ORIGINAL: Mr Cox
Well that's why I'm asking which engines we are talking about that would need 25% nitro?
Here are a few examples where I have personal experience. For running on 5.7-6x3 props;
Brodak MkI (CS); the two I have were overcompressed even on 0% nitro. It needed 3-4 head shims to run on 10% nitro.
AP wasp; runs fine on 0% nitro, can handle 10% but I don't think it needs any more nitro.
Cox 05 RC; overcompressed for 10% nitro with the TD head and one head shim.
MP Jet; comes with three head shims installed and seem fine for 10%nitro, I haven't tried higher
Cox engines with the Merlin head; needs about 3-4 head shims to run on 10% nitro (I haven't tried it on 0%)
HB .12; set-up for 0% nitro, needs headshims to run with more nitro
MDS .18; set-up for 0% nitro, needs headshims to run with more nitro