Originally posted by crossup:
what are the concerns for extending 4 stroke pipes?
Generally, 4-strokes (and diesels) are very tolerant of extended exhausts. As with 2-strokes, rigid pipe is best (I normally use thin-wall aluminium tube). One problem with 4-strokes is the exhaust gas temperature ... anything connected directly to the manifold needs to be metal really (silicone tubing will burn through quite quickly) - there are some nice after-market metal 'bendy pipe' systems that work well. On sport models I extend after the standard silencer ... silicone tube (with cable ties to retain) is fine for joints here. Typically I'll run some wide-bore aluminium tube down to the u/c, near the wheel. So long as it exits outside the prop arc (that's the key) 99% of the gunk will fly clear of the model. None of my 4-strokes spray much out of the crankcase breather, but if you do have a problem with this you can run a short pipe into a 35mm film can mounted on the bulkhead to trap it.
With 2-strokes I generally use quiet pipes (tuned), all silicone tube joints are secured with cable ties so they don't leak, and I fit a card or paper gasket between the exhaust stack and manifold.
I have film-covered IC models that are up to 15 years old and still looking good ... keeping them free from exhaust spray makes a huge difference to the lifespan (provided you don't crash them of course