carbon buildup creates a rough surface on its own.
A good three-angle valve job will allow up to 15% improvement in flow on a GX160/200 head. Unless running huge Mikunis or Tilly’s and turning 7000+ rpm, the stock valve sizes work very well and are capable of making up to 16+ hp. Stainless valves do flow a bit better than stock stuff. For high RPM operation (with big carbs & cams) the 27.5 mm intake valve will help and will allow 20 hp. We have tested 28.5 & larger valves but have not seen any improvement. In fact, we have seen power decreases on the dyno using these. The exhaust valve size should be left alone as it is too big to begin with. A few years ago we installed a smaller seat along a 120 intake valve in a GX160 exhaust port. The motor showed improvement, esp. on the low end.
It is important to know that actual flow through the head is also a function of valve lift. Needless to say, the more lift, the more flow. (Note: higher lifts can decrease velocity, which in turn decreases torque). For the most part, GX200s show improvement as lift is increased. However, on the flow bench a GX160 race head (big valve, ported, milled) does not show any additional flow after about .350 lift. Dyno testing combos above .350 lift (i.e. .400 lift) has shown only minimal high RPM improvement usually at great losses at the low end. Also, the Honda valve train does not do well with a lot of lift. There are a lot of geometry issues that one must address. In fact 1.3 rockers should be avoided as they really play havoc on the geometry. We have seen many failures from using 1.3 rockers with big cams. You can get them to work, but custom valves, spring pocket, and other head work is needed. Stock with stock or 1.2 ratios if possible.
The best combinations for max power tend to be big duration cams (~260 at .050) at .330 to .350 lift. Cam with .280 to .290 lift using 1.2 rockers or .308 cams with 1.1 rockers tend to work the best. However for most applications (i.e. all around performance), a .270 to .280 lift 220 to 230 duration cam combined with 1.1 or 1.2 rockers will be the best set-up. Cam timing is also critical…..
TT
