Propwash October 2019 web.pub

The plug seal is another area for performance gains. The so called “turbo” or Nelson plug that seats on a taper at the base and not on a gasket has been shown to add several thousand rpm. See the Profi engine cross section for an example. Head cooling is another neglected factor. Small engines, especially on outboards, are over cooled. Turning off the cooling water as well as trimming fins on air cooled heads can add power. Ideally the squish area and the plug should be cool, but the combustion chamber needs to be hot to prevent heat loss. This was done in the head button pictured with a broad area over the squish and narrow grooves over the combustion chamber leading to a groove around the plug. Racers often drill a hole that directs the water leaving the head over the outside of the plug for cooling. The best squish action should come from a wide squish band with a sharp corner between the squish area and the central volume. The best scavenging should come from a narrow or no squish band with a blend radius into the combustion chamber like the trumpet head. Shallow chambers should also scavenge better. Toroidal and flat toroidal (bathtub) heads often are superior to the standard hemi head because of this. So what works best? It depends on a lot of factors that vary from engine to engine. Modern 5 port transfers are very good at directing the incoming mixture. With this and a really effective pipe, you can run a wide, sharp edge squish with high compression and get good power. However, only one nitro engine I know of, the Profi MB 40, has modern transfer passages. It has a standard hemi head with a wide squish. All others use transfer passage shapes that were found to be very poor in tests at the Queens University of Belfast decades ago. Still, the trend is toward a squish area nearly 50% of the bore area with sharp edges. Some testers have found power gains with the torodial or flat toroidal head designs and even trumpet shaped combustion chambers with very little squish band. PROPWASH 6 October 2019 UB Cylinders for Yamaha showing a poor #12 and an excellent #14 transfer design Squish Band Cooling How can you find the right combination? A series of standard air propellers have been used by engine builders to measure power for a very long time. Since the propeller absorbs power as the rpm cubed, a small rpm increase with a given propeller shows a large increase in power. An inertial dynamometer is more complicated but even better. In the field you can read the plug and head to get an idea of what is happening. Test with a new plug. The plug element should be lightly frosted and not distorted. A shiny plug element indicates a rich mixture. If the element is distorted the compression may be too high. Try adding a head shim. A broken or missing element usually means the mixture was too lean. The engine will often continue to run with a broken element, but the plug won’t glow with the glow igniter. Detonation shows up as sand blasted appearance on the squish and piston crown. Detonation happens when the compression is too high and the mixture explodes rather than burning with an advancing flame front as in the squish action example. This explosion not only greatly increases heat flow, but also scours away the cool boundary layer protecting the piston and combustion chamber. The greatly increased heat flow to the piston can result in a hole or melted area on the exhaust side. Most engines can stand small amounts of detonation without failure. The K&B head with the light frosting pictured ran without problems.