Propwash October 2019 web.pub

performance trick. This brings the ignition point more into the center of a hemi combustion chamber. The heat range of the plug also affects the ignition timing. Higher nitro fuels usually run best with a colder plug. Again, experiment is the usual way to find what works best. A huge number of plug combinations and head designs have been tried. I’m only going to look at designs that work with modern Schnuerle scavenging engines. The distance from the plug element to the piston effects combustion. Pulling the element out of the plug for a few coils is a standard PROPWASH October 2019 5 This is the size of the flame front at TDC . The flame front continues into the squish area at 20 degrees after TDC. Combustion is complete at 40 degrees after TDC and the burnt mixture keeps expanding until the exhaust opens around 80 to 90 degrees ATDC. Head Shapes Keeley head The other issue in head design is scavenging. A two stroke engine’s power is an excellent measure of scavenging efficiency. The flow from the transfers needs to sweep burned combustion products from the head into the exhaust. Typically the flow from the boost port does this. The head shape originated by Terry Keeley and “borrowed” by K&B is an example. The boost port flow sweeps up the small fingers and out the wider exhaust side. The angle between the narrow squish band and the central volume also helps. This design is reported to add a few thousand rpm over the standard head button. (Continued on page 6)