I think that the biggest risk of running an engine without any exhaust is not being able to hear something going wrong mechanically. I can't say I've ever ran and engine without an exhaust manifold or a header on it, but I think the idea is just plain stupid, as in just don't do it. I can't offer up any physics to the why not but the flip side, why would you? We ran a dirt late model in the 80's and early 90's and in those days all we ran were "zoomies" one pipe for each cylinder that went straight down toward the ground, they were about 12" long. We didn't worry about bending valves even running in the fall with temps in the 40's. Then in the 90's the engine guys realized how much horsepower those darn zoomies were robbing and we went to chassis headers, equal length tubes with a collector that was 3-4" and usually around 18" if we could make it fit. The drivers all could tell the increased throttle response with the change.
I guess it all comes down to, if it sounds stupid it usually is stupid.
