Okay, you wanted to do your part in conserving water, so you bought a Caroma toilet, that expensive Australian brand that's does both 3L and 6L flushes. And then you discovered that the toilet bowl design is really badly flawed. When you flush, a bit of the contents of the bowl go flying into air and coat the underside of the lid of the toilet if you have the lid down, and we really don't want to consider what's happening if the lid is up.

You can see the cause of the problem in the attached picture. When you flush, water from the tank jets out along the underside of the rim at fairly high pressure from both sides. When the two streams collide, they change direction and ARC through the air to the backside of the toilet, where they lose their velocity with a great deal of violence. This splashes the toilet bowl contents into the air. In addition to being a mess to clean up, this poorly directed flow does very little to keep the bowl clean and that spray is distinctly unsanitary, particularly if your digestive system is a bit on the runny side (my fellow vegetarians are probably nodding knowingly).

Step 1: The Desired Flow

What we'd really like to have happen is the two jets of water spiral along side the surface of the tank in the same direction. Of course to do that, we'd need a device (the green shape in the attached picture) that would completely reverse the direction of one stream, and gently alter the direction of the other stream.

Step 2: Constructing the Flow Modification Device

My flow modification device is cut from a gardener's kneeling pad. It's stiff, closed cell foam and cost about $5 at a local hardware. It just happens be the perfect thickness to fit snugly inside the lip of the bowl. Note: the lip of the bowl is wider on the sides than at the front.

I used a pair of cheap scissors to cut the the fin shaped piece below.

Step 3: Installing the Flow Modification Device

The flow modification device is inserted into the rim of the toilet bowl, with the orientation shown in the diagram below.

Note that the rim isn't of uniform design; the back half of each side is actually closed off into a pipe. You need to insert the flow modification device just after the end of the pipe section. If you feel inside the rim with a finger (or use a dental mirror), this will become obvious.

Step 4: The Improved Flow

The attached image shows a flush after installing the flow modification device (the green line is to represent flow modification device hidden underneath the lip of the bowl). Sadly, a still image doesn't do the device justice. What you will notice though is that there is no "Caroma waterfall"., the water is actually sluicing around the bowl in a wonderful manner.

Caveats: the foam is wedged in there pretty firmly, I don't think it's going to come out anytime soon (at least until the foam seriously degrades). However, for all I know, the pressure exerted by the foam on the lip of the bowl may stress it and cause fracturing in the long term. Unlikely, but possible so considered yourself warned.