I'm planning to use the OS91DF with Dynamax fan unit for the Yellow Aircraft Starfire. Wondering what the optimal tuned pipe length is from glow head to part where pipe reaches maximal diameter. I know the distance for use in ByroJet fan. Is it the same for Dynamax fan set up?

Your best bet for reliable success is to use either a Jet Model Products or BVM fixed length pipe. Either of these pipes provide optimal performance with your setup with no fiddling or guesswork. Neither are cheap but will be far better and less trouble than any other setup.

I had a fan powered Starfire once upon a time that had your engine and fan in it. I believe it was setup with the BVM pipe.

http://www.jetmodelproducts.com/exhaust.htm

http://bvmjets.com/Pages/not%20used/exhaust.htm

I need advice regarding set up:
1) JMP makes a pressure vent valve that goes with their large bore in-flight mixture valve, instead of using pipe pressure.
2) BVM uses pipe pressure with their smaller bore in-flight mixture valve.
Either will work well with os91/Dynamax set up in the Starfire?

I've used both the JMP and the BVM setup (and also the BVM puffer pipe with the JMP in-flight mixture) and all three of these combos work well. Personally, I prefer the BVM puffer pipe (though you have the added minor hassle of routing a length of fuel tubing into the duct) for simple reasons of cleanup.

The tanks will expand under pressure from both the pressure vent and the puffer pipe. If you roll inverted and pull back on the throttle (for instance when at the top of a Split-S), the fuel will enter the vent line and be exhausted from the vent/puffer. The puffer is much closer to the rear of the duct and aft of the engine so the majority of the fuel will be expelled without getting all over the inside of the duct and around the engine.

As for the length of tuned pipe, I would agree that the best course of action is to get a fixed length pipe. Two piece pipes with headers and couplers are a pain and they tend to come apart. BVM no longer makes pipes as they seem to be strictly supporting electric and turbine power, but you do run across them from time to time. JMP still seems to still make all the required pieces and they certainly do have a really great adaptor for the OS91 and a strong pipe hanger.

Good luck,

Jim

Set your engine, fan and pipe up in a test stand. Run the thing WFO while moving the the pipe back and forth. With your ear find peak rpm lengthen the pipe about .125" to .188" and you have your length. If you have a freind with a leath and a tig welder, you can make your pipe a fixed lenght at this point by leathing out a header form 6061 bar stock and heliarc welding it the pipe at the right lenght.

Dont try welding a cast header to a spun aluminum pipe. It dont work.

If you want to get more precise with the rpm paint a .25" wide line across the spinner dome on the fan a use a rpm gauge to read the rpm. Find the peak rpm and lengthen the pipe tell the rpm drops off 300 to 500 rpm.