RS2K,
The two most popular glow engines on the Sig Cap 231EX's that I have seen are the 4-stroke Saito 180 and the OS 160 2-stroke. I know a flyer who has had 3 of these Caps all powered by the Saito 180. He has also flown another flier's Cap 231EX that is powered by an OS 160 and in his opinion it is overpowered. He put a 22oz tank in his Saito 180 powered Cap and gets 8 minutes safe flying time. The 180 really sucks up fuel.

My Cap is powered by an OS 160 2-stroke and has a fast vertical climb but I don't find this plane to be easy to hover. It appears that the OS 160 uses less fuel than the Saito 180. I'm using the tank that came with the kit and I set my timer for 9-minute flights but there is always lots of fuel left when I land. Sometimes I fly it for a couple minutes longer.

Initially I followed the guidelines in the OS manual for prop selection and used a 16x10 APC prop. It made the plane fly much too fast and made it difficult to slow down and land at a tight flying field. This is not a plane you want to go blasting around at high speeds with, the ailerons will flutter and come off. I tried to experiment with an 18x6 wood Zinger prop but the plane was making unusual noises in the air so I landed in about a minute. I put my hand on the plane and ran it up to high throttle and felt it vibrating so badly that I feared it might come all unglued. I'm currently using an 18x8 wood Zinger that I had on hand but I think the plane needs to slowed even more.

All servos except for the throttle are Futaba S9304 ball bearing core-less. Throttle uses a standard no-ball bearing Futaba S3003.

Throttle management is the key to longevity with this airplane.