Piston and liner fit and performance
All my beliefs were stood on their head last weekend at our Rockhampton Q500 competition and I would appreciate any comments.
After the comp, the top three TT46 engines were inspected for any illegal modifications. There were none. The winning engine posted Australian record times of 76 seconds on the short course. On inspection of this remarkable engine, there was no shiny narrow band around the top of the piston side wall where it would normally pinch at TDC. The piston was badly carboned - real sooty black - indicating significant blow by on the down stroke. My TT46 is very tight at TDC and has a 1 to 2mm shiny band around the top of the piston side wall where it pinches at TDC.
The loose engine is performing better, with lower race times, on a standard muffler and 10x6 prop than my engine does on a Magic Muffler and 9x7 prop. The standard engine was running without a shim while mine had 0.005" under the head with the MM and 9x7 prop.
I had always thought that the tighter the TDC fit, the better the engine would perform. Not so in our case. In fact, its quite the opposite.
Any comments would be appreciated.[&:]