Hi Jim! As you may recall, I was experimenting with 40-sized diesel stunt a year ago. I had to look up my log book on my PAW40 to get this info, but here it is, for what it's worth. On a 12X6 RevUp prop, using both the small (so-called "stunt") venturi and the large "regular" c/l venturi, I tried everything from 8.1K to 9.1K at launch. (These numbers come from an old Tower digital-read tach I've been using for years, and may only be useful for comparing performance using the same tach, although I have calibrated it in the past.) Anyway, I ended up shifting to an 11X6 Power Point prop and the larger venturi to get adequate power and fuel suction at the same time (higher revs, but higher fuel consumption, too). This is on Dr. Diesel's ball bearing mix. John