If your question is:
"If I run 30% nitro in my OS 91HZ, do I need the head shims?"
The conservative answer is "Yes, you do".
But, it will depend on many factors. Start with the shims installed and then take them out after you get it tuned up and have a few flights.
The compression is lower with the shims.
Without the shims, you are getting more compression and the higher nitro can cause a bent rod or burned piston.
No shims would be the way to run 15% nitro.
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