Define "solid".  Combat and speed planes get "solid" runs all the time with humongous venturies and pressure systems.  At least, they get solid runs in a screaming-fast sort of way.

For a stunt run you want the engine to be running somewhere close to it's maximum torque rather than it's maximum power, because you want it to self regulate, or better yet, you want it to actually go faster the heavier it's loaded.  You don't get that from a wide open venturi and a pressure system.

Guys,
A question came up about the largest size venturi that can be used to get a engine to run consistent. If you run crankcase pressure can you actually have a venturi that is to big for the engine (say a 40) to run solid even if you have solid fuel supply to the spraybar?

  Of course. The more you count on the pressure and not the suction, the more touchy everything gets.

     Brett

The 21/40s run good on the 9mm intake ( G-15 late - 21/35 intake ) with crankcase pressure . Sound menacing on a 11 x 4 unmuffled .

The Cox Tee Dee .049 is an engine that thrives on a pressure bladder made from small surgical tubing and a fine(128tpi) needle valve. NVs, Tubing, fittings, barbed tips for Luer Lock fueling syringes, etc. can be purchased from Hank Nystrom at Texastimers.com. All of these parts work equally well with Norvel, Cox and all other engines that are capable of high output. Larger engines like a .25 may benefit from larger heavier bladder tubing. For these, the "Redcaps" used to seal caulking tubes make good bladders. In earlier bladder days, fountain-pen bladders and baby pacifiers were widely used too.

Boring out the Tee Dee venturi works only when the engine rpm is expected to go over 20,000. For this I would never go wider than a 5x3 Going up to .140" I.D. is the widely accepted number for the TD. A 9/64" drill bit will get you in that area. The late Joe Klause's K/K pressurised venturi's went as high as .150", but to answer your question(at least for TD .049s), larger bored TDs may lose power or become erratic, so bigger is not always better, and can cause unnecessary stress on the crankshaft and piston/rod ball socket. However Norvels work great if you just remove the venturi and put the spraybar right through the raised boss(bathtub) that formerly held the carb or venturi. I use the venturi on Norvels without boring anything, mainly because I like to have the screen to keep dirt out of the engine. There is obviously no bad effect from running those engines with only the bathtub.

Once you find your needle setting, you almost never have to vary much, excepting weather conditions and fuel differences. I leave my needle set and it always cranks up at that same setting. I prefer to use 35% nitro on all of my bladder fed machines. The higher nitro facilitates easy cranking and exceptional power output. This system also protects an engine from accidental lean hot runs. If it goes lean, the engine heats up slowing it, but the continuous in-rush of pressurized fuel "refrigerates" it, cooling it, and it resuming power. It'll sound like an up and down surge, but the action of the incoming fuel cooling a hot sagging run keeps it from damaging itself. Just add a small richening turn to the needle and fly again. It works on Cox reed valve engines very well too.  And it's a quick easy learning curve.

It's a remarkably dependable easy to use system, and I always say to my sport flying friends, once you go bladder, you'll never go back to tanks.
I hope that answers some of your questions.
Rusty