<div style="margin: 0cm 0cm 10pt">Mattk</div><div style="margin: 0cm 0cm 10pt">Thanks for sharing all your info.</div><div style="margin: 0cm 0cm 10pt">I have not read all your posts, so please forgive me if I ask what has been before.</div><div style="margin: 0cm 0cm 10pt">As for the mintor Vs OS compression &ndash; you cant really compare this by turning the engines over cold. Mintor have alu pistons in steel liners hence the design is such that the bore tolerances are larger to allow for the piston to swell when the engine runs and heats up. Not sure though on the OS design though. I belive that ring and ring land design can also play a part here, meaning some designs have low compression until the force of the explosion actually pushes the ring outwards against the cylinder walls. I am no expert with regard to this though.</div><div style="margin: 0cm 0cm 10pt">What pipe setup incl header length are you using. Have you played with the header lengths. I have a Mintor 38cc on the test bench and brand new out the box after 10mins got 7600 rpm using a JXF / Xoar 19x8 wood prop. The JXF wood are higher load props than APC to my knowledge. This is also on the std exhaust system and not a pipe.</div>