RE: Smallest RIRE engine
I have a UCCUM 09 with pipe thats rear exhaust. I am at work now but can get pics up later tonight. I think it was right around 30,000 on a 7x3
It might be out of your size range but i dont think anything out there can beat the Jett BSE 35 LX FIRE with higher timing.
You can also can some of the R/C car engines in rear exhaust that can turn a 7x6 APC in the 23,000 range. the traxxis engine has the longest stroke and makes great power.