I'm ignorant and in need of educating. What is meant by blending the valves, such as in a 1200 conversion with 1200 valves?

couple of points...

The steps between the seats and the ports can be quite dramatic in production heads..

Blending the seats is just blending the transition point so the steps are removed...

Most people would do passable job when the valves are kept the same size and the transition areas in the head are cleaned up.

DO NOT ENLARGE THE PORTS.

Unfortunately puttin 1200 conversion valves into an 883 head requires that the bowl shape be changed and enlarged, this is an area where you can go backwards EASIER than going forwards. If no work is done and the seats are just cut to take the 1200 valves then you may end up with LESS flow than you started with.

Basicly if ya wanna go 1200 valves it's mainly for high speed use and as such needs to be done by someone who knows what they are doing... gains can be had down low with larger valves as well but again it is as much from the correct port/seat shape as it is the larger valves...

Aarons got some dyno charts up as a comparison on one of the 883-1200 threads.... have a search

Blending is making a smooth transition from the port wall to the seat.

The seat is just a ring of cast iron or steel alloy that's pressed into a pocket in the aluminum that's been cut around the perimeter of where the port meets the chamber. Problem is, it's not put in there directly on-center with the port, nor is it's i.d. necessarily cut to the same i.d. as the port. Hence you get overhang. The port may overhang the seat i.d. or vice-versa. These overhangs can cause turbulence and resultant losses. It varies from head to head.

By carefully cutting the seat and port i.d. together, you can blend them together nicely. You can also increase the i.d. of the seat and port beyond where the factory had it. And the size of that hole has a big impact on flow. Put in a bigger valve and you can cut the hole even bigger and get even more flow.

You also have to cut the sealing surface on the seat, where the valve actually rests on the seat. We call that "the 45" because it's at a 45 degree angle to the valve stem. It needs to be flat and clean and perfectly centered to the valve so they valve will seal well against it. And you want to lead the air up to and away from that sealing surface smoothly, so as not to introduce turbulence, so you also cut a radius or more angles above and below the 45. Cutting the 45 and the area above and below it is called the "valve job" and it's absolutely critical.

There's a pretty wide area on the valve where it can rest against the 45 degree cut on the seat. What you see in performance valve jobs is that the contact point will be pushed out more, toward the edge of the valve. That's done to make the hole as big as possible and get more flow. But you don't want to put it out there so far that a little valve recession with age starts making the contact point fall off the valve. There's a longevity vs. performance trade-off here.

What you see in poorly done conversions is they'll just do the valve job out to an appropriate diameter to the valve, but they won't cut the seat and port i.d. out to the right size for the valve. This leaves the i.d. of the seat in something of a cone shape, where at the base of the seat (where it meets the aluminum) it's still basically sized for the little valve.