High Performance, Animal / 206 Ported, High compression, Cylinder heads
Installing a 1.250” intake valve or bigger in a stock Briggs Animal or 206 cylinder head provides little to no improvement in performance!
Here’s why: When installing a bigger valve the side of the valve closest to the combustion chamber wall is so close that it actually restricts the flow of the intake charge.
When the vacuum in the cylinder is available to the intake port the incoming charge wants to take the shortest route into the cylinder, which is the short turn radius. That is the area directly under where the port enters the bowl.
You might think that the larger valve area on the opposite side (towards the center of the cylinder) will compensate but it doesn’t. The air wants to take the shortest, easiest and fastest route into the cylinder! That route is on the short turn side.
Here's an analogy for you. If you watch water flowing over a dam (or Niagra Falls) you see it immediately falling. You don't see it extending several feet from the dams edge.
Another reason not to put a 1 ¼” intake in a stock bore engine is that the valve may even hit the edge of cylinder wall. Needless to say, that can be a real problem.
If you look at a good billet Briggs head you will see that the intake valve has been moved towards the center of the combustion chamber to help reduce or eliminate the restriction on the short turn side.
There are ways to fix this problem but that starts with a big piece of aluminum.