I have had the opportunity to have a few M8’s on the dyno the end of this week. Two of them didn’t have the oil cap vented. One was a 128 with head work putting out right under 150/150. Another was a 124 drop on putting out roughly 125/135. The last one was a HD stage 4 114 w/ a different cam choice putting out 130 something and right under 130 tq. All of these bikes did not have the prior sumping issues. All of them did have after market pump and plate. Both the blue and red brands. Only the 114 had the oil cap vented and on my suggestion. He did spend the four dollars to put a one way in it.
My testing consisted of 100% throttle runs. I would make 3 pull with oil cap on. Pull oil cap and leave hole open and make 3 pull back to back. The one with ventilator got the hose pulled off of cap and plugged. This actually had three test. Cap removed from bike, a plugged oil cap, and ventilator with check valve in place.
My findings? Absolutely nothing. No change in those back to back runs. I suspected that there would be and I was wrong. I also saw nothing gained. So, the idea that it isn’t breathing enough is also debunked for me. Anyway, findings posted either way.