Interesting stuff - a "how to" guide is given here:
Homework
And some dyno results from someone who did it on a YZ250F with good effect:
I tried Motoman's porting... - Page 3 - General Dirt Bike Discussion - ThumperTalk
Another similar concept:
CVN High Performance Cylinder Head Design
Tend to agree with some of the statements about "If you can get past how annoying he is (and his website is), there is actually a hint of good information there." and from somewhere else "thinks he invented critical thinking"... but there are lots of positive reports - mostly anecdotal and I don't doubt there's selection bias in there, but a few have data (dyno comparisons) to back them up.
Not that I'm planning to modify my ports (especially on the exhaust side, where even JB weld isn't going to be that great
) but it makes sense that
1) There's an optimal port size for any particular application, and the ports on a given engine may well be either too big
or too small for what you want to do with it.
2) Car manufacturers would, in most cases, go for 'safe' rather than 'optimal'.
3) For castings like the P2J head, which is used for the power-VTEC motors and they probably expect will be modded with cams, turbos etc., 'safe' probably means 'big' rather than 'small'...
Another piece of corroboration is that VTEC-e 12-valve mode is not just about swirl, but also maintaining intake velocity. So if you can increase intake velocity without (a) losing your fuel on the bends or (b) overly restricting airflow at WOT (and WOT do we care about that
?), then it may well be worth it.
The HotVWs Mileage Motor was a 2 valve per cylinder design, so I'm not hugely surprised they used larger valves - 4 valve per cylinder engines like the honda D series have a
lot more area, in fact if you tried to fit the equivalent valve area with a 2 valve design, you'd have a 42.4mm intake and a 36.8mm exhaust valve. Good luck fitting those in a 75mm bore without them hitting eachother on overlap
(you'd need a pretty steep roof on your combustion chamber to start with)
I've seen some stuff on extrude honing - makes me wonder if you could do it with a standard pug mill (I suspect you could)...