For our 23" 20 HP Baker Burner with 2 1/2" OD venturi mixing tubes, I have installed 4" diameter venturi flares. They act like a trumpet flare in guiding more air molecules into the mixing tubes. I have used the flares for years but these are the largest yet that I have tried. With our other cars, I have found a remarkable increase in steam recovery and increased fire reliability while using these flares. I made mine out of some stainless steel shafting that was laying around our shop. It took about 90 minutes each to whittle these out on our South Bend lathe. To find out more about these flares, go the the front page of stanleysteamers.com and look under technical.