I did not install a sniffle valve when I built my RV7 and was lucky I did not ruin my engine. I was in St Louis one time and had to delay my takeoff due to very heavy blowing rain of several hours duration. When I went to start my engine, the prop rotated about half a turn and stopped. I got out and tried to pull it by hand but it would not turn. I took the cowling off and could not see anything wrong. Tried starter again with same result. I didn't know what else to try so I removed a plug from each cylinder and imagine my surprise when water came out of one cylinder. The engine was hydrolocked. I was lucky cause if one cylinder had fired up, I probably would have bent a connecting rod or worse. Anyway, I removed the plug that I had installed in the sniffle valve hole, drained a bunch or water out, pulled the prop through several times to get water out of the cylinders, started it up and all was well.
I have the Vans vertical induction on my IO360 that goes to the front of the baffle and had never heard of a sniffle valve till I called Vans and informed them of what happened. I bought a sniffle valve and also suggested that they put something an the RVator but they never did.