I believe this is the five port? If it is the five port or the three port, here are some practical starting points:

1. From the center of the head, measure 14" to the widest part of the tuned pipe. Sometimes the pipe has a lathe line denoting this, but I'm sure you know what I'm talking about. This is a very conservative pipe setting.

2. Tach readings at this length on an 11 X 7 will be in the 13,500 range on 10% nitro.

3. Start trimming 1/2" pieces off of the header and keep taching the engine. 1/2" pieces could account for as much as 500 rpm gain or little as zero. Expect to see noticeable power increase by 1 1/2" of trimming.

4. My experience with my OPS many years ago was the length ended up being right at 12 1/2" to 12 3/4", center of head to wide part of the pipe. Ground tach readings @ 15,500 on 10% nitro with a wood 11 X 7. I loved to hear it unload in the air! Any shorter on the pipe and plugs popped after one flight. Not knowing what size prop or the amount of nitro could affect these parameters, so consider these figures ballpark.

I hope this helps.
Regards,
Mike