Blast Pipe Diameter

[greasemonkey]

Hi All
What calculations do you use to calculate the blast pipe diameter? The cylinders are 1.6875" diameter by 2.4375" stroke. This scales up very closely with the full size engine dimensions of 19" diameter by 28" stroke. Someone suggested that I scaled the full size blast pipe which at 5.125" would give me a dimension of 0.454" diameter however this seems a little large. What does everyone think? The model design calls for 0.25" but that gives a fair bit of back pressure and a very strong blast. How about using the 0.25" designed diameter with a multiple jet nozzle? Would that offer any advantage?
Andy

[waggy]

Andy,
As far as I'm aware the full size calculation doesn't apply to us, the reason being explained to me that "steam is steam" no matter what size or scale of the engine.
I have two 5" loco's, both have blast nozzles of 9/32" diameter and work well. I suggest you start off with an easily removeable nozzle with a 1/4" hole. If the blast is too fierce remove the nozzle and open it out, refit and try again. Taper reamers are excellent tools for this work as they open the hole out and give a useful taper to the nozzle. A small alteration can make a big difference!
Splitting your blast with a multi jet nozzle will make things worse for you as there is a greater surface area to the blast so a higher vacuum is created.
Let me know how you go on.

Regards,

Dave.