I'm with RJConnet on this one. I have a number of K&B Sportsters and they run just fine without muffler pressure, but there are cases where pressure can help. Like RJConnet stated, if the tank is positioned in a reasonable fashion, they run fine without pressure. I had one aircraft where I needed to position the tank a bit further aft. I ran pressure on that one and it was a happy camper. The needle valve was a bit more sensitive, but very consistent.